EFFECTS OF CONVENTIONAL PASSIVE EARMUFFS, UNIFORMLY ATTENUATING PASSIVE EARMUFFS, AND HEARING AIDS ON SPEECH INTELLIGIBILITY IN NOISE
DISSERTATION
Presented in Partial Fulfillment of the Requirements for
The Degree Doctor of Philosophy in the Graduate
School of The Ohio State University
By
Babette L. Verbsky, M.S.
* * * * *
The Ohio State University 2002
Dissertation Committee: Approved by Dr. Lawrence L. Feth, Advisor
Dr. Gail Whitelaw ______Advisor Dr. Mark Stephenson Speech and Hearing Science Department
ABSTRACT
Occupational hearing conservation regulations neither address issues related to
speech intelligibility in noise for normal-hearing or hearing-impaired workers, nor do the
regulations comment on the safety of hearing aid use by hearing-impaired workers. Do certain types of hearing protection devices (HPDs) allow for better speech intelligibility than others? Would use of hearing aids with earmuffs provide better speech intelligibility for hearing-impaired workers? Is this method of accommodation safe?
To answer these questions, a method for evaluating speech intelligibility with
HPDs was developed through a series of pilot tests. The test method allows for evaluation of both normal-hearing and hearing-impaired listeners. Speech intelligibility for normal-hearing listeners who wore uniformly attenuating earmuffs was found to be significantly better than for the same listeners who wore conventional earmuffs.
Hearing-impaired listeners were tested with each type of earmuff and while wearing their own hearing aids in combination with each earmuff. Unlike the normal hearing listener group, the hearing-impaired listener group did not exhibit better speech intelligibility with the uniformly attenuating earmuffs than with the conventional earmuffs. However, earmuffs worn in combination with hearing aids allowed for significantly better speech intelligibility than with either earmuff alone.
ii
To determine the safety of hearing aid use under earmuffs, a model was developed to predict occupational noise exposure for the aided-protected worker. Data
from real ear measures with an acoustic mannequin was found to be in agreement with
model predictions.
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Dedicated to the memory of my grandmother, Antoinette Elizabeth Ontko Dziak Fish,
who taught her daughters and grandchildren to value and pursue a good education
Dedicated also to my mother, Janet Marie Dziak Eaton, who has always given me the
encouragement, support and freedom to follow my dreams
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ACKNOWLEDGMENTS
I wish to thank my research colleagues at the National Institute for Occupational
Safety and Health (NIOSH) in the Hearing Loss Prevention Section at Taft Laboratories in Cincinnati, Ohio. My primary mentor at NIOSH, John R. Franks, spent many hours with me providing technical and informational support throughout the project. I am also grateful to Dr. Franks for extending the Fellowship to work on the Hearing-Impaired
Worker Project in conjunction with my dissertation. My colleagues on the Hearing-
Impaired Worker project, Thais C. Morata and Christa L. Themann, participated in discussions regarding the test protocol. Ms. Themann also made excellent analytical observations, shared writing suggestions, and encouraged me on a daily basis. William J.
Murphy provided technical support for the ATF measurements done in the Hearing
Protector Laboratory. Dr. Murphy also engaged me in challenging and thought- provoking discussions regarding this research, and was always available for technical assistance. Chucri A. (Chuck) Kardous developed a custom MatLab program for digital file analysis which was used in this research project. And finally, Edward Krieg, Jr and
Peter B. Shaw, provided statistical support for this project.
I am indebted to The Ohio State University Speech-Language-Hearing Clinic
Director, Gail Whitelaw, and her office and audiology staff for providing recruiting assistance, equipment and facilities for this research. Without their co-operation this v
project may not have been possible. In particular, a special thank you goes to Shannon
Hand for scheduling support.
John Zimmer and Lawrence L. Feth assisted in the laboratory setup for the preliminary work with normal hearing listeners. Many thanks go to fellow students for volunteering to participate in the normative studies. I am also grateful to Dr. Feth’s lab group, B.A.N.G., for helpful comments on this research.
My dissertation committee included three remarkable people, Mark Stephenson,
Gail Whitelaw, and Lawrence Feth, who always made time for me when I needed them.
Dr. Stephenson has been a mentor, colleague and friend. His insightful comments have kept me focused on the important issues in hearing conservation. Dr. Whitelaw has given me excellent advice during my time at OSU and has always been willing to stand in the gap when needs would arise. Dr. Feth, my advisor, has invested countless hours in my education both in the classroom and in the laboratory. More importantly, he taught me to use my knowledge of the discipline to be an independent researcher.
And finally thank you to my husband, Mark, for his unconditional love and many acts of kindness throughout the entire dissertation process.
vi
VITA
July 19, 1959………. Born – Ashland, Ohio
1982………………... Bachelor of Music Education, Bowling Green State University
1990 – 1991………... Teaching Assistant, Bowling Green State University
1991………………... Master of Science, Communication Disorders, Bowling Green
State University
1991 – 1994………... Clinical Audiologist, West Central Ohio Hearing & Speech Center
1995……………….. Clinical Audiologist, Mercy Speech & Hearing Center
1996 – 1999……….. Chief Audiologist, Mary Rutan Hospital
1999 – 2000……….. Teaching Assistant, The Ohio State University
PUBLICATIONS
Research Publication
None
FIELDS OF STUDY
Major Field: Speech and Hearing Science
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TABLE OF CONTENTS
Page Abstract ...……………………………………………………………………………... ii
Dedication ……………………………………………………………………………... iv
Acknowledgments ……………………………………………………………………... v
Vita …………………………………………………………………...... vii
List of Tables ………………………………………………………………...... xi
List of Figures.………………………………………………………………………….xiii
Chapters:
1. Introduction ………………………………………………………………………... 1
1.1 Measurement of Speech Intelligibility with HPDs …………………...... 3 1.1.1 Effects of Hearing Impairment on Speech Intelligibility with HPDs...... 6 1.1.2 Effects of Presentation Level on Speech Intelligibility with HPDs….... 8 1.1.3 Effects of SNR on Speech Intelligibility with HPDs...... 10 1.1.4 Effects of Test Methods on Speech Intelligibility with HPDs...... 11 1.1.5 Effects of Type of Noise Masker on Speech Intelligibility with HPDs.. 12 1.1.6 Effects of Reverberation Time on Speech Intelligibility with HPDs...... 13 1.1.7 Effects of Type of HPD on Speech Intelligibility with HPDs...... 14 1.2 Audibility with HPDs: Job Safety...... 17 1.3 Balancing Speech Intelligibility and Audibility in Specific Work Environments...... 17 1.4 HAs in Noisy Environments...... 18 1.5 Prediction of Speech Intelligibility with HPDs and Hearing Aids...... 18 1.6 Project Overview...... 20
2. Preliminary Work...... ……………………...………………………………...... 21
2.1 Test Protocol Development ...... …...... ……….……………………...... 21
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2.2 Test Protocol Validation ...... 23
2.2.1 Phase I...... 24 2.2.1.1 Hypotheses...... 24 2.2.1.2 Study Design...... 27 2.2.1.3 Equipment...... 27 2.2.1.4 Subjects...... 35 2.2.1.5 Test Procedures...... 37 2.2.1.6 Results...... 41 2.2.1.7 Discussion...... 55
2.2.2 Phase II...... 59 2.2.2.1 Purpose of the Study...... 59 2.2.2.2 Study Design...... 59 2.2.2.3 Equipment...... 59 2.2.2.4 Subjects...... 60 2.2.2.5 Test Procedures...... 61 2.2.2.6 Results...... 64 2.2.2.7 Discussion...... 70
2.3 Prediction of Noise Exposure with Passive Earmuffs and Hearing Aids Worn in Combination...... 71 2.3.1 Aided-Protected Noise Exposure Model...... 72 2.3.2 Validation of Model Predictions...... 79
3. Methods...... 82 3.1 Introduction...... 82 3.2 Research Questions...... 82 3.3 Study Design...... 83 3.4 Equipment...... 83 3.5 Subjects...... 86 3.6 Procedures...... 94
4. Results...... 99 4.1 Audiological Evaluation...... 99 4.2 Hearing Aids...... 109 4.3 Experimental Test Protocol...... 111 4.3.1 Repeated Measures ANOVA...... 115 4.3.2 Correlation Analyses...... 121
5. Discussion...... 129 5.1 Protected versus Aided-Protected Speech Intelligibility...... 129 5.2 Conventional versus Uniformly-attenuating Earmuffs...... 130 5.3 Speech Intelligibility Predictions...... 135 5.4 Study Limitations...... 136 ix
5.5 Conclusions...... 138 5.6 Future Research...... 138
Reference List...... 141
Appendix A TEXT OF DIRECT MAIL SUBJECT RECRUITMENT LETTER… 149
Appendix B SUBJECT RECRUITMENT NEWSPAPER ADVERTISEMENT (ENLARGED)……………………………………………………….. 150
Appendix C MEDICAL SCREENING QUESTIONS……………………………. 151
Appendix D SUBJECT CONSENT FORM………………………………………... 152
Appendix E NORMAL HEARING SUBJECTS’ HEARING THRESHOLD LEVELS……………………………………………………………… 154
Appendix F HEARING-IMPAIRED SUBJECTS’ HEARING THRESHOLD LEVELS……………………………………………………………… 155
Appendix G QUICK SIN TEST SCORES (SNR50 in dB) BY LISTENING CONDITION………………………………………………………… 156
Appendix H HEARING-IMPAIRED SUBJECTS’ DATA USED FOR CORRELATION WITH QSIN SCORES IN EXPERIMENTAL LISTENING CONDITIONS…………………………………….…... 158
Appendix I DATA RECORDING FORM………………………………………... 159
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LIST OF TABLES
Table Page
1.1 Codes for HPD are 1-conventional plug, 2-uniformly attenuating plug, 3- conventional earmuffs, 4-uniformly attenuating earmuffs, 5-automatic noise reduction (ANR), 6-level dependent earmuffs, 7-communication system, 8-flight helmet...... 4
2.1 Assumed Protection Value = mean - 1 standard deviation. Methods used are per ISO 4869-1 and 4869-2...... 35
2.2 ANOVA Table. Fixed Speech versus Fixed Noise in Adaptive Tracking Test Protocol...... 45
2.3 ANOVA Table. Headphone versus Sound Field Testing with HINT...... 47
2.4 Model for Aided-Predicted Noise Exposure – Step 1...... 74
2.5 Model for Aided-Predicted Noise Exposure – Step 2...... 75
2.6 Model for Aided-Predicted Noise Exposure – Step 3...... 76
2.7 Model for Aided-Predicted Noise Exposure – Step 4...... 78
3.1 Maximum permissible hearing aid gain levels with exposure to speech at 90 dB SPL while wearing Bilsom’s model 817 earmuffs...... 91
3.2 Maximum permissible hearing aid gain levels with exposure to speech at 90 dB SPL while wearing Bilsom’s model 717 earmuffs...... 92
3.3 Maximum permissible hearing aid gain levels with exposure to speech at 90 dB SPL while wearing the lesser protective (by frequency) of the experimental earmuffs...... 93
4.1 Hearing aids worn by subjects...... 110
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Table Page
4.2 Hearing aid gain by subject identifier as a function of center frequency. All gain values were within acceptable study limits (compared to Maximum Permissible Hearing Aid Gain)...... 111
4.3 The relationship between sound field measures of speech intelligibility and the experimental listening conditions. Values shown are in units of R-squared. Bolding indicates the strongest correlation for the listening condition...... 123
4.4 Table shows correlations between headphone measures of speech intelligibility and the experimental listening conditions. Values shown are in units of R-squared. Bolding indicates the strongest correlation for the listening condition...... 124
4.5 The relationship between selected HTLs obtained under headphones and the experimental listening conditions. Values shown are in units of R- squared. Bolding indicates the strongest correlation for the listening condition with that group of predictors...... 126
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LIST OF FIGURES
Figure Page
2.1 Ambient noise levels in audiometric booth used in Phase I and II compared to ANSI S3.1-1991...... 28
2.2 Frequency response curve for the GSI speaker box used in Phase I and II...... 29
2.3 Conventional earmuffs on left (Bilsom's model 717) and uniformly- attenuating earmuffs on right (Bilsom's model 817)...... 32
2.4 Bilsom's model 717 earmuffs were representative of conventional earmuffs. Bilsom's model 817 earmuffs provided more uniform attenuation characteristics between the low- and high-frequency regions...... 33
2.5 Average Hearing Threshold Levels (HTL) for Phase I subjects. The circles on the red line show the right ear average HTLs and the X’s on the blue line indicate the left ear average HTLs...... 36
2.6 The HINT has published norms for the headphone (HP) condition, but requires the user to establish site-specific norms for each sound field (SF)...... 42
2.7 While the HINT does not provide means for the sound field (SF), it does provide the standard deviation from the normative sample...... 43
2.8 Means +/- 95% confidence interval for each HINT adaptive tracking method.. 44
2.9 Means +/- 95% confidence intervals for each HINT presentation method...... 46
2.10 Normal hearing adults' SNR50 scores and group averages plotted as a function of speech level in dB SPL. Individual subject scores are represented by connected data points...... 49
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Figure Page
2.11 For conditions of reduced speech audibility (70 dB SPL with earmuffs), the difference between the earmuffs was significant. When speech audibility was not reduced (90 dB SPL with earmuffs), the difference between the earmuffs was not significant...... 51
2.12 Slope values of experimental masker noises. Frequency analysis of the file containing the HINT speech spectrum waveform was not possible due to file format incompatibility with the analysis software...... 52
2.13 Noise type effects on DRT scores: A case study. DRT scores in percent correct for one subject in each of the experimental noises. Speech and noise were presented at 70 dB SPL from the same loud speaker in front of the subject...... 54
2.14 Average hearing threshold level (HTL) by frequency. Right ear HTLs are indicated by circles connected with a red line, and left ear HTLs are indicated with Xs connected with a blue line...... 61
2.15 SNR50 for the DRT was equal to the estimated 75% correct point on the psychometric function. Estimation was based on least squares linear trend line fit to the measured data points...... 64
2.16 Average SNR50s plotted as a function of listening condition for each test tool...... 66
2.17 DRT features analysis for listening in quiet. Bars show average performance for eight normal hearing subjects with a speech level of 70 dB SPL...... 67
2.18 DRT features of speech scores in percent correct (corrected for guessing). Unoccluded listening (blue squares), Conventional Earmuffs (red circles), Uniformly-attenuating Earmuffs (green triangles)...... 69
2.19 Predicted versus measured noise exposure for linear hearing aids under uniformly-attenuating earmuffs. Measurements were made with an ER 7-C probe microphone in the ear canal of the ATF. Noise was presented in 1/3-octave bands at 83 dB SPL (equivalent to 90 dBA pink noise)...... 80
3.1 Ambient noise levels in audiometric booth used in the final study with equipment turned on compared to ANSI S3.1-1991...... 84
3.2 Frequency response curve to white noise at 70 dB HL from the audiometer for the OSU Clinic’s GSI speaker box...... 85 xiv
Figure Page
3.3 Average Hearing Thresholds (HTLs). Phase I normal hearing listeners are represented with Xs and Os. Final study hearing-impaired listeners are represented with squares and triangles...... 88
3.4 Maximum hearing aid insertion gain plotted as a function of frequency for 8 of 10 subjects. Maximum allowable gain values are from Table 3.3...... 94
4.1 Normal hearing subjects’ right ear HTLs. Box plots indicate the distribution of HTLs as a function of frequency. A line in the box indicates the median value, while the 25th and 75th percentiles are at the edges of the box. The 10th and 90th percentiles are the ends of the error bars. Outliers are shown with a plus (+)...... 100
4.2 Normal hearing subjects’ left ear HTLs. Box plots indicate the distribution of HTLs as a function of frequency. A line in the box indicates the median value, while the 25th and 75th percentiles are at the edges of the box. The 10th and 90th percentiles are the ends of the error bars. Outliers are shown with a plus (+)...... 101
4.3 Hearing-impaired subjects’ right ear HTLs. Box plots indicate the distribution of HTLs as a function of frequency. A line in the box indicates the median value, while the 25th and 75th percentiles are at the edges of the box. The 10th and 90th percentiles are the ends of the error bars. Outliers are shown with a plus (+)...... 102
4.4 Hearing-impaired subjects’ left ear HTLs. Box plots indicate the distribution of HTLs as a function of frequency. A line in the box indicates the median value, while the 25th and 75th percentiles are at the edges of the box. The 10th and 90th percentiles are the ends of the error bars. Outliers are shown with a plus (+)...... 103
4.5 Hearing-impaired subjects’ speech MCLs with TDH-39 headphones. MCL = Most Comfortable Loudness Level. The MCLs shown here were the speech levels used to present the NU-6 word lists in Figure 4.6...... 105
4.6 Hearing-impaired subjects’ NU-6 scores under THD-39 headphones. Speech presentation levels were the MCLs from Figure 4.5...... 106
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Figure Page
4.7 Hearing-impaired subjects’ NU-6 scores in the sound field with and without hearing aids. Speech presentation level was 70 dB SPL. Hearing aids, if adjustable, were set for typical one-on-one conversation. Subject HI1 did not return for this testing. Subject HI4 was tested in both conditions, and scored 0% in the unaided condition...... 108
4.8 QSIN scores for hearing-impaired subjects. Speech presentation level was 70 dB SPL. Hearing aids, if adjustable, were set for typical one-on- one conversation...... 109
4.9 QSIN group means for normal hearing subjects (Phase I) and hearing- impaired subjects (final study). Speech level was 90 dB SPL for the solid squares and circles. Speech level was 70 dB SPL for the open triangles, the X, and the *. Significant Difference = p < 0.05...... 115
4.10 Interaction of Listening Condition and Gender...... 118
4.11 Interaction of Age and Gender. Overall QSIN Scores are individual subject’s scores averaged across all listening conditions. Lines are linear trend lines...... 119
4.12 Male versus Female Subjects by Hearing Status: Average Audiograms...... 120
4.13 Hearing Aid Insertion Gain Measurements at Use Settings with 70 dB SPL Swept Pure Tones...... 121
5.1 Individual speech intelligibility at 90 dB SPL with and without earmuffs as a function of HTL at 2 KHz...... 132
5.2 Better Ear Audiogram: Subject HI3 versus Subject HI8...... 134
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CHAPTER 1
INTRODUCTION
Current hearing conservation regulations and standards were developed to protect workers with normal hearing from the adverse effects of noise exposure. First, noise reduction ratings (NRRs) were required for hearing protection device (HPD) labels (U.S.EPA, 1979). Second, noise level measurements, audiometric monitoring, and the provision of HPDs were mandated (Occupational Safety and Health
Administration, 1983a). Finally, standard methods for measuring the real-ear attenuation of HPDs were described in ANSI S12.6-1997 (American National Standards
Institute, 1997b). Each of these documents discusses the use of HPDs to attenuate noise to safer levels in an effort to prevent workers from acquiring noise-induced hearing loss
(NIHL).
In addition to protection from NIHL, many workers require good speech intelligibility for the safe performance of their job duties. Occupational sound environments often contain speech in addition to noise. HPDs do not selectively attenuate the noise; rather, they attenuate both noise and speech. Early studies established that for high levels, the attenuation of speech by HPDs did not impair speech intelligibility, and for some acoustic environments, speech intelligibility was actually improved (Kryter, 1946; Pollack, 1957). More recent investigations
1 confirmed these assertions in normal hearing subjects, but have found evidence that
HPDs may degrade speech intelligibility in hearing-impaired subjects (Abel, Alberti,
Haythornthwaite, & Riko, 1982; Lindeman, 1976; Wilde & Humes, 1990; Abel,
Armstrong, & Giguere, 1993; Abel & Spencer, 1997). Current hearing conservation regulations and standards do not make provisions for the accommodation of hearing- impaired workers.
Different types of HPDs can affect speech intelligibility differently depending on the frequency spectrum of the noise, temporal characteristics of the noise, level of the noise (Abel et al., 1982), speech to noise ratio (Chung & Gannon, 1979), and attenuation characteristics of the HPD (Abel & Spencer, 1997; Reeves, 1998; Van
Wijngaarden & Rots, 2001; Wagstaff & Woxen, 2001). Currently, no standard test method exists for the evaluation of an HPD’s effect on speech intelligibility. Therefore, depending on a worker’s hearing status, the acoustic environment, and the attenuation characteristics of the HPD, workplace communications could be rendered unintelligible or inaudible for some workers.
Data regarding the effects of particular HPDs on speech intelligibility are not available from the manufacturers, and hearing conservationists typically do not have access to the necessary equipment for the measurement of speech intelligibility, which requires a calibrated sound field. Without the benefit of knowing to what extent each
HPD affects speech intelligibility for a given acoustic environment and hearing loss, hearing conservationists are left to fit HPDs based mainly on NRRs, documented workplace noise levels, worker preference (usually based on physical comfort) and, ultimately, the inventory of employer-provided HPDs.
2 Clinical audiologists usually do have a calibrated sound field. However, clinical audiologists do not have a validated method for testing speech intelligibility differences between HPDs, nor do they typically have the noise measurement spectrum and/or level data from the workplace with which to make valid predictions of real world speech intelligibility for a given type and degree of hearing loss.
1.1 Measurement of Speech Intelligibility with HPDs
A review of the literature from 1990 to 2001 revealed 15 studies (Table 1.1) in which at least one dependent variable was speech intelligibility with HPDs. The effects of HPDs on speech intelligibility were measured by these researchers using various methodologies. The test protocols used included varying types, numbers, and levels of independent variables. These studies showed that hearing status of the subjects, presentation level, speech-to-noise ratio (SNR), test methods (linguistic unit, response set, and/or psychophysical procedure), background noise masker, reverberation time, and HPD are among the variables that can affect speech intelligibility with HPDs.
3
WNL HI Test Study Citation Masker HPD (n) (n) Method
Van Wijngaarden & nonsense pink and helicopter 16 0 1, 2, 7, 8 Rots, 2001 monosyllables noises
Wagstaff & Woxen, 9 0 monosyllables helicopter noise 1, 2, 7 2001
Abel & Spencer, speech spectrum and 24 0 FAAF Test 1, 5, 7 1999 cable swager noises high, mid, and low 1 and 3 Reeves, 1998 16 28 MRT frequency-emphasis (emulated) noises Abel & Spencer, 32 16 SPIN cable swager noise 1, 2, 3, 5 1997 Hashimoto, Kumashiro, & 10 0 monosyllables pink noise 1 and 3 Miyake, 1996 Wagstaff, Tvete, & monosyllables 10 0 helicopter noise 7 Ludvigsen, 1996 and spondees Gower, Jr. & Casali, 9 0 MRT pink and tank noises 5 and 7 1994 Shilling RD & variation of 9 0 simulated aircraft noise 1 and 8 Thomas GB, 1994 MRT Abel, Armstrong, & FAAF Test and quiet and cable swager 40 20 3, 4, 6 Giguere, 1993 SPIN noise white, low pass (-12 words in Arlinger, 1992 15 0 dB/oct) and high pass 3, 4, 6 sentences (12 dB/oct) noises
Nixon, McKinley, & 10 0 MRT pink noise 5 Steuver, 1992
Table 1.1: Codes for HPD are 1-conventional plug, 2-uniformly attenuating plug, 3- conventional earmuffs, 4-uniformly attenuating earmuffs, 5-automatic noise reduction (ANR), 6-level dependent earmuffs, 7-communication system, 8-flight helmet. (Continued)
4 Table 1.1: Continued
WNL HI Test Study Citation Masker HPD (n) (n) Method Total subjects = CID W-22 12 (pilots Chan JW & Simpson words, PB Recorded HA-1S with "varying 5 and 8 CA, 1990 words, PD-100 (Cobra) cockpit noise degrees of words hearing loss") nonsense 193 Pekkarinen, monosyllables, (10 to Viljanen, Salmivalli, 0 spondees, and white noise 3 28 per & Suonpaa, 1990 words in group) sentences 12 Pekkarinen, groups disyllables and Salmivalli, & of 10- 0 "wideband" noise 3 trisyllables Suonpaa, 1990 40 per group
Of these 15 studies, eight used speech tests in English (Abel & Spencer, 1997;
Gower, Jr. & Casali, 1994; Abel et al., 1993; Abel & Spencer, 1999; Chan & Simpson,
1990; Reeves, 1998; Shilling RD & Thomas GB, 1994; Nixon, McKinley, & Steuver,
1992), three in Norwegian (Van Wijngaarden & Rots, 2001; Wagstaff & Woxen, 2001;
Wagstaff, Tvete, & Ludvigsen, 1996), one in Japanese (Hashimoto, Kumashiro, &
Miyake, 1996), one in Swedish (Arlinger, 1992), and two in Finnish (Pekkarinen,
Salmivalli, & Suonpaa, 1990a; Pekkarinen, Viljanen, Salmivalli, & Suonpaa, 1990b).
Of the English language studies, the most commonly used speech intelligibility test was the Modified Rhyme Test (MRT) (House, Williams, Hecker, & Kryter, 1965) or variations of the MRT. The Four Alternative Auditory Feature Test (FAAF) (Foster &
5 Haggard, 1979) and the Speech Perception in Noise Test (SPIN) (Bilger, 1994) were the next most commonly-used tests of speech intelligibility used to evaluate HPDs. Other
English word lists used in these studies included the Central Institute for the Deaf (CID)
W-22 Word Lists (Hirsh et al., 1952) and Phonetically Balanced (PB) words (per MIL-
STD-1472C). A more in-depth discussion of speech intelligibility test tools is found in
Chapter 2.
1.1.1 Effects of Hearing Impairment on Speech Intelligibility with HPDs
Only three of the 15 studies compared hearing-impaired subjects to normal hearing subjects (Reeves, 1998; Abel & Spencer, 1997; Abel et al., 1993). Reeves’
(1998) data showed lower speech intelligibility scores for the hearing-impaired subjects than for the normal hearing subjects in every listening condition. Moreover, the moderate to severe hearing loss group performed more poorly than the mild to moderate hearing loss group. Statistically, hearing status interacted with hearing protector, presentation level, and noise type. Hearing-impaired subjects did not perform like their normal hearing counterparts.
Abel and Spencer (1997) tested subjects at higher presentation levels (80 or 90 dB SPL) than in Reeves’ study (40 to 70 dB SPL). They reported that normal hearing subjects’ speech intelligibility increased with HPD use versus unoccluded listening in all conditions; however, the hearing-impaired group’s performance was qualified by presence or absence of ANR, amount of HPD attenuation, and linguistic context of speech stimuli. In general, the hearing-impaired subjects scored better with ANR than without ANR. Low context word recognition decreased as the amount of HPD
6 attenuation increase. Similar results were not seen for high context words or with normal hearing subjects. Hearing-impaired subjects were adversely affected by variables that did not affect normal hearing subjects in terms of speech intelligibility performance.
Abel et al. (1993) compared speech intelligibility performance of a group of twenty mild to moderately hearing-impaired subjects with the same size groups of
“young” (ages 20 to 35) and “old” (ages 40 to 60) normal hearing subjects. Overall, hearing-impaired subjects performed more poorly as the amount of HPD attenuation increased; whereas the normal hearing subjects’ performance between HPDs remained unaffected.
In a fourth study (Pekkarinen et al., 1990a), normal hearing subjects listened to filtered words to simulate performance by hearing-impaired subjects. This simulation of hearing impairment also showed that conventional earmuffs differentially affected speech intelligibility at 0 SNR for presentation levels of 60 and 85 dBA. While earmuffs improved speech intelligibility for unfiltered words, which represented normal hearing, they did not improve speech intelligibility for filtered words (simulated hearing impairment).
Chan and Simpson (1990) did not group subjects’ speech intelligibility data on the basis of hearing sensitivity, but stated that the subjects had “varying degrees of hearing loss.” However, even though hearing-impaired subjects participated in the study, no effects of hearing loss on speech intelligibility were reported. It is not possible to directly compare the speech intelligibility data from this study’s mixed
7 hearing status group with that of a normal hearing group due to the inconsistency of variables between studies.
1.1.2 Effects of Presentation Level on Speech Intelligibility with HPDs
For the studies which tested the effects of speech presentation level on speech intelligibility with HPDs, speech presentation levels ranged from 40 dBA (Reeves,
1998) to 85 dBA (Hashimoto et al., 1996; Pekkarinen et al., 1990b; Pekkarinen et al.,
1990a). These levels also encompassed the range of speech presentation levels used in the studies with HPDs which did not specifically test for level effects.
Reeves (1998) presented speech at 40, 46, 52, 58, 64, 70 dBA with noise at -3, -
3, -8 SNR for high-, mid-, and low-frequency emphasis noise, respectively. These
SNRs were determined by averaging three normal hearing listeners’ 70 – 75% correct
SNR for each noise type at a speech presentation level of 70 dBA. These levels were chosen to prevent ceiling effects in the normal hearing data, and thus, created a more sensitive test of speech intelligibility. However, it should be noted that the maximum noise levels (67, 67, and 62 dBA for high-, mid-, and low-frequency emphasis noise, respectively) were well below the level at which HPDs would typically be worn.
The effects of presentation level on speech intelligibility in noise varied by hearing status, frequency spectrum of the noise (high-, mid-, or low-frequency emphasis), and emulated HPD. For the emulated HPD conditions, average earplug and average earmuff transfer functions were applied to the speech and noise stimuli and presented over binaural headphones.
8 Normal hearing subjects in high-frequency emphasis noise performed best at 64 dBA in both of the emulated HPD conditions, best in mid-frequency emphasis noise at
58 dBA with the emulated earplugs, and best in low-frequency emphasis noise at 64 dBA with the emulated earplugs and 52 dBA with the emulated earmuffs. These results are consistent with reported levels for optimal speech intelligibility (American National
Standards Institute, 1969).
Hearing-impaired subjects in high- and mid-frequency noise showed no difference between presentation levels for either of the emulated HPD conditions. Mild to moderately hearing-impaired subjects in low-frequency emphasis noise performed best at 52 dBA with the emulated earplugs, but showed no difference between presentation levels with the emulated earmuffs. The moderate to severely hearing- impaired subjects in low-frequency emphasis noise showed no difference between presentation levels for either of the emulated HPD conditions.
The results for the hearing-impaired groups suggest that even at the highest speech presentation level (70 dBA), audibility may have been reduced sufficiently by the subjects’ hearing losses and the emulated HPDs that the lack of audible speech may have impaired speech intelligibility. For all but one condition, further incremental reduction of the speech presentation level failed to show any significant decrease in speech intelligibility for either of the hearing-impaired groups.
Hashimoto et al. (1996) tested ten normal hearing subjects with three HPDs at speech presentation levels of 65 and 85 dBA. The pink noise masker was presented at
0, +5, and +10 dB SNR. The authors stated that these SNRs were chosen based on earlier findings that for these conditions, less than 0 dB SNR would yield near chance
9 performance and greater than +10 dB SNR would result in near perfect performance.
Results of this investigation were affected by speech presentation level. When speech was presented at 85 dBA, speech intelligibility was improved over the unoccluded condition; however, when speech was presented at 65 dBA, speech intelligibility remained unchanged in noise and worse in quiet relative to the unoccluded condition.
In two similar studies, young normal hearing subjects (mean age 20.6 years, standard deviation 2.0 years) were tested with conventional earmuffs at speech presentation levels of 60 and 85 dBA (Pekkarinen et al., 1990a; Pekkarinen et al.,
1990b). White noise or “wideband” noise was presented at 0, +5, and +10 dB SNR.
When speech was presented at 60 dBA, the unoccluded listening condition provided for better speech intelligibility than listening with earmuffs. However, when the speech was presented at 85 dBA, listening with earmuffs provided better speech intelligibility relative to the unoccluded condition.
While these studies differed in many respects, each provided some evidence that for protected listening conditions, higher speech presentation levels provided enhanced speech intelligibility in certain listening conditions. Beyond that observation, it is not appropriate to generalize the findings.
1.1.3 Effects of SNR on Speech Intelligibility with HPDs
Chan and Simpson (1990) found that SNR had a larger effect on speech intelligibility in 85 dBA of helicopter noise than did the presence of ANR in flight helmets. Without the benefit of ANR, speech intelligibility increased 21% due to an increase in speech level from 85 dBA (0 SNR) to 95 dBA (+10 SNR). The addition of
10 ANR to the 0 SNR condition netted a 13% increase in speech intelligibility, and at +10
SNR, only a 6% increase in speech intelligibility was realized.
In a study with 24 normal hearing subjects (ages 20-50), speech was presented at
85 dB SPL in backgrounds of speech spectrum or cable swager noise at +5, 0, -5, and -
10 dB SNR (Abel & Spencer, 1999). Averaged across all HPD conditions, speech intelligibility decreased as the SNR decreased. The rate of decrease in speech intelligibility was faster for the cable swager listening conditions.
Pekkarinen et al. (1990a) found that for speech presentation levels at 60 and 85 dBA and 0 dB SNR, speech intelligibility was improved for both filtered and unfiltered
(hearing loss simulation) words. When SNRs were +5 or +10 dB at the lower speech presentation level, earmuffs impaired speech intelligibility for both filtered and unfiltered words. For the same conditions at the higher speech presentation level, speech intelligibility was unaffected.
Overall, the results of these three studies suggest that at lower speech presentation levels, higher SNRs do not improve speech intelligibility and may actually prove to be detrimental. For higher speech levels, the results were no different between levels of SNR for protected speech intelligibility.
1.1.4 Effects of Test Methods on Speech Intelligibility with HPDs
Only two of the studies looked at the effects of test methods on the measurement of protected speech intelligibility (Abel et al., 1993; Wagstaff et al., 1996). While the other studies did not compare different test methods, a list of the types of speech stimuli or published tests used in these 15 review studies is included in Table 1.1.
11 Abel et al. (1993) used two English language tests, the Four Alternative
Auditory Feature Test (FAAF) (Foster & Haggard, 1979) and the Speech Perception in
Noise Test (SPIN) (Bilger, 1994). The FAAF test has 80 target consonant-vowel- consonant (CVC) combinations which are presented in a four-choice closed set rhyming format. The SPIN test is a sentence test with a target word at the end of each sentence that may be high probability or low probability compared with the context of the sentence. The essentially “open set” format of the SPIN test proved to be the more sensitive of the two test tools for detecting speech intelligibility differences between
HPDs.
Wagstaff et al. (1996) plotted performance-intensity functions for three tests in
Norwegian. The word stimuli in each test were in open set format and included phonetically balanced lists of monosyllables, spondees, or lists of three-digit combinations. The monosyllable word lists were shown to be the best test for discriminating speech intelligibility differences between listening conditions. The spondees were the next best test, followed by the three-digit combination word lists.
Apart from these results which compared particular published tests or word lists, other methodological considerations such as effect of the size of the response set or the psychophysical procedure on measures of protected speech intelligibility were not considered.
1.1.5 Effects of Type of Noise Masker on Speech Intelligibility with HPDs
Within the 15 studies, several types of background noises were used to assess the HPDs’ effects on speech intelligibility. Noise recorded or simulated from a specific
12 work setting was the background noise most commonly employed in these studies.
Work place noise types included helicopter noise (Van Wijngaarden & Rots, 2001;
Wagstaff & Woxen, 2001; Wagstaff et al., 1996; Chan & Simpson, 1990), aircraft noise
(Shilling RD & Thomas GB, 1994), tank noise (Gower, Jr. & Casali, 1994), and cable swager noise (Abel & Spencer, 1999; Abel & Spencer, 1997; Abel et al., 1993). The primary purpose for the majority of studies using workplace noise was to find the optimum HPD for a particular work setting.
Some listening conditions were devised using speech spectrum or talker babble backgrounds (Abel & Spencer, 1999), since a speech spectrum background will be the most efficient masker of speech. White noise (Arlinger, 1992; Pekkarinen et al., 1990b) and pink noise (Van Wijngaarden & Rots, 2001; Hashimoto et al., 1996; Gower, Jr. &
Casali, 1994; Nixon et al., 1992) backgrounds were used in order to compare speech intelligibility for HPDs with different attenuation characteristics in a standardized noise background. Although most long term speech spectra are nearly identical, differences do exist between studies; whereas white noise and pink noise have defined spectra
(American National Standards Institute, 1994).
Depending on the purpose for a given study, any of these background noises could be the optimal noise for an evaluation of an HPD’s effect on speech intelligibility.
1.1.6 Effects of Reverberation Time on Speech Intelligibility with HPDs
The two previously-mentioned studies by Pekkarinen et al. (1990a, 1990b) were the only studies in this review to measure and report the effects of reverberation time on protected speech intelligibility. Both studies showed significant improvement in all
13 speech intelligibility measures with a shorter (1.6s) rather than a longer (2.1s) reverberation time.
1.1.7 Effects of Type of HPD on Speech Intelligibility with HPDs
Conventional earplugs and conventional earmuffs were emulated based on the average transfer functions of each type of HPD (Reeves, 1998), and compared for their effects on speech intelligibility in noise. Results varied by hearing status and spectrum of the noise (high-, mid-, or low-frequency emphasis). Normal hearing subjects performed better with the emulated conventional earmuffs than with the emulated conventional earplugs in each of the noise types. In subjects with a mild to moderate hearing loss or a moderate to severe hearing loss, emulated earmuffs allowed for better speech intelligibility than emulated earplugs in high-frequency emphasis noise, but emulated earplugs were better than emulated earmuffs in mid- and low-frequency emphasis noise.
Another study which compared speech intelligibility (in pink noise) with actual conventional earplugs and conventional earmuffs found normal hearing subjects performed better with conventional earplugs in a high (85 dBA) speech level at 0, +5, and +10 dB SNR than with conventional earmuffs (Hashimoto et al., 1996). However, this difference did not hold when the speech presentation level was 65 dBA.
Other studies compared the speech intelligibility performance of conventional
HPDs with that of electronic noise-limiting HPDs. In a study which compared the effects of a conventional earmuff, a nonlinear uniformly-attenuating earmuff, and two level-dependent electronic earmuffs on speech intelligibility, the electronic earmuffs
14 were significantly better than either of the passive earmuffs (Arlinger, 1992). When compared to the unoccluded listening condition, the electronic muffs increased speech intelligibility; whereas the passive earmuffs decreased speech intelligibility.
Abel et al. (1993) found that amplifying level-dependent earmuffs improved speech intelligibility for hearing-impaired listeners beyond that measured with conventional or nonlinear uniformly-attenuating earmuffs. This differential finding was not found with the normal hearing group.
In a comparison of ANR earmuffs, conventional earplugs, uniformly-attenuating earplugs, and conventional earmuffs, Abel et al. (1997) found that speech intelligibility in noise varied between groups by hearing status, age, and type of speech stimuli, and within groups by amount of HPD attenuation.
The effects on speech intelligibility with ANR switched “on” versus ANR switched “off” were examined within the same flight helmet (Chan & Simpson, 1990) and the same set of earmuffs (Nixon et al., 1992). Both studies found an improvement in speech intelligibility in noise with ANR; however, the effect sizes reported by Chan and Simpson (1990) (6% to 21% depending on SNR) were larger than those of Nixon et al. (1992) (5% to 9% depending on type of earmuffs). Another difference between these two studies was the type of background noise used to mask the speech. Chan and
Simpson (1990) used helicopter noise (low-frequency emphasis); whereas Nixon et al.
(1992) used a pink noise spectrum (less low-frequency emphasis). ANR differentially attenuates by frequency, with the lower frequencies being attenuated more than the higher frequencies. In an environment with more low-frequency noise, the attenuation will be greater, and speech intelligibility should increase.
15 More recently, studies have focused on speech intelligibility in noise with communications headsets or earplugs (Van Wijngaarden & Rots, 2001) (Wagstaff &
Woxen, 2001; Abel & Spencer, 1999). Each of these studies was concerned with providing adequate speech intelligibility with a communication headset worn in a high noise environment while maintaining sufficient hearing protection.
Abel and Spencer (1999) tested an aviation communication headset equipped with ANR worn with and without conventional earplugs and compared with unoccluded speech intelligibility. They found that the type of noise background and SNR had larger effects on speech intelligibility than the HPD condition, which showed no significant differences between HPD conditions.
Wagstaff and Woxen (2001) also paired earplugs with an aviation communication headset. Conventional foam, custom, and uniformly-attenuating earplugs were tested in helicopter noise with an aviation communication headset. All three earplug types resulted in decreased speech intelligibility when compared with the headset alone. However, the custom earplug provided significantly better speech intelligibility with the headset than either of the other two earplug types.
Van Wijngaarden & Rots (2001) asked the question, “Which HPDs give the best speech intelligibility while providing adequate hearing protection in helicopter noise?”
The answer came in the form of a new earplug called the Communications Earplug, an electronic device, which was paired with an aviation headset.
16 1.2 Audibility with HPDs: Job Safety
A perceived threat to physical safety is often the reason given by noise-exposed workers for not wearing HPDs; however, very few studies have examined the role that hearing ability plays in occupational accident rate. In a retrospective study of job- related injuries at a cotton yarn plant in North Carolina (Schmidt, Royster, & Pearson,
1980), injury rates were compared for equal times periods before and after the implementation of a hearing conservation program. Results showed a significant decrease in the rates of injuries after the advent of the hearing conservation program. In a more recent study of occupational injuries among older workers with disabilities
(Zwerling et al., 1998), hearing impairment was shown to be a risk factor for occupational injury.
1.3 Balancing Speech Intelligibility and Protection in Specific Work Environments
Some jobs require better speech intelligibility for safe and successful completion of tasks than other jobs. Speech intelligibility in general aviation, but primarily in helicopters, has been the subject of several research efforts in the area of balancing speech intelligibility needs with protection from NIHL (Ribera, Mozo, Mason, &
Murphy, 1996; Shilling RD & Thomas GB, 1994; Van Wijngaarden & Rots, 2001;
Wagstaff et al., 1996; Wagstaff & Woxen, 2001). The balancing act becomes more difficult as the amount of hearing loss in the listener increases.
17 1.4 Hearing Aids in Noisy Environments
In addition to the communication and physical safety concerns, hearing- impaired workers often depend on hearing aids (HAs) in order to increase speech audibility and speech intelligibility, even in quiet surroundings. In the noisy work environment, hearing aids can contribute to greater noise-induced hearing loss. When measurement of real ear levels with a probe microphone were performed, it was found that when noise exposures from around 80 dB(A) were amplified by a mild gain hearing aid, the resulting sound levels were well above the OSHA maximum of 90 dB(A)
(Dolan & Maurer, 1996). When a HA is worn by a worker, the “exposure level” is the combination of the sound field measure and the HA output to the ear.
Hearing aids are the primary means of hearing rehabilitation for persons with noise-induced hearing loss, who often continue to be employed in the same hazardous conditions in which they acquired their hearing losses. Under these conditions, hearing aid use without attenuation of the overall sound level would likely lead to increased noise-induced hearing loss. Since many workers with noise-induced hearing loss have hearing aids, hearing conservationists need data that describe the conditions under which hearing aids worn in combination with earmuffs would not be hazardous to the remaining hearing.
1.5 Prediction of Speech Intelligibility with HPDs and Hearing Aids
Wilde and Humes (1990) investigated the validity of the Articulation Index (AI)
(American National Standards Institute, 1969) as a predictive method for the purpose of predicting speech intelligibility with HPDs. Their mean group results for normal-
18 hearing and high frequency hearing-impaired HPD wearers in all study conditions were accurately described by the AI. However, not all laboratory experiments have found the same predictive accuracy with the AI. Gower and Casali (1994) used the AI to predict speech intelligibility with a conventional headset versus an ANR headset. The AI predictions were in agreement with their empirical findings; however, subjects performed at higher levels than the AI predicted. This was attributed to the limited vocabulary of the test tool (MRT) and the skill, experience, and practice level of the subjects.
The current ANSI Standard for the prediction of speech intelligibility, ANSI
S3.5-1997, is a major revision of and has replaced ANSI S3.5-1969 (American National
Standards Institute, 1997a). The revised Standard has updated several factors in the model to reflect research findings since the last revision. These factors include spread of masking, the standard speech spectrum level, and relative importance of various frequencies to speech intelligibility. To differentiate the older model AI from the revised model, it has been renamed the Speech Intelligibility Index (SII). More recently, investigators have used for the SII to predict speech intelligibility in noise with filtered speech (Eisenberg, Dirks, Takayanagi, & Martinez, 1998), for younger versus older listeners (Hargus & Gordon-Salant, 1995; Magnusson, 1996), with hearing aids (Magnusson, Karlsson, & Leijon, 2001; Magnusson, Karlsson, Ringdahl, &
Israelsson, 2001), and for helium-affected speech in divers (Mendel et al., 1998).
However, a review of literature did not reveal a study in which speech intelligibility with HPDs was predicted with the SII.
19 1.6 Project Overview
The first goal of this project was to develop a clinical test protocol for use in the evaluation of speech intelligibility in noise with HPDs. Chapter 2 describes the test protocol development process with normal hearing subjects in Phase I and Phase II. A second purpose for this project was to provide a quantitative model for the prediction of
“safe” amounts of hearing aid gain based on the acoustic environment. This model is provided at the end of Chapter 2.
Following the preliminary work described above, the final study tested hearing- impaired subjects with the test protocol implemented in Phase II. Chapter 3 describes how uniformly-attenuating earmuffs were compared with conventional earmuffs in terms of speech intelligibility for both normal hearing and hearing-impaired subjects.
Additionally, hearing-impaired subjects were tested with their own hearing aids worn in combination with each of the earmuffs in order to evaluate their combined effects on speech intelligibility in noise. Study results as well as methods for predicting speech intelligibility in noise for the experimental listening conditions are reported in Chapter
4. A discussion of the findings from this project, their limitations, and recommendations for future research are found in Chapter 5.
20
CHAPTER 2
PRELIMINARY WORK
2.1 Test Protocol Development
For the NIOSH Hearing-Impaired Worker Project, it was essential that the final test protocol incorporated the needs and limitations of potential industrial and/or clinical audiology applications. Therefore, the first goal was to identify the most sensitive, time-efficient, widely available test of speech intelligibility in noise.
A test that is sensitive to differences (i.e. has high statistical power) which is to be used with a small sample in five experimental conditions would need to have many items and/or a large effect size. Two commercially-available clinical tests of speech intelligibility in noise, the Quick Speech in Noise Test (QSIN) (Etymotic Research,
2001) and the Hearing in Noise Test (HINT) (Nilsson, Soli, & Sullivan, 1994; House
Ear Institute, 2001), were judged to have a sufficient number of test items for the preliminary work and the final study. The Speech Perception in Noise Test (SPIN)
(Kalikow, Stevens, & Elliott, 1977; Bilger, 1994), also commercially-available, lacked a sufficient number of novel test stimuli for the final test protocol. Since one of the goals of this study was to evaluate commercially-available clinical tests as they were
21 published, it was decided not to randomize the SPIN test stimuli in an attempt to provide an adequate stimulus pool from which to draw for the final study conditions.
Time-efficiency requires a test to have a short test item per unit time ratio (i.e. rapid presentation of stimuli and/or fewer items to present). Both the QSIN and the
HINT were time-efficient to administer. The QSIN, with five test items per sentence, required approximately two minutes per listening condition; whereas the HINT, with the sentence as the test item, lasted about five minutes for each listening condition.
In addition to commercially-available clinical tests of speech intelligibility in noise, widely-available laboratory tests of speech intelligibility are listed in the standard for measuring the intelligibility of speech over communication systems (American
National Standards Institute, 1989). This standard requires the use of one of three sets of test materials – the Phonetically Balanced Word Lists (PB word lists) (Egan, 1948), the Modified Rhyme Test (MRT) (House et al., 1965)or the Diagnostic Rhyme Test
(DRT) (Voiers, 1983).
The MRT was used in four of the reviewed studies of speech intelligibility in noise with HPDs from Chapter 1 (Nixon et al., 1992; Shilling RD & Thomas GB, 1994;
Gower, Jr. & Casali, 1994; Reeves, 1998). The MRT has also been the test of choice in several more investigations concerning speech intelligibility in noise with or without
HPDs (Williams, Mosko, & Greene, 1976; Mackersie, Neuman, & Levitt, 1999b;
Mackersie, Neuman, & Levitt, 1999a; Hiselius, 2000).
The DRT is similar to the MRT in that it employs a closed response set of rhyming words. However, the DRT uses a two-alternative forced choice (2AFC) paradigm whereas the MRT provides six alternatives from which to choose. There are
22 more items (232) in a DRT list than in an MRT list (50). However, the DRT provides an analysis of the features of speech, which is not available from the MRT. Both tests’ speech stimuli were available on audio CD, but the response sets were not available in a ready-to-use format. The experimenter had to create and display the response sets to the subjects either via a computer monitor screen or printed test forms. Both the DRT and the MRT require a longer administration time than either the QSIN or the HINT.
The speech features analysis portion of the DRT was thought to have potential application to rehabilitation strategies for hearing-impaired workers. Therefore, the decision was made to evaluate the DRT along with the QSIN and the HINT for possible use in the final test protocol.
Test protocols from the review of literature employed a variety of test methods, speech levels, noise maskers, and SNRs in the assessment of HPD effects on speech intelligibility in noise. As described in Chapter 1, significant differences were found between levels of each variable. Therefore, in order to obtain a valid assessment of the effects of hearing impairment on speech intelligibility with HPDs, one must account for the possible effects of each variable on speech intelligibility. Alternatively, a variable can be controlled in the study design by holding its value constant across experimental listening conditions.
2.2 Test Protocol Validation
Test protocol validation was divided into two phases. In Phase I, normative data was collected with each of the clinical speech intelligibility tests chosen for evaluation, and the effects of speech level, test method, and background noise type were explored.
23 Reporting speech intelligibility in terms of the mean SNR for 50% correct for each clinical test controlled for the effects of SNR. The purpose of Phase II was to test normal-hearing subjects with the proposed final test protocol on each of the three speech intelligibility tests to determine which test was the most sensitive indicator.
2.2.1 Phase I
2.2.1.1 Hypotheses
The first hypothesis tested in Phase I was that for each of the three HINT test protocols and for the QSIN, the OSU sample’s means would fall within the 95% confidence intervals from the publishers.
The second set of hypotheses dealt with the area of test methods. The HINT provided the means to evaluate 1) the effect of holding constant the speech level versus the noise level in an adaptive tracking test protocol and 2) the effect of using headphone versus sound field testing of speech intelligibility in noise. There were several reasons why it was important to test whether holding constant the speech versus the noise in an adaptive tracking protocol would influence the results. Given the predicted effects of speech level on speech intelligibility, it was decided to hold the speech level fixed in each test protocol. However, the normative data for the HINT was collected with the noise held constant and the speech level varied. According to the HINT manual (House
Ear Institute, 2001), “Test data may be compared to HINT norms only if the standard test parameters were used during the test. Comparing data collected with non-standard test parameters to HINT norms may lead to erroneous conclusions.” The normative data provided by the manufacturer indicated that the mean SNR50 for normal hearing
24 listeners on the headphone test was –2.6 dB. Since the SNR50 was so close to 0, the difference in speech levels between the test protocols was thought to be minimal.
Therefore, the hypothesis tested was that for normal hearing listeners, there would be no difference between HINT protocols with fixed speech level versus fixed noise level.
The HINT headphone method used an average head-related transfer function
(HRTF) to simulate how the listener’s head would affect the sound in a sound field presentation. The HINT sound field was created using one speaker (0 degrees Azimuth) or two speakers oriented at a 90-degree angle (90 and 270 degrees Azimuth). The headphone version of the HINT precluded the use of HPDs; however, if it were shown that unoccluded sound field testing was predictive of occluded performance, it would be important to know if the headphone version and the sound field version produced similar results within subjects. Given the need of industrial hearing test providers to keep equipment costs to a minimum, a headphone administered test would be more likely to be acquired than a sound field administered test.
Thirdly, Phase I sought to find the optimal speech level for testing speech intelligibility in noise with earmuffs. According to Fletcher and Galt’s method of calculating the articulation index, normal hearing listeners reportedly show maximum speech intelligibility at a speech level of 68 dB SPL (Fletcher & Galt, 1950). This model also predicts a decrease in speech intelligibility at higher speech levels which, in clinical audiology, is referred to as ‘rollover’.
Using the QSIN, the Fletcher and Galt model’s predictions were tested in noise with earmuffs at 70 dB SPL and 90 dB SPL and without earmuffs at 40 dB SPL, 70 dB
SPL, and 90 dB SPL. The QSIN manual recommended setting the audiometer’s
25 attenuator dial to 70 dB HL for normal hearing listeners. In order to approximate the
Fletcher and Galt model’s prediction of optimal speech intelligibility level, 70 dB SPL
(66 dB HL on the audiometer) was chosen instead of 70 dB HL as recommended in the test manual. To quantify the effects of reduced speech and noise levels on SNR50 (in preparation for the study with hearing-impaired subjects), speech at 40 dB SPL was presented in the unoccluded listening condition. Since the final test protocol was intended to obtain an estimate of a worker’s speech understanding ability in high noise levels, speech was also presented at 90 dB SPL.
The earmuffs tested (Bilsom 717 and Bilsom 817) were identical in volume and shape. The Bilsom 717 earmuffs provided passive sound attenuation with frequency attenuation characteristics consistent with that of many conventional earmuffs. The
Bilsom 817 earmuffs provided passive sound attenuation, but with a more uniform frequency attenuation curve than conventional earmuffs. The hypothesis was that the more uniformly-attenuating earmuffs would allow for better speech intelligibility in noise than the conventional earmuffs.
The fourth variable tested was background noise type. The HINT and QSIN each had its background noise included with the test materials. The HINT used speech spectrum noise while the QSIN used four-talker babble. The DRT word stimuli could have been presented in whatever background noise the experimenter deemed optimal for a particular research question. Since the goal of this study was to identify the most sensitive test protocol for detecting speech intelligibility differences in noise, the background noise masker would need to be the most efficient masker of the speech stimuli. A case study with one of the subjects from the Phase I subject pool was
26 conducted to quantify the differences between speech spectrum noise, four-talker babble, and pink noise on speech intelligibility with the DRT word stimuli.
2.2.1.2 Study Design
In each study, a within subjects design was used to evaluate the effects of the manipulated variables. The manipulated variables in Phase I were speech level (40, 70, and 90 dB SPL), adaptive tracking method (fixed noise level versus fixed speech level), and listening condition (unoccluded, conventional passive earmuffs, and uniformly- attenuating earmuffs). Background noise type (speech spectrum, four-talker babble, and pink noise) was informally evaluated with a case study during protocol development for the DRT.
2.2.1.3 Equipment
The test protocol development and validation studies for Phases I and II were conducted in a double-walled IAC audiological test booth located in Room 20 of
Pressey Hall at The Ohio State University. The inside chamber dimensions measured
108” by 100” by 78” high. Due to a broken latch on the booth door, the ambient noise levels did not meet the specifications for threshold testing with ears uncovered in an audiometric booth as illustrated in Figure 2.1 (American National Standards Institute,
1991). However, sound field hearing thresholds were not part of the experimental data, and the ambient noise levels were judged to be acceptable for the suprathreshold sound levels used in the experiments.
27 Audio Booths Ambient Noise Levels (equipment ON) 30 Ears Uncovered ANSI S3.1-1991
25 Room 20 Booth (Normal Hearing Subjects)
20
15 dB SPL
10
5
0 125 250 500 800 1000 1600 2000 3150 4000 6300 8000 Frequency (Hz)
Figure 2.1: Ambient noise levels in audiometric booth used in Phase I and II compared to ANSI S3.1-1991.
The QSIN CD was played on an ADC model CD-100x compact disc player.
The output from the CD player was routed through a model SD20 Interacoustics diagnostic audiometer and a McIntosh 250 Audio Autoformer Type 043-667/668 amplifier to the GSI sound field speakers inside the test booth. The sound field speakers were calibrated with a Bruel & Kjaer Type I sound level meter in accordance
28 with recommended procedures for speech sound field testing (American Speech-
Language-Hearing Association, 1991). Figure 2.2 shows the frequency response of the sound field speaker used with the QSIN and the DRT.
Speaker Response Curve to W hite Noise
10
0
-10
-20
-30 dB
-40
-50
-60
-70 100 600 1100 1600 2100 2600 3100 3600 4100 4600 5100 5600 6100 6600 7050 7550 9000 19000 Center Frequency (Hz)
Figure 2.2: Frequency response curve for the GSI speaker box used in Phase I and II.
29 The HINT for Windows version 6.0 was a beta version manufactured by Maico in conjunction with the House Ear Institute and included computer software and proprietary hardware. The HINT for Windows software was installed on a custom-built personal computer with an Intel Pentium processor and 64 MB of RAM. The computer ran on a Windows 98 operating system. The HINT hardware consisted of a Hearing
Test Device (HTD), which connected to the computer via a USB port, two loudspeakers
(Optimus XTS 4.0), TDH-39 subject headphones, a talkback microphone and monitor headset for the tester and a lapel microphone for the subject. The loudspeakers were positioned at a 90-degree angle, one meter from the center of the subject’s head, with the center of the speaker box located 45 inches above the floor. The sound field was calibrated as specified in the HINT for Windows User Manual (House Ear Institute,
2001).
Experimental Earmuffs
The earmuffs used in these studies, Bilsom’s model 717 and 817, were tested on an acoustic test fixture (ATF) at the NIOSH Hearing Protector Laboratory in Cincinnati,
OH (Figure 2.3). The ATF was a model of an adult-size head with a Bruel & Kjaer model 4165 ½” microphone and a model 4157 ear simulator mounted in the location of the right ear. It was designed to simulate head shadow effects as well as the acoustical and mechanical properties of the human outer and middle ears. Since high levels of acoustic energy can enter the cochlea through two pathways, the outer ear/middle ear system and vibration of the skull (bone conduction), it was important to control for possible bone conduction of the stimuli. The NIOSH ATF was built with a greater head
30 density than the more commonly used Knowles Electronics Mannequin for Acoustic
Research (KEMAR). The greater head density of the NIOSH ATF allowed for a closer approximation of the human bone conduction response to high sound pressure levels. A personal computer running customized software provided calibration functionality and controlled the Tucker-Davis Technologies programmable attenuators. The digital signals were relayed to the Tucker-Davis D/A converter and the programmable attenuators, through the Sherbourne amplifiers and played out over custom-built loudspeakers in a diffuse sound field. One-third octave noise bands were presented for unoccluded and occluded conditions. The resulting frequency attenuation curves are shown in Figure 2.4.
Bilsom’s model 717 earmuffs, like many conventional earmuffs, provided more noise reduction in the high-frequency region relative to the low frequency region.
Words filtered by such a frequency attenuation pattern would be perceived by a listener as muffled sounding or lacking clarity since consonant sounds contain more high- frequency energy than low-frequency energy. Vowel sounds, which are primarily low- frequency, would receive very little attenuation from conventional earmuffs. This could result in a further degradation of the speech signal, since low-frequency sounds mask not only other low-frequency sounds, but also can mask high-frequency sounds as well.
This auditory phenomenon has been termed the “upward spread of masking.” Bilsom’s model 817 earmuffs produced more low-frequency attenuation and less high-frequency attenuation. This more uniform attenuation pattern should provide greater speech clarity since acoustic energy from consonant sounds is greater and the low-frequency acoustic energy is less.
31
Figure 2.3: Conventional earmuffs on left (Bilsom's model 717) and uniformly- attenuating earmuffs on right (Bilsom's model 817).
32 Insertion Loss Values: Bilsom's Model 717 versus 817 Earmuffs
717 (Conventional) Muffs 817 (Uniformly-attenuating) Muffs
45
40
35
30
25
20
15 Insertion Loss (dB) 10
5
0 125 250 500 1000 2000 3150 4000 6300 8000 Frequency (Hz)
Figure 2.4: Bilsom's model 717 earmuffs were representative of conventional earmuffs. Bilsom's model 817 earmuffs provided more uniform attenuation characteristics between the low- and high-frequency regions.
Insertion loss values for earmuffs, while much less variable than for earplugs, do show some test-retest variability that is primarily due to the placement of the HPD.
Because of this inherent variability, it is not possible to predict the exact amount of insertion loss. Instead, manufacturers are required to provide results of behavioral test data from a panel of subjects. These average test results are obtained through different
33 test methods and reported in various ways depending on the standard adopted by the country in which the HPD is sold.
In the United States, at the time of this writing, manufacturers are required to provide average test data from a panel of subjects using the experimenter-fit method of
HPD placement (U.S.EPA, 1979; American National Standards Institute, 1974). It has been shown that these data overestimate the real-world attenuation values achieved by workers in the field (Berger et al., 1998). The current standard has been updated to include the subject-fit method of HPD placement (American National Standards
Institute, 1997c), but is not yet required by law. In Europe and Australia, manufacturers follow requirements based on standards that describe a subject-fit method of HPD placement (International Standards Organization, 1990; Standards Australia and
Standards New Zealand, 1999). Therefore, to better estimate the real-world insertion loss values in this study, it was decided to adopt the International Standards
Organization’s (ISO) Assumed Protection Value (APV) (International Standards
Organization, 1994). Table 2.1 shows the APV by octave band frequency calculated from the means and standard deviations obtained in accordance with ISO 4869-1 and reported by the manufacturer for the two experimental earmuffs.
34 Assumed Protection Values for Experimental Earmuffs
Octave Band Center Frequency (Hz) 125 250 500 1000 2000 4000 8000
717 Mean Attenuation (dB) 14.2 15.5 25.1 35.2 34.8 37.9 37.3
717 Standard deviation (dB) 3.2 2.3 2.6 1.6 2.7 2.9 3.7
717 Assumed Protection Value (dB) 11.0 13.2 22.5 33.6 32.1 35.0 33.6
817 Mean HPD Attenuation (dB) 14.8 24.2 27.1 24.3 28.6 30.6 33.2
817 Standard deviation (dB) 3.3 2.7 2.6 1.5 2.3 2.5 2.4
817 Assumed Protection Value (dB) 11.5 21.5 24.5 22.8 26.3 28.1 30.8
Table 2.1: Assumed Protection Value = mean - 1 standard deviation. Methods used are per ISO 4869-1 and 4869-2.
2.2.1.4 Subjects
In Phase I, a convenience sample of ten normal hearing adult volunteers (4 males, 6 females) aged 22 to 44 years (mean = 34.6 years, standard deviation = 7.4 years) were recruited into the study. Normal hearing was defined as having pure-tone thresholds less than or equal to 20 dB HL at the octave frequencies from 250 Hz through 8000 Hz, bilaterally. Average hearing threshold levels (HTLs) are shown in
Figure 2.5. Participants were required to be native English speakers, since non-native
35 speakers have been shown to have a decreased ability to understand speech in noise
(Abel et al., 1982; Mayo, Florentine, & Buus, 1997; Meador, Flege, & MacKay, 2000;
Van Wijngaarden, Steeneken, & Houtgast, 2002).
Phase I Normal Hearing Subjects Average Audiogram
Frequency (Hz) 250 500 1000 2000 3000 4000 6000 8000 -10 0 10 20 30 40 50 60
HTL (dBHL) 70 80 90 100 110 120
Figure 2.5: Average Hearing Threshold Levels (HTL) for Phase I subjects. The circles on the red line show the right ear average HTLs and the X’s on the blue line indicate the left ear average HTLs.
36 2.2.1.5 Test Procedures
Subjects were required to read and sign the Subject Consent Form before participating in the study. The subject’s date of birth, self-reported otologic history, and language background were entered into the HINT software. The subject was escorted to a chair in the test booth, and fit with TDH-39 headphones and a lapel microphone by the experimenter. Instructions for pure-tone testing were given over the headphones. A pure tone audiogram was performed for each ear, and the test results were entered into the HINT software’s database. Once it was confirmed that the subject met the audiometric requirements for normal hearing, s/he was accepted into the experimental portion of the study. Following completion of the experimental protocol, the subject was shown his or her test results and any questions concerning the results and/or test protocol were answered.
HINT Protocol
In the first portion of the study, each subject was tested with the HINT. The subject repeated the entire sentence, and the experimenter audio-visually monitored the subject’s verbal response and scored the sentence as one test item. Certain articles of speech (e.g. a, an, the) could be substituted and the sentence still be considered correct.
These substitutions were noted in the test key on the computer screen. Based on the subject’s response, the speech or noise level was raised or lowered by the computer to increase or decrease the task difficulty in an adaptive tracking paradigm.
The HINT determined SNR50 by an up-down adaptive tracking procedure which employed an initial 4-dB step size on the first 5 sentences followed by a 2-dB
37 step size on the remaining 15 sentences. The clinical protocol on which the norms were based held the noise level constant and varied the speech level. For purposes of this study, this method was called the standard tracking method. The HINT software also provided for the inverse procedure in which the speech level was held constant and the noise level was varied. This method was called the alternative tracking method.
In this study, the HINT was administered over TDH-39 headphones and in the sound field utilizing the standard tracking method at 65 dB SPL. The alternative tracking method was also performed in the sound field at 65 dB SPL. These three test paradigms were presented in counterbalanced order to control for potential practice effects. Data were collected for speech in quiet (front), and for speech front with noise right, noise left, and noise front for each test paradigm.
QSIN Protocol
The QSIN was administered with the subject seated at the calibration location in the center of the booth and directly facing a sound field speaker. The speaker was positioned in the corner to the right of the observation window (experimenter’s view) with the middle of the speaker 48” above the floor and 62” in front of the subject. The subject’s task was to repeat the entire sentence; however, unlike the HINT, the QSIN sentences contained five target words within each sentence. Each target word was scored as a separate item.
In the QSIN, SNR50 was determined by a method after the Tillman-Olsen method for obtaining spondee thresholds (Tillman & Olsen, 1973). In the Tillman-
Olsen method, two spondees were presented at each level starting at the level where
38 100% correct was attained. The presentations were decreased in 2 dB steps until no responses were obtained for several words. Since two words were presented at each step in 2 dB steps, the formula for calculating threshold for spondees was:
Threshold = (starting level + 1 dB [half the step size]) – total # correct
In the QSIN, a sentence with five key words was presented to the listener at a
+25 dB SNR. The listener repeated as much of the sentence as s/he heard. The test administrator listened to the response and scored one point for each correct key word.
A second sentence was presented at a +20 dB SNR, and the process continued through 0
SNR. The number of correct key words was summed. With five key words in each of six sentences, a perfect score would have been 30. Following Tillman-Olsen’s method, since there were 5 words presented at each step in 5 dB steps, the formula for calculating threshold (or SNR50) was:
SNR50 = (25 + 2.5 dB [half the step size]) – total # correct
The SNR50 reported by the manufacturer for normal-hearing persons was 2 dB
(Etymotic Research, 2001) so one would subtract 2 dB from the SNR50 to calculate
SNR loss. If a listener repeated all 30 words correctly, his SNR loss would be calculated:
SNR loss = SNR50 – 2 dB
39 SNR50 = (25 + 2.5 dB) – 30
= 27.5 – 30
= -2.5 dB
SNR loss = -2.5 – 2
= -4.5 dB
Theoretically, the best score a normal-hearing listener can achieve on the QSIN is –4.5 dB SNR. None of the normal hearing subjects in the OSU sample demonstrated this ceiling effect.
The QSIN was used to assess the effects of speech level and listening condition on speech intelligibility. Each subject was tested in the unoccluded listening condition at each of three speech levels (40, 70, and 90 dB SPL) and in two earmuff listening conditions (conventional earmuffs and uniformly-attenuating earmuffs) at each of two speech levels (70 and 90 dB SPL). The order of the listening conditions was counterbalanced to control for possible practice effects.
DRT Protocol
In the DRT, the speech and noise were presented with the same sound field speaker configuration that was used in the QSIN protocol. One randomized list of 232 words was presented at 70 dB SPL for both the speech level and the noise level in each of three background noise maskers – speech spectrum noise from the HINT, four-talker babble from the QSIN, and pink noise. The response paradigm was a 2AFC task. The
40 subject saw two monosyllabic words on a computer screen. One of the words was the target word, and the other word differed from the target by either the initial or the final consonant sound. Therefore, the subject had a 50% chance of guessing the correct response. The subject indicated her response by pressing the numeral “1” or “2” on the computer’s keyboard. Responses were saved in a computerized data file and scored against a coded template. Since the SNR was set at 0 and 50% correct would have been chance performance, the scores for each background noise masker condition were compared in units of percent correct instead of SNR50.
2.2.1.6 Results
Published Norms versus OSU Data: HINT and QSIN
Below is a comparison of the published means (Figure 2.6) and standard deviations (Figure 2.7) with this study. In each case, the recommended clinical protocol was observed. The only significant difference noted between the normative data and the OSU sample was the mean for the QSIN. The 95% confidence interval (CI) for the
QSIN (-0.55, 4.35) did not contain the Ohio State University (OSU) sample mean
(-1.05); therefore, there was a statistically significant difference between the two means.
The standard deviations between the OSU sample and the QSIN compared favorably.
The 95% CI for the HINT headphone condition (-4.952, -0.248) contained the OSU sample mean (-3.11); therefore, there was no statistically significant difference between the two means. While there was no published mean for the HINT sound field condition, the OSU mean (-2.33) fell within the HINT headphone condition 95% CI and the standard deviations between the sound field conditions were quite similar as well.
41 OSU versus Published Test Means
3.00
2.00
1.00 OSU 0.00 PUB
-1.00 Mean SNR50Mean
-2.00 SIGNIFICANT DIFFERENCE -3.00
-4.00 QSIN HINT (HP) HINT (SF) OSU -1.05 -3.11 -2.33 PUB 1.9 -2.6
Test (HP = Head Phones, SF = Sound Field)
Figure 2.6: The HINT has published norms for the headphone (HP) condition, but requires the user to establish site-specific norms for each sound field (SF).
42 OSU versus Published Test Standard Deviations
2.00
1.00 OSU 0.00 PUB
-1.00
-2.00 Standard Deviation (SNR50)Standard Deviation
-3.00 QSIN HINT (HP) HINT (SF) OSU 0.96 0.84 1.00 PUB 1.25 1.20 1.20
Test (HP = Head Phones, SF = Sound Field)
Figure 2.7: While the HINT does not provide means for the sound field (SF), it does provide the standard deviation from the normative sample.
HINT: Fixed Speech versus Fixed Noise
The “standard” tracking method and the “alternative” tracking method were compared using the HINT test protocols. Mean SNR50 scores obtained in the noise front (0 degrees Azimuth) presentation for each method are plotted in Figure 2.8.
43 HINT Tracking Methods -1.5
-2.0
-2.5 SNR50 (dB)
-3.0
-3.5 Standard Alternative
Figure 2.8: Means +/- 95% confidence interval for each HINT adaptive tracking method.
A repeated measures ANOVA revealed no significant differences between the means for the two methods (Table 2.2). However, due to the small amount of observed power of the test (6%), it is possible that statistically significant differences were present but undetected by the test. According to the HINT for Windows manual (House
Ear Institute, 2001), “Each 1 dB of threshold elevation over that of the population with normal pure-tone thresholds in the noise tests corresponds to approximately 10% poorer intelligibility in noisy listening conditions, based on studies performed at the House Ear
Institute.” Therefore, in terms of practicable application, the difference between the
44 two means (0.145 dB) would result in approximately 1.45% poorer intelligibility, which would not be noticeable to a listener in most noisy communication conditions.
Tests of Within-Subjects Effects
Type III Partial Sum of Mean Eta Noncent. Observed Source Squares df Square F Sig. Squared Parameter Power Fixed 0.105 1 .105 .170 .690 .018 .170 .066 Variable
Error 5.581 9 .620
Observed Power computed using alpha = .05. No significant difference between holding speech level fixed versus noise level fixed in the HINT adaptive tracking protocol with normal hearing subjects.
Table 2.2: ANOVA Table. Fixed Speech versus Fixed Noise in Adaptive Tracking Test Protocol.
HINT: Headphone versus Sound Field Testing
The question of whether headphone testing with an average HRTF would yield the same results as unoccluded testing in a sound field was also addressed using the
HINT. The means for the headphone-simulated noise front condition and the sound
45 field noise front condition (0 degrees Azimuth) are shown in Figure 2.9. Data from the standard tracking method are reported.
HINT: Headphone vs Sound Field -1.0
-1.5
-2.0
-2.5 SNR50 (dB) -3.0
-3.5
-4.0 Headphone Sound Field
Presentation Transducer
Figure 2.9: Means +/- 95% confidence intervals for each HINT presentation method.
No statistically significant differences were found; however, the observed power of the test was low (44%), indicating a possible failure to detect a difference between
46 the means (Table 2.3). This test showed a larger effect size (31%) than the previous test
(2%), which resulted in the more powerful test.
The difference between the two means (0.785 dB SNR50) would be predicted to result in approximately 7.85% poorer intelligibility. From a practical standpoint, this amount of change in speech intelligibility, depending on the listening situation, may be important to the listener.
Type III Partial Sum of Mean Eta Noncent. Observed Source Squares df Square F Sig. Squared Parameter Power Sound 3.081 1 3.081 4.099 .074 .313 4.099 .440 Source
Error 6.765 9 .752
Observed Power computed using alpha = .05. No significant difference between mean speech intelligibility scores for HINT headphone versus sound field sound sources.
Table 2.3: ANOVA Table. Headphone versus Sound Field Testing with HINT.
QSIN: Effects of Speech Level on Unoccluded Speech Intelligibility in Noise
In this study, the QSIN was used to assess the effects of speech level on speech intelligibility in normal hearing listeners without HPDs. Speech was presented at 40, 70 dB, and 90 dB SPL. The distribution of individual scores and the mean for each level
47 are shown in Figure 2.10. When the level of the speech signal was decreased from the optimally audible level of 70 dB SPL to 40 dB SPL, the standard deviation of the
SNR50 scores increased from 0.96 dB to 2.88 dB. Six of the ten subjects showed decreased speech intelligibility in noise with a 90 dB SPL speech level relative to performance with a 70 dB SPL speech level, which is a positive indication of the rollover phenomenon. A repeated measures analysis of variance (ANOVA) of the data was statistically significant (p < 0.001) for the main effect of speech level. In a post hoc paired comparison analysis, each level was found to be significantly different from the others. These empirical findings are consistent with the predictions from Fletcher and
Galt’s articulation index model.
48 Effects of Speech Level on SNR50 (Quick SIN Test)
10 SN1
8 SN2
SN3 6 SN4
SN5 4
SN6 SNR50 2 SN7
SN8 0
SN9
-2 SN10
AVG -4 40 dB SPL 70 dB SPL 90 dB SPL KEY Speech Level
Figure 2.10: Normal hearing adults' SNR50 scores and group averages plotted as a function of speech level in dB SPL. Individual subject scores are represented by connected data points.
49 QSIN: Effects of Speech Level and Earmuff Type on Speech Intelligibility in Noise
Speech levels and attenuation characteristics of HPDs both affect speech intelligibility. In normal hearing listeners, would a more uniformly-attenuating HPD enhance speech intelligibility? Would enhanced speech intelligibility be speech level dependent?
For this group of normal hearing listeners, results showed no difference between the earmuffs when speech was presented at 90 dB SPL. However, the results revealed a significant reduction in speech intelligibility with the conventional earmuffs when speech was presented at 70 dB SPL (Figure 2.11).
50 Level Effects on Speech-to-Noise Ratio in Unprotected and Protected Listening
Speech @ 70 dBListening SPL ConditionSpeech @ 90 dB SPL -2
-1.5 -1.05 -1 -1.2 -0.5 -0.6 0 -0.1 0.5 0.83 Average SNR50Average 1
1.5 Conventional than 2 Muff @ 70 dB is Uniformly-attenuating significantly 2.25 Muff @ 70 dB 2.5 WORSE
3 Unoccluded Conventional Muff Uniformly-Attenuating Muff
Figure 2.11: For conditions of reduced speech audibility (70 dB SPL with earmuffs), the difference between the earmuffs was significant. When speech audibility was not reduced (90 dB SPL with earmuffs), the difference between the earmuffs was not significant.
DRT: Effects of Noise Type on Speech Intelligibility
The three experimental noise slopes are shown in Figure 2.12. The digital waveforms from the QSIN CD (pink noise and 4-talker babble) were converted from audio CD format with CDEX software and the resulting wave files were analyzed with
Cool Edit software. However, the HINT noise file could not be analyzed directly.
51 Noise samples from the loudspeaker output in the sound booth were analyzed with a
Larson-Davis 800B sound level meter and a Hewlett-Packard 3561A dynamic signal analyzer.
10.0 9.0 8.5 8.0
7.0 7.0 6.0
5.0 5.0 4.5 4.0
3.0 3.0 Slope (dB/octave) Slope 2.0 1.0 0.0 Pink Noise HINT Speech Spectrum QSIN 4-Talker Babble Noise Noise Type Slope of Waveform Slope of Speaker Output
Figure 2.12: Slope values of experimental masker noises. Frequency analysis of the file containing the HINT speech spectrum waveform was not possible due to file format incompatibility with the analysis software.
52 The results of a case study with one subject on the DRT in the experimental noises are presented in Figure 2.13. It is notable that there is an inverse relationship between the slope of the experimental noise and the speech intelligibility score.
Practice effects alone are an unlikely explanation of the linear trend displayed since the noise type presentation order was HINT noise followed by QSIN noise and finally pink noise.
53 100
95
90 88.8
84.9
% Correct 85 81.9
80
75 Pink Noise HINT Speech Spectrum QSIN 4-Talker Babble Noise Noise Type
Figure 2.13: Noise type effects on DRT scores: A case study. DRT scores in percent correct for one subject in each of the experimental noises. Speech and noise were presented at 70 dB SPL from the same loud speaker in front of the subject.
The trend in the data suggested that the QSIN four-talker babble presented the most difficult listening environment. However, due to the single subject design of this study, it was not possible to determine if the resulting differences in scores between the listening conditions were significant or not. No additional testing beyond this exploratory case study was done.
54 2.2.1.7 Discussion
In comparing the OSU sample’s means to the manufacturers’ data, it was found that the OSU sample’s mean was not significantly different for the HINT, but that the
OSU sample’s mean for the QSIN was significantly better than that of the manufacturer’s sample mean. The reason for the difference is unknown. It is possible that the hearing thresholds, while still considered being within normal limits, were significantly different between the QSIN sample and the OSU sample. The mean hearing threshold values for the OSU sample did not exceed 10 dB HL. Hearing threshold data were not available for the QSIN sample. Since the OSU sample’s standard deviation data did not show a significant difference from the manufacturers’ data for the HINT or the QSIN, it was concluded that both tests were valid and reliable test tools for speech intelligibility in noise evaluation.
The HINT was used to compare the effects of fixed speech level versus fixed noise level in an adaptive tracking test paradigm. Results showed no difference between the test methods. However, the test had very low statistical power, which would increase the probability that if there were a significant difference, the test would be unable to detect it. Therefore, it was decided that only the standard tracking method would be used in Phase II in order to permit comparisons with the manufacturer’s normative data.
The HINT was also evaluated as a possible headphone test of unoccluded speech intelligibility in noise. Results again showed low statistical power for this study. If, indeed, the unoccluded HINT sound field standard tracking protocol in Phase II were found to be predictive of occluded speech intelligibility, further testing with a more
55 powerful test protocol (more test items or more subjects) would need to be done before it could be concluded that there was truly no difference between the headphone and sound field presentations.
The QSIN was used to measure the effects of speech level on unoccluded speech intelligibility in noise for normal-hearing listeners. Results were consistent with
Fletcher and Galt’s articulation index model, which predicts optimum speech intelligibility at 68 dB SPL. Measured speech intelligibility for speech presented at 40 dB SPL was significantly poorer than that elicited by speech presented at either 70 or 90 dB SPL.
The QSIN was also used to evaluate the experimental earmuffs’ effects on speech intelligibility at 70 dB SPL and 90 dB SPL. The use of a 70 dB SPL speech in noise level, while not representative of a typical work environment, was included to provide a basis for predicting speech intelligibility performance by hearing-impaired listeners. Depending on the amount of hearing loss, a hearing-impaired listener wearing earmuffs might receive a less than optimal level of the speech signal even if the speech level would be optimal for a normal hearing listener wearing earmuffs. While the results revealed no significant effect on speech intelligibility with the speech level at 90 dB SPL, a significant reduction in speech intelligibility was seen with the conventional earmuffs as compared to the uniformly-attenuating earmuffs at 70 dB SPL.
Taken together, the QSIN unoccluded and occluded study results provided information on which the speech level for the final test protocol was based. First, speech stimuli presented at 70 dB SPL increased the sensitivity of the QSIN to differences between the earmuffs as compared with speech stimuli presented at 90 dB
56 SPL. The relatively small attenuation differences between the earmuffs became more important as the audibility of the speech signal was reduced. Second, unoccluded speech intelligibility was significantly reduced as a result of audible, but low-level speech in noise. Since there was an effect of decreased audibility at 70 dB SPL in occluded, normal hearing listeners, justification of the speech presentation levels for the final test protocol would need to include consideration of the effects of decreased audibility as a result of hearing loss in the hearing-impaired subjects.
Two methods of determining the speech presentation level(s) for comparing normal hearing and hearing-impaired subjects were considered. The first method for defining the presentation level was to use equal dB levels defined in terms of sensation level (SL) with all subjects. Sensation level is the number of decibels above the subject’s threshold. For example, if the test protocol for speech level was set at 40 dB
SL, the speech presentation level for a normal hearing subject with a speech threshold of 5 dB HL would be 45 dB SPL. For the same test protocol, if a hearing-impaired subject had a speech threshold of 40 dB HL, that subject would be tested at 80 dB SPL.
This method, while accounting for the overall differences in level effect on audibility
(and, ultimately, speech intelligibility), does not control for the differences in slope of individual hearing losses, which would affect audibility (and, ultimately, speech intelligibility). This method also would not allow for comparison with other studies in the literature, since most studies that used HPDs defined a fixed noise level or speech level in dB SPL.
The second method under consideration, instead of using equal SL, was to define speech level in terms of equal SPL. For example, all subjects would be tested at
57 90 dB SPL regardless of their hearing status. Since the dependent variable would be speech intelligibility, as long as the speech signal would be audible to every subject in each listening condition, this method could be used. This method does not attempt to control for differences in audibility; therefore, the differences in degree or slope of hearing loss would not be considered a confounder. Rather, these differences would be quantifiable variables that would need to be built into the design and analysis of the study.
The equal SPL method was chosen for the final study protocol. Since earmuffs are not typically worn in noise less than 90 dB(A), it was decided to use the higher noise level in Phase II. This level was achievable with the QSIN and DRT. However, the HINT was not able to produce this level without modification of the hardware.
Therefore, the decision was made to administer the HINT in Phase II using the standard adaptive tracking paradigm (noise fixed) with the noise level set to the maximum setting (65 dB). If the results at this lower level were comparable to those at the higher level, it would provide evidence to support a lower sound level test protocol.
Results from the case study of background noise effects on speech intelligibility as measured with the DRT suggested that of the three noises tested, the 4-talker babble from the QSIN provided the most difficult listening environment. Therefore, it was decided to present the DRT word stimuli in the 4-talker noise babble from the QSIN test for the Phase II study.
58 2.2.2 Phase II
2.2.2.1 Purpose of the Study
The purpose of Phase II was to test normal-hearing subjects with the proposed final test protocol on each of the three speech intelligibility tests to determine which test was the most sensitive indicator. By eliminating two tests from the final study, it would reduce test time and study costs.
2.2.2.2 Study Design
A within subjects design was used to evaluate the effects of the manipulated variables. The manipulated variables in Phase II were 1) listening condition
(unoccluded, conventional earmuffs, uniformly-attenuating earmuffs), and 2) test
(QSIN, HINT, DRT). The dependent variable, speech intelligibility, was reported in units of dB at SNR50 for all three tests.
2.2.2.3 Equipment
The equipment described for use in Phase I was also used in Phase II. In addition, the DRT was programmed in MatLab version 6.0 to randomly present the 232 stimulus sets recorded on wave files through the sound card of an IBM ThinkPad i1400 series notebook computer. The speech output from the soundcard was directed into channel 1 of the audiometer, routed through the amplifier, and played out to the sound field speaker. Masker noise was played out of the CD player into channel 2 of the audiometer, and routed through the amplifier to the same sound field speaker as the
59 speech stimuli. The stimuli sets were displayed on the screen of the computer and the subject response buttons were the “1” and “2” keys on the computer keyboard.
2.2.2.4 Subjects
A convenience sample of eight normal hearing adult subjects (4 males, 4 females), aged 22 to 41 years (mean = 33.25 years, standard deviation = 7.22 years), were paid for their participation in the Phase II study. Three of the eight subjects also participated in the first pilot study. As in Phase I, normal hearing was defined as having pure-tone thresholds less than or equal to 20 dB HL at the octave frequencies from 250
Hz through 8000 Hz, bilaterally. Average HTLs are shown in Figure 2.14. Also as in
Phase I, participants were required to be native English speakers.
60 Phase II Normal Hearing Subjects (n=8) Average Audiogram
Frequency (Hz) 250 500 1000 2000 3000 4000 6000 8000 -10 0 10 20 30 40 50 60
HTL (dBHL) 70 80 90 100 110 120
Figure 2.14: Average hearing threshold level (HTL) by frequency. Right ear HTLs are indicated by circles connected with a red line, and left ear HTLs are indicated with Xs connected with a blue line.
2.2.2.5 Test Procedures
Subjects were required to read and sign the Subject Consent Form before participating in the study. The subject’s date of birth, self-reported otologic history, and language background were entered into the HINT software. The subject was escorted to a chair in the test booth, and fit with TDH-39 headphones and a lapel microphone by the experimenter. Instructions for pure-tone testing were given over the headphones. A 61 pure tone audiogram was performed for each ear, and the test results were entered into the HINT software’s database. Once it was confirmed that the subject met the audiometric requirements for normal hearing, s/he was accepted into the experimental portion of the study. Following completion of the experimental protocol, the subject was shown his or her test results and any questions concerning the results and/or test protocol were answered. Subjects were paid for their time and signed the receipt log.
In this study, all listening conditions presented the speech and noise from one loudspeaker positioned in front of the subject (0 degrees Azimuth). Due to differences in test methodology and safety considerations, the HINT and the DRT used a fixed noise level, whereas the QSIN used a fixed speech level. Since it was desirable to control for the effects of SNR, the dependent variable was reported in dB at SNR50.
This point on the psychometric function of the DRT was found in pilot testing to be between -10 and -15 dB. In order to fix the speech level at 90 dB SPL, the noise levels would have had to be at least 105 dB SPL. It was decided to fix the noise level instead of the speech level in order to avoid this high-level noise exposure.
Due to previously described output limitations, the fixed level for the HINT was
65 dB SPL; whereas the fixed level for the QSIN and DRT was 90 dB SPL. The three listening conditions (unoccluded, conventional earmuffs, uniformly-attenuating earmuffs) were presented in counterbalanced order to control for any practice effects.
The HINT was administered first, since the audiometric evaluation was performed using the HINT. Utilizing the standard tracking method at 65 dB SPL, subjects repeated one list of 20 sentences in each listening condition. Next, the subject was repositioned to face the GSI loudspeaker in the right corner of the booth. The QSIN was administered
62 using a 90 dB SPL speech level. Subjects repeated two lists of six sentences for each listening condition.
Using the same loudspeaker and subject positioning, the DRT was administered in quiet and with the QSIN 4-talker babble noise presented at 90 dB SPL at 0, -5, -10, -
15, and -20 dB SNR for each listening condition. The quiet condition provided baseline data for the features of speech analysis. The resulting percent correct scores were plotted as a function of SNR. For a 2AFC task, chance performance would have been
50%; therefore, the SNR50 was defined as the 75% correct point on the psychometric function. This value was estimated by fitting a least squares linear trend line to the data
(Figure 2.15). Three listening conditions multiplied by six noise conditions equaled 18 test runs. Subjects performed the 2AFC task with 232 monosyllabic word stimuli for each of the 18 test runs. Subjects were instructed to request breaks as needed and/or were scheduled for two separate test sessions. The total time required for the complete test battery was about five hours on average.
63 Estimating SNR50 for the DRT: An Example 100 Measured Data Points in the Unoccluded Condition
90 Linear Trendline
y = -10.123x + 107.86 80 R2 = 0.9783 Estimated 75% Correct Point (-11.28) 70 DRT Score (% Correct) Correct) (% Score DRT 60
50 -20 -15 -10 -5 0 Speech to Noise Ratio (dB)
Figure 2.15: SNR50 for the DRT was equal to the estimated 75% correct point on the psychometric function. Estimation was based on least squares linear trend line fit to the measured data points.
2.2.2.6 Results
The mean SNR50 values for each test were plotted as a function of listening condition in Figure 2.16. A numerically higher SNR50 value indicated that the speech signal must be higher relative to the noise in the given test parameters in order for half the speech stimuli to be understood. Conversely, a numerically lower (more negative)
SNR50 value indicated that the speech signal can be lower than the noise in the given 64 test parameters in order for half the speech stimuli to be understood. Higher SNR50 values were a result of a more adverse listening condition; whereas lower SNR50 values were a result of a relatively easier listening condition. Therefore, given each test’s unique test protocol, test results suggested that the QSIN was the most difficult test, followed by the HINT, and then the DRT.
A repeated measures ANOVA revealed a statistically significant difference between the earmuffs (Figure 2.16). Since the QSIN showed a significant difference between the earmuffs, it was concluded that the QSIN was the most sensitive test for this application.
65 4
2 2.22
0.19 0 -0.28
-2 -2.01 -2.86 -2.70 -4 SNR50 -6
-8
-10 -10.67 -10.38 -11.28 -12 Unoccluded Conventional earmuffs Uniformly-attenuating earmuffs Listening Condition
QSIN Pilot 2 @ 90 HINT Pilot 2 subjects @ 65 DRT Pilot 2 @ 90
Figure 2.16: Average SNR50s plotted as a function of listening condition for each test tool.
The DRT, while not the most sensitive test, had an additional features analysis for possible rehabilitation applications. Since no noise was used, the test scores are reported in percent correct rather than SNR50. The three listening conditions were analyzed for effects of listening condition on six distinctive features of speech. First,
66 baseline data were collected in quiet with speech at the same level as presented in the noise conditions (70 dB SPL). Group means are plotted as a function of speech feature for each listening condition in Figure 2.17.
98 98 99 100 99 99 100 98 99 100 96 100 93 94 95 88 89 90 87 85
80
70
60
50
40
30 Average Percent Correct 20
(corrected for chance performance) 10
0 Voicing Nasality Sustention Sibilation Graveness Compactness Speech Features
Unoccluded Conventional Earmuffs Uniformly-attenuating Earmuffs
Figure 2.17: DRT features analysis for listening in quiet. Bars show average performance for eight normal hearing subjects with a speech level of 70 dB SPL.
67 A repeated measures ANVOA of the quiet listening condition showed only a few differences between listening to speech at 70 dB SPL unoccluded, with the conventional earmuffs or with the uniformly-attenuating earmuffs. For the features of compactness and graveness, conventional earmuffs significantly reduced speech intelligibility (p=0.033 and 0.006 for compactness and graveness, respectively) while the uniformly-attenuating earmuffs were not found to be significantly different from the unoccluded condition (p=0.064 and 0.051 for compactness and graveness, respectively).
Although scores were reduced for the feature of sustention, neither the conventional earmuffs nor the uniformly-attenuating earmuffs were found to be significantly different than the unoccluded condition (p=0.101).
Figure 2.18 shows psychometric functions by feature as a function of listening condition in noise. As with the total DRT SNR50 measure, speech intelligibility decreased monotonically with decreasing SNR. With the speech held constant at 70 dB
SPL and the noise increased to the maximum test condition of 90 dB SPL, improvements in speech intelligibility with earmuffs as compared to unoccluded listening were noted in sustention, sibilation and compactness over the less challenging masking conditions.
The interaction patterns in the figures below show how the perception of different features of speech may be differentially affected by the HPD and/or the SNR.
For some SNRs, the conventional earmuffs were better than the uniformly-attenuating earmuffs for a given feature of speech, but worse on another. For example, at a SNR of
68 –15 dB, the conventional earmuffs yielded better speech intelligibility for nasality, compactness and sustention, but the uniformly-attenuating earmuffs yielded better speech intelligibility for graveness.
Voicing Nasality
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Figure 2.18: DRT features of speech scores in percent correct (corrected for guessing). Unoccluded listening (blue squares), Conventional Earmuffs (red circles), Uniformly- attenuating Earmuffs (green triangles). (Continued)
69 Figure 2.19: Continued
Graveness Compactness
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2.2.2.7 Discussion
The QSIN emerged as the only one of the three tests evaluated that objectively validated the subjects’ subjective opinions of speech clarity with each of the earmuffs.
Therefore, it was decided to retain the QSIN for the final test protocol.
The HINT did not show statistically significant test results with this sample of eight normal hearing subjects. However, the trend of the HINT data was consistent with the subjective reports of better clarity with the uniformly-attenuating earmuffs than with the conventional earmuffs. The results may not have shown statistical significance due to the low power of the test (power = 0.23). Due to the small effect size of the
HINT in detecting speech intelligibility differences between the earmuffs (eta squared =
0.15), a larger sample size would be required to show good (80%) test power.
70 In addition to the above statistical difficulties with the HINT, other HINT characteristics were considered as would relate to the final test protocol. First, hearing- impaired listeners would require a higher SNR than normal hearing listeners in order to achieve a score of 50% correct. The standard HINT test protocol would need to be run at a fixed noise level much lower than that of the normal hearing subjects. Therefore, the hearing-impaired and normal hearing groups would not be comparable at equal
SPLs nor equal SLs. Secondly, the final test protocol should have the ability to test hearing-impaired subjects in representative occupational noise levels. As stated earlier, without equipment modification from the clinical calibration settings, the HINT would not have adequate loudspeaker output levels for this requirement. Given these limitations, the decision was made to eliminate the HINT from the final test protocol.
The DRT results for the listening conditions were not statistically different from one another, nor did the trend reflect the subjective observations of the subjects regarding speech clarity with the two types of earmuffs as did the other two tests. The
DRT speech features analyses gave only minimal objective evidence of the speech clarity differences between the earmuffs. Given these results, it was decided to eliminate the DRT from the final test protocol.
2.3 Prediction of Noise Exposure with Passive Earmuffs and Hearing Aids Worn in
Combination
The review of literature failed to discover published experimental data regarding the effects of hearing aids worn in conjunction with passive earmuffs on speech intelligibility or the resulting sound levels. Researchers may have assumed that hearing
71 aids worn in combination with earmuffs at or above the action level for HPD use (90 dBA) would necessarily expose the worker to unsafe sound levels. Peak clipping, a common output limiting strategy used in analog hearing aids, distorts high-level input.
However, state-of-the-art digital hearing aids now make use of various types of output limiting strategies, which are used for improved speech intelligibility across different communication settings – even with high-level input.
Before the speech intelligibility question could be added to the final test protocol, it was necessary to develop a computational model for the prediction of noise exposure with hearing aids and earmuffs worn in combination and to empirically test the model.
2.3.1 Aided-Protected Noise Exposure Model
This model for the calculation of Aided-Protected (AP) noise exposure was initially developed to define the maximum hearing loss criterion for hearing-impaired subject selection. The hearing aid gain, since unknown, was to be estimated by the
NAL-R prescription (Byrne & Dillon, 1986) for the given HTLs. The NAL-R hearing aid fitting formula estimates the necessary linear gain by frequency for specified hearing thresholds. However, many hearing aids fit during the last decade have been equipped with compression amplification rather than linear amplification.
Compression amplification, instead of providing the same amount of gain regardless of the input level (linear gain), generally provides linear gain at low input levels up to a predetermined level called the threshold kneepoint. At the threshold kneepoint, the gain is compressed relative to the input by a chosen ratio, called the
72 compression ratio. Currently, there are many different compression ratios used in hearing aid fitting since no one strategy has found universal acceptance. However, for high level inputs, compression amplification always generates less gain than linear amplification. Additionally, the recent introduction of digital signal processing to hearing aid amplification has made it possible for the amount of gain at high input levels to be reduced to zero automatically and then returned to the proper gain levels for good speech intelligibility during quieter conversation. Therefore, in order to account for the “worst case scenario” in terms of noise exposure, it was decided to base any unknown hearing aid gain on a widely accepted linear hearing aid fitting formula. It was decided to use the NAL-R for this model due to its long-standing use by clinicians and researchers as well as its ease of calculation.
The final protocol required subjects to wear their personal hearing aids, whatever the signal processing, in the experiment. Therefore, the amount of gain would vary, not by the amount of hearing loss of the subject, but as a function of the input level under the earmuffs and the type of amplification used. Since the most valid measure of gain for this study was the amount of gain at noise levels under the earmuffs, the subject selection criterion was changed from an audiogram-based requirement to a maximum hearing aid gain limitation. While the subject selection criteria for the final study did not limit participation based on the subject’s hearing thresholds, the model retained the audiogram variable. This was done so that if in future applications (e.g. occupational hearing conservation programs), the actual hearing aid gain was not available, the NAL-R values (or other linear amplification fitting formula) could be substituted.
73 The model requires data for all variables at the center frequencies (CFs) of 125
Hz, 250 Hz, 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz, and 8000 Hz. The variables include noise levels, earmuff attenuation, and use hearing aid gain or calculated NAL-R values.
Correction factors for microphone response and concha resonance characteristics are also incorporated into the model predictions.
Prediction of Aided-Protected Noise Exposure: An Example
First, noise levels measured in dB SPL for the center frequencies of 125 Hz, 250
Hz, 500 Hz, 1000 Hz, 2000 Hz, 4000 Hz, and 8000 Hz are entered into the model. For this example, noise levels were held constant across the frequency spectrum (pink noise). The total level of the octave band noise levels indicated in Table 2.4 is equivalent to 91.5 dB SPL or 90.0 dBA.
Center Frequency 125 250 500 1000 2000 4000 8000 (Hz) Noise 83 83 83 83 83 83 83 (dB SPL)
Table 2.4: Model for Aided-Predicted Noise Exposure – Step 1
74 Secondly, the Assumed Protection Values (APVs) are entered into the model.
This measurement was previously defined in the equipment section’s description of the experimental earmuffs. For this example, the APVs are for Bilsom model 817 earmuffs. The APVs are subtracted from the noise levels, which give the Unaided-
Protected (UP) noise exposure levels shown in Table 2.5.
Center Frequency 125 250 500 1000 2000 4000 8000 (Hz) Noise (N) 83 83 83 83 83 83 83 (dB SPL) APV 11.5 21.5 24.5 22.8 26.3 28.1 30.8 (dB) Unaided- Protected (dB SPL) 71.5 61.5 58.5 60.2 56.7 54.9 52.2 (UP=N- APV)
Table 2.5: Model for Aided-Predicted Noise Exposure – Step 2
In the third step, the measured hearing aid gain values (or calculated NAL-R values) are entered into the model. These values are added to the Unaided-Protected noise exposure values to obtain the Aided-Protected (AP) noise exposure (before correction factors) in Table 2.6.
75 Center Frequency 125 250 500 1000 2000 4000 8000 (Hz) Noise (N) 83 83 83 83 83 83 83 (dB SPL) APV 11.5 21.5 24.5 22.8 26.3 28.1 30.8 (dB) Unaided- Protected 71.5 61.5 58.5 60.2 56.7 54.9 52.2 (dB SPL) (UP=N-APV) Hearing Aid Gain* 0 0 6 19 22 23 0 (dB) (measured) * These gain values are the measured values for a linear hearing aid in the ear of the ATF which was set to the NAL-R prescription for the mild to moderately severe sloping hearing loss indicated below. HTLs (dB HL) 25 25 25 40 55 60 55 (for NAL-R calculation) Σ UP + Gain = Aided- 71.5 61.5 64.5 79.2 78.7 77.9 52.2 Protected (dB SPL) (AP)
Table 2.6: Model for Aided-Predicted Noise Exposure – Step 3
76 In the last step, the model corrects for the microphone insertion loss and response differences between the real ear probe microphone (ER 7-C) and the room microphone. In addition, the model accounts for the resonance effects of the concha
(Shaw, 1974). The final Aided-Protected noise exposure (after correction factors) is shown in Table 2.7.
77 Center Frequency 125 250 500 1000 2000 4000 8000 (Hz) Noise (N) 83 83 83 83 83 83 83 (dB SPL) APV 11.5 21.5 24.5 22.8 26.3 28.1 30.8 (dB) Unaided- Protected 71.5 61.5 58.5 60.2 56.7 54.9 52.2 (dB SPL) (UP=N-APV) Hearing Aid Gain* (dB) 0 0 6 19 22 23 0 (measured) * These gain values are the measured values for a linear hearing aid in the ear of the ATF which was set to the NAL-R prescription for the mild to moderately severe sloping hearing loss indicated below. HTLs (dB HL) (for NAL-R 25 25 25 40 55 60 55 calculation) Σ UP + Gain = Aided- 71.5 61.5 64.5 79.2 78.7 77.9 52.2 Protected (dB SPL) (AP) Microphone Correction -0.7 1.0 1.3 1.1 1.8 5.2 8.8 Factors (dB) (MCF) Σ AP + MCF 70.8 62.5 65.8 80.3 80.5 83.1 61.0 Concha Resonance 0 0 0 0 1.5 5 7 Values (dB) (CRV) Σ AP + MCF +CRV = FINAL Corrected AP 70.8 62.5 65.8 80.3 82.0 88.1 68.0 Noise Exposure (dB SPL)
Table 2.7: Model for Aided-Predicted Noise Exposure – Step 4
78 2.3.2 Validation of Model Predictions
A validation study of the noise exposure model was conducted in the NIOSH
Hearing Protector Laboratory. Predictions from the example above and the measured noise levels are plotted in Figure 2.19. The maximum acceptable exposure level was defined as the measured unoccluded ear response to a flat, 85 dBA noise. This level of noise exposure does not require the use of HPDs in the workplace (Occupational Safety and Health Administration, 1983b). Therefore, as long as the hearing aid output in the ear canal did not exceed these levels, the noise exposure should not put the hearing aid user at risk for noise induced hearing loss.
79 100
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Unoccluded Measured Levels @ 85 dBA 'Maximum Acceptable Exposure' Measured Noise Levels Predicted Noise Levels
Figure 2.19: Predicted versus measured noise exposure for linear hearing aids under uniformly-attenuating earmuffs. Measurements were made with an ER 7-C probe microphone in the ear canal of the ATF. Noise was presented in 1/3-octave bands at 83 dB SPL (equivalent to 90 dBA pink noise).
The measured noise exposure levels were very close to the predicted levels.
Both the measured and the predicted levels did not exceed the maximum acceptable exposure limit. Therefore, it was concluded that for this noise level (90 dBA) and spectrum (flat), with a linear hearing aid gain appropriate for a mild to moderately severe hearing loss, and Bilsom 817 earmuffs the listener could be adequately protected
80 from excessive noise exposure. This conclusion does not necessarily apply to all noise levels, noise types, hearing aid gain settings, or earmuffs. Each of these variables must be quantified in the model in order to obtain a valid prediction of noise exposure.
The validation study suggests that, at least for some combinations of noise levels, noise types, hearing aid gain settings, and earmuffs, the resulting noise exposure can be kept within acceptable limits. Whether or not this amplification strategy would prove beneficial in understanding speech in noise was a question asked in the final study.
81
CHAPTER 3
METHODS
3.1 Introduction
The final study was conducted to obtain speech intelligibility data from hearing- impaired listeners for comparison with those of the normal hearing listeners in Phase I.
Additionally, the hearing-impaired listeners were tested with their own hearing aids worn under the experimental earmuffs. The data collected in the final study was used to test two hypotheses.
3.2 Research Questions
First, do certain types of HPDs allow for better speech intelligibility than others?
For normal hearing listeners, the preliminary work showed a significant difference in speech intelligibility between two sets of passive earmuffs when the levels were at 70 dB SPL, but not when the levels were at 90 dB SPL. Reduced audibility of the speech signal was thought to have been the major factor in this outcome. Therefore, it was hypothesized that, due to reduced audibility from hearing loss, hearing-impaired listeners would exhibit a significant difference in speech intelligibility between the two sets of passive earmuffs even when the levels were at 90 dB SPL.
82 Secondly, would use of hearing aids worn in combination with earmuffs instead of wearing earmuffs alone provide better speech intelligibility for hearing-impaired workers? Since reduced audibility of the speech signal was thought to be the major contributor to the poorer speech intelligibility in the normal hearing listeners at 70 dB
SPL, logically it would follow that hearing-impaired listeners would show an even greater effect size as a result of their hearing losses. It was hypothesized that an amplified speech signal would significantly improve speech intelligibility under earmuffs in hearing-impaired listeners.
3.3 Study Design
In the final study, a within subjects design was used to evaluate the effects of the manipulated variables. The manipulated variables were earmuff type (conventional versus uniformly-attenuating) and hearing aid use (aided versus unaided). The resulting listening conditions were: unoccluded, conventional earmuffs unaided, conventional earmuffs aided, uniformly-attenuating earmuffs unaided, and uniformly-attenuating earmuffs aided. The final study’s data was statistically analyzed with repeated measures ANOVA. Also, clinical hearing threshold and speech intelligibility measures were correlated with the experimental QSIN scores.
3.4 Equipment
The final study was conducted in a double-walled, two-chamber IAC audiological test booth located in the OSU Speech-Language-Hearing Clinic. The inside chamber dimensions were 112” by 120” by 78” high. The ambient noise levels
83 met the specifications for threshold testing with ears uncovered in an audiometric booth as illustrated in Figure 3.1 (American National Standards Institute, 1991).
25 Ears Uncovered ANSI S3.1-1991
20 OSU Clinic Booth (Hearing-Impaired Subjects)
15
10
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0
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Figure 3.1: Ambient noise levels in audiometric booth used in the final study with equipment turned on compared to ANSI S3.1-1991.
The QSIN CD was played on an Onkyo compact disc player. The output from the CD player was routed through a GSI 61 diagnostic audiometer to a GSI sound field
84 speaker. The speaker was mounted in the left front corner of the booth with the middle of the speaker box positioned 48” above the floor. The sound field speakers were calibrated in accordance with recommended procedures for speech sound field testing
(American Speech-Language-Hearing Association, 1991). Figure 3.2 shows the frequency response of the sound field speaker used with the QSIN.
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Figure 3.2: Frequency response curve to white noise at 70 dB HL from the audiometer for the OSU Clinic’s GSI speaker box.
85 The experimental earmuffs were the same earmuffs used in the preliminary work. See Chapter 2 for a complete description of the conventional (Bilsom 717) and uniformly-attenuating (Bilsom 817) earmuffs.
The equipment used in the audiological evaluation portion of the study included a Welch Allyn otoscope, GSI-61 Clinical Audiometer with TDH-39 headphones, GSI
Tympstar Middle Ear Analyzer with probe #20010831, and an Audioscan Real-Ear
Hearing Aid Analyzer. Calibration of this equipment was conducted annually through a contract with a local service provider. Experimenter documentation of the speaker output levels was performed with a Bruel & Kjaer Type I sound level meter before the study began, and via digital recordings (44,100 Hz, 16 bit, stereo) with a Dell Latitude
C810 laptop computer and Music Match Version 7.2 software at the conclusion of the study.
3.5 Subjects
Ten (5 males, 5 females) hearing-impaired adults, who were ages 45 to 65 years
(mean = 55.1 years, standard deviation = 6.2 years) and wore binaural in-the-ear hearing aids, were recruited into the final study. As in the preliminary studies, participants were required to be native English speakers. Additionally, potential subjects were required to pass a telephone-administered screening questionnaire (Appendix C) to rule out otologic pathology prior to setting the study appointment. Subject recruitment was accomplished by sending a direct mail solicitation (Appendix A) to OSU Speech-
Language-Hearing Clinic patients who met the subject selection criteria and by placing an advertisement in the Columbus Dispatch, the major daily newspaper in Columbus,
86 OH, reaching 255,390 households daily (Appendix B). Subjects were paid $20 for the two-hour session and were offered a copy of their clinical hearing test results.
Hearing impairment was defined as a 25 dB HL or greater sensori-neural hearing loss at 2000 Hz and above. Persons with a conductive or mixed (conductive plus sensori-neural) hearing loss were excluded from the subject pool, since speech intelligibility is typically not impaired by a conductive hearing loss when speech is presented at audible levels. Inclusion of conductive or mixed hearing losses could have confounded the results. Average hearing threshold levels (HTLs) for the Phase I normal hearing group and for the final study hearing-impaired group are shown in Figure 3.3.
87 FREQUENCY (Hz) 250 500 1000 2000 3000 4000 6000 8000 -10 0 10 20 30 40 50 60 70 HTL (dBHL) 80 90 100 110 120
WNL Left AVG WNL Right AVG HI Left AVG HI Right AVG
Figure 3.3: Average Hearing Thresholds (HTLs). Phase I normal hearing listeners are represented with Xs and Os. Final study hearing-impaired listeners are represented with squares and triangles.
Setting the age requirements for the hearing-impaired subject group involved some compromise. When recruiting efforts failed to attract ‘young’ (ages 21 to 40 years) hearing-aid wearers to participate in the study, the benefits of age matching to the normal hearing group were lost. In a review of the literature, studies suggested that
‘elderly’ (mean age >65 years) hearing-impaired listeners score lower on speech intelligibility in noise tests than can be accounted for by audibility alone (Magnusson et 88 al., 2001; Schum, Matthews, & Lee, 1991). When ‘older’ (ages 40 to 60 years) hearing- impaired subjects have been tested, it was found that aging effects on speech intelligibility in noise may begin before age 65(Abel, Krever, & Alberti, 1990; Abel et al., 1993). However, it was important to the NIOSH Hearing-Impaired Worker Project to have test results, which could be generalized to workers of all ages. Therefore, taking the subject recruitment issues and research needs as well as the experimental evidence into account, it was decided to limit the age of participants to the common retirement age of 65 years.
Hearing aid gain limitations were based on the model for the prediction of aid- protected noise exposure described in Chapter 2. In this model, the maximum acceptable exposure curve was based on the fact that for a work shift of 8 hours, exposure to noise levels of 85 dBA does not require the use of HPDs. However, exposure to 8-hour durations of 85 dBA noise may, over time, cause hearing loss in those workers who are highly susceptible to noise-induced hearing loss (NIHL)
(NIOSH, 1998). It is important to note that the duration of the noise exposure as well as the level of the noise exposure is needed to calculate the noise dose:
Dose = [C1/T1 + C2/T2 + ... + Cn/Tn ] x 100, where, Cn is the total time of exposure at a specified noise level, and Tn is the exposure time at which noise for this level becomes hazardous.
In the final protocol, the duration of exposure to noise in excess of the maximum acceptable exposure curve was only two minutes. This occurred during the unoccluded listening condition, in which speech was presented at 90 dB SPL. Model predictions for the protected and aided-protected listening conditions revealed exposure levels below
89 the maximum acceptable exposure curve. The calculations for maximum hearing aid gain in the aided-protected listening conditions for Bilsom’s model 817 and 717 earmuffs are shown in Tables 3.1 and 3.2, respectively. The lower of the two permissible hearing aid gain values for each octave band was set as the maximum hearing aid gain for this study (Table 3.3).
90 Center 125 250 500 1000 2000 4000 8000 Frequency (Hz) Calculate Unaided-Protected Exposure: Octave Band Speech Levels at 90 83 85 85 80 77 71 67 dB SPLa) (S) (dB SPL) Model 817 APV 11.5 21.5 24.5 22.8 26.3 28.1 30.8 (dB) Microphone Correction Factors -0.7 1.0 1.3 1.1 1.8 5.2 8.8 (dB) (MCF) Concha Resonance 0 0 0 0 1.5 5 7 Values (dB) (CRV) Unaided-Protected (dB SPL) 72.2 62.5 59.2 56.1 47.4 32.7 20.4 UP=S-APV-MCF- CRV Calculate Maximum Permissible Gain Levels: Maximum Acceptable Exposure Levelsb) 77.3 79.0 80.7 82.2 89.0 93.6 81.7 (MAEs) (dB SPL) Unaided-Protected 72.2 62.5 59.2 56.1 47.4 32.7 20.4 (UP) (dB SPL) Maximum Permissible Gain 5.1 16.5 21.5 26.1 41.6 60.9 61.3 (MPG) (dB) (MPG=MAE-UP) a) Long term average speech spectrum (Fig. 3, Pg. 12 ANSI S3.5-1969) + 17 dB = 90 dB SPL b) Real ear measurements in ear canal of NIOSH ATF at 85 dBA.
Table 3.1: Maximum permissible hearing aid gain levels with exposure to speech at 90 dB SPL while wearing Bilsom’s model 817 earmuffs.
91 Center 125 250 500 1000 2000 4000 8000 Frequency (Hz) Calculate Unaided-Protected Exposure: Octave Band Speech Levels at 90 83 85 85 80 77 71 67 dB SPLa) (S) (dB SPL) Model 717 APV 11 13.2 22.5 33.6 32.1 35 33.6 (dB) Microphone Correction Factors -0.7 1.0 1.3 1.1 1.8 5.2 8.8 (dB) (MCF) Concha Resonance 0 0 0 0 1.5 5 7 Values (dB) (CRV) Unaided-Protected (dB SPL) 72.7 70.8 61.2 45.3 41.6 25.8 17.6 UP=S-APV-MCF- CRV Calculate Maximum Permissible Gain Levels: Maximum Acceptable Exposure Levelsb) 77.3 79.0 80.7 82.2 89.0 93.6 81.7 (MAEs) (dB SPL) Unaided-Protected 72.7 70.8 61.2 45.3 41.6 25.8 17.6 (UP) (dB SPL) Maximum Permissible Gain 4.6 8.2 19.5 36.9 47.4 67.8 64.1 (MPG) (dB) (MPG=MAE-UP) a) Long term average speech spectrum (Fig. 3, Pg. 12 ANSI S3.5-1969) + 17 dB = 90 dB SPL b) Real ear measurements in ear canal of NIOSH ATF at 85 dBA.
Table 3.2: Maximum permissible hearing aid gain levels with exposure to speech at 90 dB SPL while wearing Bilsom’s model 717 earmuffs.
92 Center 125 250 500 1000 2000 4000 8000 Frequency (Hz) Maximum Permissible Hearing Aid Gain for 4.6 8.2 19.5 26.1 41.6 60.9 61.3 Final Study
Table 3.3: Maximum permissible hearing aid gain levels with exposure to speech at 90 dB SPL while wearing the lesser protective (by frequency) of the experimental earmuffs.
93 Hearing Aid Insertion Gain Measurements at Use Settings (Input = 70 dB SPL swept pure tones)
Frequency (Hz) 125 250 500 1000 2000 4000 8000 -10 Maximum Allowable Gain 0 Average 10 HI1 20 HI2 30 HI3 40 HI4 HI5 50 HI6 60 HI7 Gain (dB) Gain 70 HI8 80 90 100 110 120
Figure 3.4: Maximum hearing aid insertion gain plotted as a function of frequency for 8 of 10 subjects. Maximum allowable gain values are from Table 3.3.
3.6 Procedures
Potential subjects, whether responding to the direct mail or newspaper advertisement, were instructed to contact the secretary at the OSU Speech-Language-
Hearing Clinic. The secretary would then contact the primary investigator, who would return the potential subject’s phone call. During this phone call, any questions about the study were answered, and the test protocol was explained. If the potential subject
94 indicated that s/he would like to participate in the study, the subject selection requirements were confirmed and a brief screening questionnaire was administered to rule out the presence of otologic abnormalities (Appendix C). If the potential subject met all the criteria for the study, contact information was recorded, and s/he was given an appointment time and directions to the OSU Speech-Language-Hearing Clinic.
When the subject arrived for the study, s/he was given an OSU Speech-
Language-Hearing Clinic Parking Permit to place in the front window of the car. The subject returned to the car with the permit, and put enough money in the parking meter to assure adequate time to complete the test protocol (two hours). A few subjects who were not clients at the OSU Speech-Language-Hearing Clinic expressed irritation with the parking situation even though it had been explained to them during the scheduling phone call. In an effort to appease these subjects, the parking meter fee was paid for them by either the OSU Speech-Language-Hearing Clinic or the primary investigator.
Audiological Evaluation
Before testing was performed, the subject was asked to read and sign the Subject
Consent Form (Appendix D). The initial portion of the clinical audiological evaluation consisted of an otoscopic exam, tympanograms, ipsilateral, and contralateral acoustic reflexes to 500 Hz and 1000 Hz pure tones. Upon confirmation of normal outer and middle ear status, the subject was seated in the sound booth and fit with TDH-39 headphones. Instructions for pure-tone testing were given over the headphones, and an air and bone conduction pure tone audiogram was performed for each ear to confirm the audiometric subject selection criteria. Next, instructions for the Speech Reception
95 Threshold Test (SRT Test) were given over the headphones. Recorded spondees were presented monaurally to each ear in 5-dB steps using an up-down adaptive tracking method. Threshold was defined as the lowest level for 3 out of 6 correct responses.
This value was compared to the pure tone average (500 Hz, 1000 Hz, and 2000 Hz) for validation purposes. Plus or minus 7 dB was considered to be acceptable variability
(Schill, 1985; Hall & Mueller, 1997). If the variability was unacceptable, the test protocol called for a retest of pure tone thresholds and SRTs. Word recognition testing was performed at the subject’s most comfortable loudness level (MCL). One fifty-word list of phonetically balanced monosyllabic words was taken from the Department of
Veterans Affairs CD recording (1989) of the Northwestern University NU-6 Word Lists and was presented to each ear under TDH-39 headphones.
In addition to the standard speech testing described above, binaural word discrimination in quiet was measured using the NU-6 Word Lists in the sound field at
70 dB SPL with subjects unaided and aided. To assess binaural speech in noise performance, the QSIN was administered at 70 dB SPL in the unaided and aided conditions. All data was recorded on the Data Recording Form (Appendix I).
Hearing Aid Evaluation
Hearing aids were visibly inspected for occluding cerumen and general condition. A listening check was performed, followed by an electroacoustic analysis.
Several of the hearing aids could not be set to full on without the use of proprietary software and equipment. Therefore, to obtain comparable data, all measures were done at user settings for typical conversation. The 2-cc coupler tests were run according to
96 the Audioscan Manual for ANSI S3.22-1987 testing (American National Standards
Institute, 1987). Probe microphone measurements were made at 70 dB SPL to quantify the insertion gain of each hearing aid. These values were compared to the maximum acceptable gain levels from the model, and the results were explained to the subject. If time and hearing aid processing permitted, the effects of compression processing on loud versus soft sounds were demonstrated to the subject.
Experimental Test Protocol
The QSIN was administered with the subject seated at the calibration location in the center of the booth and directly facing the left sound field speaker. The speaker was positioned in the corner to the right of the observation window (experimenter’s view), and the middle of the speaker was 48” above the floor and 74” in front of the subject.
The subject’s task was to repeat the entire sentence. There were five target words in each sentence, and each target word was scored as a separate item. The assignment of
QSIN sentence lists to listening conditions was randomized and recorded on each score form prior to the start of data collection. Two lists of six sentences each were presented at 90 dB SPL in each of the five listening conditions: unoccluded, conventional earmuffs unaided, conventional earmuffs aided, uniformly attenuating earmuffs unaided, and uniformly attenuating earmuffs aided. The order of the listening conditions was counterbalanced to control for a possible practice effect. Earmuffs were fit using the experimenter fit method in order to minimize differences due to earmuff placement; however, in the aided conditions, subjects inserted their own hearing aids.
97 Following the completion of the experimental protocol, each subject was shown his or her test results. At this time, the difference between the earmuffs was revealed.
Any questions the subject raised were answered or referred to the proper authority. The subject was offered a copy of his or her test results, received $20 for his or her time, and asked to sign a receipt of payment.
98
CHAPTER 4
RESULTS
4.1 Audiological Evaluation
Ten hearing-impaired adults presented for the qualifying audiological evaluation. All ten hearing-impaired adults met the subject selection criteria for participation. In each of the 20 ears evaluated, otoscopy revealed no excess cerumen or symptoms consistent with outer or middle ear pathology. Tympanograms were within normal limits, and acoustic reflexes were present at expected levels.
Pure tone audiometry, which was performed on normal hearing as well as hearing-impaired subjects, showed that all 20 normal hearing ears and all 20 hearing- impaired ears met the subject selection criteria for their assigned group. Hearing threshold distributions as a function of pure tone frequency are presented for the Phase I normal hearing subjects in Figures 4.1 and 4.2 for the right and left ears, respectively.
Hearing threshold distributions for the hearing-impaired subjects are shown in Figures
4.3 and 4.4 for the right and left ears, respectively. Hearing threshold levels (HTLs) by normal hearing subject are listed in Appendix E and by hearing-impaired subject in
Appendix F.
99 Normal Hearing Subjects: Right Ear HTLs
-20
-10
0
10
20
30
40
50
60
70
Hearing Threshold (dB HL) (dB Threshold Hearing 80
90
100 250 500 1000 2000 3000 4000 6000 8000 Frequency (Hz)
Figure 4.1: Normal hearing subjects’ right ear HTLs. Box plots indicate the distribution of HTLs as a function of frequency. A line in the box indicates the median value, while the 25th and 75th percentiles are at the edges of the box. The 10th and 90th percentiles are the ends of the error bars. Outliers are shown with a plus (+).
100 Normal Hearing Subjects: Left Ear HTLs
-20
-10
0
10
20
30
40
50
60
70
Hearing Threshold (dB HL) (dB Threshold Hearing 80
90
100 250 500 1000 2000 3000 4000 6000 8000 Frequency (Hz)
Figure 4.2: Normal hearing subjects’ left ear HTLs. Box plots indicate the distribution of HTLs as a function of frequency. A line in the box indicates the median value, while the 25th and 75th percentiles are at the edges of the box. The 10th and 90th percentiles are the ends of the error bars. Outliers are shown with a plus (+).
101 Hearing Impaired Subjects: Right Ear HTLs
-20
-10
0
10
20
30
40
50
60
70
Hearing Threshold (dB HL) (dB Threshold Hearing 80
90
100 250 500 1000 2000 3000 4000 6000 8000 Frequency (Hz)
Figure 4.3: Hearing-impaired subjects’ right ear HTLs. Box plots indicate the distribution of HTLs as a function of frequency. A line in the box indicates the median value, while the 25th and 75th percentiles are at the edges of the box. The 10th and 90th percentiles are the ends of the error bars. Outliers are shown with a plus (+).
102 Hearing Impaired Subjects: Left Ear HTLs
-20
-10
0
10
20
30
40
50
60
70
Hearing Threshold (dB HL) (dB Threshold Hearing 80
90
100 250 500 1000 2000 3000 4000 6000 8000 Frequency (Hz)
Figure 4.4: Hearing-impaired subjects’ left ear HTLs. Box plots indicate the distribution of HTLs as a function of frequency. A line in the box indicates the median value, while the 25th and 75th percentiles are at the edges of the box. The 10th and 90th percentiles are the ends of the error bars. Outliers are shown with a plus (+).
103 There was a greater range of HTL values within the hearing-impaired group than within the normal hearing group. This was to be expected since the audiometric subject selection criteria for the hearing-impaired group was more broadly defined than for the normal hearing group. Pure tone bone conduction audiometry confirmed the absence of conductive hearing loss within the hearing-impaired group.
Speech reception thresholds showed good agreement with the pure tone thresholds in every case. Word discrimination testing was performed at the Most
Comfortable Loudness Level (MCL) for each ear under headphones. Each hearing- impaired subject’s headphone MCLs are shown in Figure 4.5 with the NU-6 scores shown in Figure 4.6.
104 100
90
80
70
60 50 Normal Hearing Speech MCL Range 40
30
Speech Level (dB HL) Level (dB Speech 20
10
0 HI1 HI2 HI3 HI4 HI5 HI6 HI7 HI8 HI9 HI10 Subject Identifier
Left Ear MCLs Right Ear MCLs
Figure 4.5: Hearing-impaired subjects’ speech MCLs with TDH-39 headphones. MCL = Most Comfortable Loudness Level. The MCLs shown here were the speech levels used to present the NU-6 word lists in Figure 4.6.
105 100 90 80 70 60 50 40 30
NU-6 Score (% Correct) 20 10 0 HI1 HI2 HI3 HI4 HI5 HI6 HI7 HI8 HI9 HI10 Subject Identifier
Left Ear % Correct Right Ear % Correct
Figure 4.6: Hearing-impaired subjects’ NU-6 scores under THD-39 headphones. Speech presentation levels were the MCLs from Figure 4.5.
The NU-6 word lists were also presented at 70 dB SPL in the sound field in both unaided and aided listening conditions. This testing is typically done to show functional hearing aid benefit. However, another reason for including this testing in the study was to explore a possible correlation between word recognition scores obtained under headphones and those obtained in the sound field. The sound field NU-6 testing was not added to the test protocol until after data collection from the first hearing-impaired subject. Several attempts were made to have this subject return for this assessment. 106 When, at the completion of data collection, these attempts had been unsuccessful, the decision was made to proceed to the data analysis phase of the study. Since the data to be correlated were within rather than between subjects, the results would not be systematically affected by non-response bias. Therefore, the results would provide a valid estimate of the correlation between the unaided tests. Results from the unaided and aided sound field NU-6 word recognition tests are shown in Figure 4.7.
107 100 90 80 70 60 50 40 30
NU-6 Score (% Correct) 20 10 0 HI1 HI2 HI3 HI4 HI5 HI6 HI7 HI8 HI9 HI10 Subject Identifier
Unaided Aided
Figure 4.7: Hearing-impaired subjects’ NU-6 scores in the sound field with and without hearing aids. Speech presentation level was 70 dB SPL. Hearing aids, if adjustable, were set for typical one-on-one conversation. Subject HI1 did not return for this testing. Subject HI4 was tested in both conditions, and scored 0% in the unaided condition.
The QSIN test was also presented at 70 dB SPL in the sound field in both unaided and aided listening conditions. This testing was done to show functional hearing aid benefit for a moderate level, noisy listening situation. Results from the unaided and aided QSIN test are shown in Figure 4.8.
108 0 0.5 1.5 2 0.5 3 2.5 3 4 4.75 4.5 4.25 5 5.5 5 8 10 9.5 11.75 13.5 15
19.25 20
23.75 QSIN Score (SNR50) Score QSIN 25 25.25
30 HI1 HI2 HI3 HI4 HI5 HI6 HI7 HI8 HI9 HI10 Subject Identifier
Unaided Aided
Figure 4.8: QSIN scores for hearing-impaired subjects. Speech presentation level was 70 dB SPL. Hearing aids, if adjustable, were set for typical one-on-one conversation.
4.2 Hearing Aids
Each subject wore his or her own binaural in-the-ear type hearing aids in the study. Table 4.1 provides a description of each subject’s hearing aids.
109 Subject ID Shell Size Manufacturer Amplification Type HI1 CIC Oticon digital adaptive compression analog class D circuit with HI2 ITC Starkey limiting output, variable release compression HI3 CIC Oticon digital adaptive compression HI4 ITE Oticon digital adaptive compression HI5 ITC Oticon digital adaptive compression HI6 CIC Oticon digital adaptive compression HI7 ITC Oticon digital adaptive compression HI8 ITC Micro-Tech analog compression HI9 CIC Rexton analog linear HI10 Half Shell Oticon digital adaptive compression
Table 4.1: Hearing aids worn by subjects.
Hearing aid insertion gain measurements made with the hearing aids set to the user’s settings are shown in Table 4.2. The input stimulus was a swept pure tone at 70 dB SPL. Real ear analysis was not performed for subject HI9’s or HI10’s hearing aids.
The tight fit of subject HI9’s completely-in-the-canal (CIC) hearing aids did not permit proper probe tube placement without acoustic feedback. Subject HI10 was unable to wait for the test equipment to become available, but had recent electroacoustic analysis results, which were provided to the experimenter. Electroacoustic analyses per ANSI
S3.22-1987 (American National Standards Institute, 1987) showed that both subjects’ hearing aids were functioning within specifications. Given this information and a biological listening check, hearing aid insertion gain measurements were estimated to be within acceptable limits for this study.
110 Center 125 250 500 1000 2000 4000 8000 Frequency (Hz) Maximum Permissible Hearing Aid Gain for 4.6 8.2 19.5 26.1 41.6 60.9 61.3 Final Study (dB) HI1 0 2 12 10 8 3 0 HI2 0 2 12 18 18 15 12 HI3 0 2 7 10 0 0 0 HI4 0 5 12 20 20 10 0 HI5 0 2 5 18 22 15 0 HI6 0 5 16 11 15 15 0 HI7 0 7 5 12 18 0 0 HI8 0 5 10 26 12 25 0 HI9 CNT Real Ear -- too tight a fit. HI10 CNT Real Ear -- machine not available, subject had to leave.
Table 4.2: Hearing aid gain by subject identifier as a function of center frequency. All gain values were within acceptable study limits (compared to Maximum Permissible Hearing Aid Gain).
4.3 Experimental Test Protocol
The normal hearing and hearing-impaired group means from the QSIN test are plotted as a function of listening condition in Figure 4.9. Individual test scores for each listening condition are located in Appendix G. A numerically higher SNR50 value indicates that the speech signal must be higher relative to the noise in the given test parameters in order for half the speech stimuli to be understood. Conversely, a numerically lower (more negative) SNR50 value indicates that the speech signal can be lower than the noise in the given test parameters in order for half the speech stimuli to be understood.
111 The normal hearing group was tested with speech at 70 dB SPL and 90 dB SPL in each listening condition. As previously discussed in Chapter 2, results showed no difference between the earmuffs when speech was presented at 90 dB SPL. However, the results revealed a significant reduction in speech intelligibility with the conventional earmuffs when speech was presented at 70 dB SPL.
The hearing-impaired group was only tested at 90 dB SPL in the protected listening conditions as well as in the aided-protected conditions due to anticipated floor effects for the protected listening conditions at 70 dB SPL. Even so, these scores were significantly worse than those of the normal hearing group at the lower presentation level. The differences between the groups increased as a function of the increase in attenuation. In the unoccluded condition, the normal hearing group’s mean was -1.05 dB (70 dB SPL presentation level) versus the hearing-impaired group’s mean of 4.60 dB (90 dB SPL presentation level). The 5.6 dB difference between the groups may or may not be noticeable depending on the communication burden of the situation.
However, in the conventional earmuff listening condition, the means were markedly different. The normal hearing group’s mean (2.25 dB) was only 3.2 dB worse than its unoccluded mean. However, the hearing-impaired group’s mean (17.85 dB) was 13.25 dB worse than its unoccluded mean. Instead of only a 5.6 dB difference between the groups, as in the unoccluded condition, the difference increased to 15.6 dB in the conventional earmuff condition. The uniformly-attenuating earmuffs produced a similar difference between the groups (approximately 15 dB). The group means were
0.825 dB versus 15.85 dB for the normal hearing versus the hearing-impaired group, respectively.
112 The hearing-impaired group was also tested in the aided-protected conditions. It was thought that the custom-fit hearing aids might increase the protected speech intelligibility scores to more closely compare with those of the normal hearing group.
Indeed, the addition of the hearing aids did produce better protected speech intelligibility; however, the differences between the groups’ performances remained significantly larger than in the unoccluded condition. In the conventional earmuff aided-protected condition, the mean score was 12.7 dB. This was approximately 3 dB better than without hearing aids, but still fell about 10 dB short of the normal hearing group’s 2.25 dB performance with the conventional earmuffs. The uniformly- attenuating earmuff aided-protected condition yielded a mean score of 9.0 dB, which was nearly 6 dB better than in the uniformly-attenuating earmuff only condition. While this increase in speech intelligibility over the earmuff only condition was twice that of the conventional earmuffs, the hearing-impaired group still fell about 8 dB short of the normal hearing group’s 0.825 dB performance with the uniformly-attenuating earmuffs.
In addition to the experimental protocol, hearing-impaired subjects were tested in the clinical audiological evaluation for unoccluded speech intelligibility at 70 dB SPL with and without their hearing aids. The group means were 4.2 dB with the hearing aids and 10.73 dB without the hearing aids. This 6.5 dB improvement in speech intelligibility between the unaided and aided conditions was consistent with the amount of improvement seen between the protected and aided-protected conditions with the uniformly-attenuating earmuffs (6 dB improvement). With the conventional earmuffs, the improvement in speech intelligibility between the unaided and aided conditions was much smaller (3 dB).
113 It is possible that the smaller amounts of low frequency attenuation and larger amounts of high frequency attenuation inherent in the conventional earmuffs relative to the uniformly-attenuating earmuffs is what degraded speech intelligibility rather than the slightly greater amount of total attenuation. The results from the normal hearing group are consistent with this theory as well. The differences between the 70 dB SPL and 90 dB SPL means in the unoccluded condition were negligible; however, there was nearly a 3 dB difference with the conventional earmuffs versus only a 2 dB difference with the uniformly-attenuating earmuffs. While this effect size was not as pronounced as that of the hearing-impaired group, it was statistically significant.
114 -5
-1.05 -0.6 -1.2 0 -0.1 2.25 0.825
4.2 5 4.60
9.00 10 10.73 12.70 Average SNR50 (dB) 15 15.85 17.85 20 Unoccluded 717 Muffs 817 Muffs HAs + 717s HAs + 817s Listening Condition
Normal Hearing (Speech = 90) Hearing Impaired (Speech = 90) Normal Hearing (Speech = 70) Hearing Impaired (70/Unaided) Hearing Impaired (70/Aided)
Figure 4.9: QSIN group means for normal hearing subjects (Phase I) and hearing- impaired subjects (final study). Speech level was 90 dB SPL for the solid squares and circles. Speech level was 70 dB SPL for the open triangles, the X, and the *. Significant Difference = p < 0.05.
4.3.1 Repeated Measures ANOVA
As reported in Chapter 2, the normal hearing group did not show a significant difference between the conventional and uniformly attenuating earmuffs when speech was presented at 90 dB SPL. However, when speech was presented at 70 dB SPL, a significant main effect of earmuff type was shown.
115 A within subjects ANOVA of the hearing-impaired group’s experimental protocol data showed a significant main effect of listening condition (p<0.0001) with the unoccluded condition as the control condition. Results of the multiple comparisons, based on the Tukey-Kramer adjustment, indicated that every listening condition significantly degraded speech intelligibility when compared to the unoccluded condition. The type of earmuff did not show significantly different effects on speech intelligibility. However, test results showed a significant difference between unaided and aided listening conditions in terms of improved speech intelligibility with the use of hearing aids in combination with each type of earmuff. The difference between the unaided (10.73 dB) and aided (4.2 dB) unoccluded QSIN test presented at 70 dB SPL was also significant for the hearing-impaired group.
The factors of gender and age were also included in the analysis model for the hearing-impaired subjects. There was a main effect for gender (p = 0.0290) but not for age (p = 0.3080). The male subjects scored significantly better than the female subjects overall. However, the main effect of gender was qualified by interactions with listening condition (p = 0.0204) and age (0.0050). Figure 4.10 shows the interaction of listening condition and gender. The unoccluded and 717 earmuff conditions showed similar results for males and females; however, in the 817 earmuff condition and the aided/protected conditions, QSIN scores were much different between the genders.
Figure 4.11 shows the interaction of age and gender. The linear trend lines indicate that the overall QSIN score increased (became worse) with increasing age in females; whereas in males the overall QSIN score remained relatively unchanged as a function of age.
116 A possible explanation for these effects lies in the HTL differences between the male and female subjects. Figure 4.12 presents average right and left audiograms for normal hearing and hearing-impaired subjects grouped by gender. The normal hearing subjects were a homogenous group in terms of HTLs. The female hearing-impaired subjects’ HTLs were 15 – 20 dB worse than their male counterparts in the low to mid frequencies. However, the groups’ audiometric averages were similar from 3000 Hz –
8000 Hz. Although the HTLs between the hearing-impaired males and females were different, the hearing aid gain did not show the expected differences based on the HTLs
(Figure 4.13). Therefore, even in the aided conditions, the female subjects heard the sentences at a lower sensation level than the male subjects.
117 0
2 1.89 2.55 4 4.11
6 5.60
8 7.78 8.90 9.32 10 9.95
12 12.40
14 Average SNR50 (dB) (dB) SNR50 Average 16 16.50 18 Unoccluded 717 Muffs 817 Muffs 717s+HAs 817s+HAs
Listening Condition
Female Averages Male Averages
Figure 4.10: Interaction of Listening Condition and Gender.
118 0
5
10
15
20 Overall QSIN SNR50 (dB) SNR50 QSIN Overall
25 40 45 50 55 60 65 AGE (years)
Male Subject Female Subject
Figure 4.11: Interaction of Age and Gender. Overall QSIN Scores are individual subject’s scores averaged across all listening conditions. Lines are linear trend lines.
119 Frequency (Hz) 250 500 1000 2000 3000 4000 6000 8000 -10 0 10 20 30 40 50 60
HTL (dBHL) 70 80 WNL Male Left AVG WNL Males Right AVG 90 WNL Female Left AVG WNL Female Right AVG 100 HI Male Left AVG HI Male Right AVG 110 HI Female Left AVG HI Female Right AVG 120
Figure 4.12: Male versus Female Subjects by Hearing Status: Average Audiograms.
120 Frequency (Hz) 125 250 500 1000 2000 4000 8000 -10 0 10 20 30 40 50 60
Gain (dB) Gain 70 80 90 100 Maximum Allowable Gain Male Average HA Gain 110 Female Average HA Gain 120
Figure 4.13: Hearing Aid Insertion Gain Measurements at Use Settings with 70 dB SPL Swept Pure Tones.
4.3.2 Correlation Analyses
The ANOVA results showed that the QSIN test was sensitive to the differential effects of earmuffs and hearing aids on speech intelligibility in noise for groups of normal hearing and hearing-impaired listeners. A predictor of the effects of HPDs on speech intelligibility in noise for a given worker is needed in hearing conservation programs. Correlation analyses were performed on the unoccluded QSIN test and
121 the clinical hearing evaluation data in an attempt to find a predictor of protected speech intelligibility in noise performance. Was the QSIN test score in the unoccluded condition predictive of the effects of earmuffs and hearing aids for these subjects? If so, the single test would reduce the test time needed to assess a worker. Was the clinical sound field speech intelligibility test in quiet (NU-6) predictive of speech intelligibility in noise for this group of hearing-impaired listeners? If the sound field NU-6 test was predictive of speech intelligibility in noise, a worker would not need to be exposed to noise for a speech intelligibility evaluation. However, in a typical hearing conservation program, facilities are not available for sound field testing. Were hearing threshold measures or clinical speech intelligibility measures obtained under headphones predictive of the experimental listening conditions tested? In addition to these HTL and speech intelligibility measures, a modified Speech Intelligibility Index (SII) was calculated for each listening condition. Data used in the correlation analyses is in
Appendix H.
The unoccluded QSIN at 90 dB SPL was not the best predictor of protected speech intelligibility in noise. The R-squared values were 0.19 and 0.27 for the 817 and
717 earmuffs, respectively. The unoccluded QSIN was significantly positively correlated with the QSIN scores in the aided conditions, though. The majority of the variance in the aided-protected condition (62% and 53% for the 817 earmuffs and the
717 earmuffs, respectively) could be explained by the speech intelligibility performance in the unoccluded condition.
The NU-6 test score obtained in the sound field at 70 dB SPL in the unaided condition was the single best predictor across the experimental listening conditions.
122 Other sound field speech intelligibility tests that were evaluated included the aided NU-
6 test, the unaided QSIN test, and the aided QSIN test. Each of the tests had been presented at 70 dB SPL. Table 4.3 shows the R-squared values for each of these predictors. Even though the NU-6 sound field test had no competing noise, performance on the NU-6 sound field test explained 55% to 86% of the variance in the experimental speech in noise conditions as measured by the QSIN.
EXPERIMENTAL LISTENING CONDITION SPEECH 717 817 717 Muffs 817 Muffs SOUND FIELD Unoccluded Earmuffs Earmuffs + HAs + HAs PREDICTOR
NU-6 unaided 70 dB SPL 0.5506 0.7919 0.7179 0.8561 0.6315
NU-6 aided 70 dB SPL 0.6979 0.4895 0.5460 0.6978 0.6946
QSIN unaided 70 dB SPL 0.3960 0.5796 0.5704 0.6226 0.5376
QSIN aided 70 dB SPL 0.2902 0.4920 0.3379 0.5220 0.4085
QSIN Unoccluded 1.0000 0.2745 0.1911 0.5371 0.6265 90 dB SPL
Table 4.3: The relationship between sound field measures of speech intelligibility and the experimental listening conditions. Values shown are in units of R-squared. Bolding indicates the strongest correlation for the listening condition.
123 The headphone speech intelligibility tests that were evaluated included the NU-6 test score for the better ear, the worse ear, and the average of both ears. The worse ear’s performance on the NU-6 was the best predictor for three out of the five experimental listening conditions (Table 4.4). The NU-6 test score for the worse ear obtained under headphones at the subject’s MCL did not have correlations as robust as that of the sound field NU-6 test. However, with 43% to 87% of the variance explained by the headphone NU-6 test, it performed as a good predictor of speech intelligibility in each of the experimental listening conditions.
EXPERIMENTAL LISTENING CONDITION SPEECH 717 817 717 Muffs 817 Muffs HEADPHONE Unoccluded Earmuffs Earmuffs + HAs + HAs PREDICTOR
NU-6 Better Ear 0.4952 0.5860 0.3244 0.8214 0.4772 70 dB SPL
NU-6 Worse Ear 0.4805 0.7395 0.4301 0.8685 0.4761 70 dB SPL
NU-6 Average Both Ears 0.4964 0.6745 0.3836 0.8607 0.4853 70 dB SPL
Table 4.4: Table shows correlations between headphone measures of speech intelligibility and the experimental listening conditions. Values shown are in units of R- squared. Bolding indicates the strongest correlation for the listening condition.
124 Two classes of HTL predictors, pure tone averages and single-frequencies were evaluated. No one pure tone average was more predictive than the others (Table 4.5).
However, the best single-frequency HTL predictor of protected speech intelligibility in noise, the HTL at 2000 Hz for the better ear, was the best single-frequency predictor evaluated for each of the experimental listening conditions (Table 4.5). While 2000 Hz for the better ear was strongly correlated with the earmuff alone conditions (R-squared
= 0.57 and 0.77 for the 817 and 717 earmuffs, respectively) and the aided-protected 717 earmuffs condition (R-squared = 0.68), it was not a significant predictor of the more audible unoccluded and aided-protected 817 earmuffs conditions.
125
EXPERIMENTAL LISTENING CONDITION HTL 717 817 717 Muffs 817 Muffs HEADPHONE Unoccluded Earmuffs Earmuffs + HAs + HAs PREDICTOR
Pure Tone
Averages .5,1,2 kHz Both Ears 0.3241 0.8456 0.8335 0.7620 0.5308
.5,1,2 kHz Right Ear 0.3455 0.8322 0.7889 0.8102 0.5641
.5,1,2 kHz Left Ear 0.2917 0.8248 0.8439 0.6876 0.4791
1,2,3,4 kHz Both Ears 0.4625 0.5330 0.3618 0.5336 0.2085
1,2,3,4 kHz Right Ear 0.4655 0.4630 0.2938 0.4755 0.1848
1,2,3,4 kHz Left Ear 0.4299 0.5967 0.4370 0.5799 0.2280
Single
Frequency 2000 Hz Both Ears 0.2299 0.7547 0.4973 0.6215 0.2545
2000 Hz Better Ear 0.2536 0.7672 0.5749 0.6785 0.3127
2000 Hz 0.1994 0.7090 0.4103 0.5445 0.1962 Worse Ear
Table 4.5: The relationship between selected HTLs obtained under headphones and the experimental listening conditions. Values shown are in units of R-squared. Bolding indicates the strongest correlation for the listening condition with that group of predictors.
126 A modified SII was calculated for each listening condition. Variables entered into the modified SII calculations were one-third octave band SPLs for speech and noise, HTLs for the better ear, earmuff-specific APVs for the protected conditions, and real ear hearing aid gain at 70 dB SPL for the aided-protected conditions. The one-third octave band SPLs were obtained with a custom MatLab program for the analysis of digitally recorded sound files. The sound files were recorded for speech alone and noise alone in the sound field used for the study. Hearing threshold levels for the better ear of each subject and real ear hearing aid gain levels (shown in Table 4.2) were entered into the spreadsheet for the one-third octave bands that were available.
Unavailable data was linearly interpolated from the adjacent data.
Significant positive relationships were found for the protected conditions (R- squared = 0.84 and 0.67 for the 717 and 817 earmuffs, respectively); however, the unoccluded and aided-protected listening conditions were not well-predicted by the modified SII. Additional correlations using the modified SII showed significant positive relationships to the QSIN scores for the 717 earmuffs to predict the aided 717 condition (R-squared = 0.80) and likewise with the 817 earmuffs to predict the aided
817 condition (R-squared = 0.43).
Of all the predictors evaluated, the sound field NU-6 test administered at 70 dB
SPL in the unoccluded condition was the best predictor of the experimental listening conditions’ QSIN scores. The best predictor among the headphone-administered tests evaluated was the NU-6 test administered at the listener’s MCL in the worse ear. If speech testing is unavailable, the HTL at 2000 Hz in the better ear was found to be the best predictor among the HTL measures. The HTL at 2000 Hz in the better ear was
127 correlated with the three most difficult experimental conditions. However, correlations were not significant for the more audible listening conditions. The modified SII requires measurement of several variables and was not predictive of the unoccluded listening condition for this sample of hearing-impaired subjects. Therefore, the modified SII may have limited utility in actual practice.
128
CHAPTER 5
DISCUSSION
5.1 Protected versus Aided-Protected Speech Intelligibility
The primary finding from the final study was that use of hearing aids worn in combination with the Bilsom 717 and 817 earmuffs provided better speech intelligibility for hearing-impaired listeners than wearing earmuffs alone. The use of hearing aids in combination with earmuffs at 90 dB SPL provided speech intelligibility similar to that of the unaided-unoccluded condition at 70 dB SPL. While the unaided- unoccluded condition at 90 dB SPL showed significantly better speech intelligibility than the aided-protected 90 dB SPL and unaided-unoccluded 70 dB SPL conditions, unoccluded listening at 90 dB SPL for an 8-hour work shift would be contraindicated due to the significant risk for acquiring noise-induced hearing loss. Therefore, in the absence of other technology which may enhance speech audibility under earmuffs, wearing hearing aids in combination with earmuffs should be explored for individual hearing-impaired workers who need improved speech intelligibility in the work environment.
The above findings were qualified by statistically significant interactions involving gender (gender by age and gender by listening condition). Upon further
129 analysis, it was discovered that the average HTLs from 250 Hz – 2000 Hz systematically varied as a function of gender. There was no reason to assume that females in the general population would have a greater hearing loss than males in the low to mid frequencies. However, this sample of hearing-impaired subjects was not a random sample. Since the sample was drawn from binaural hearing aid users, it is possible that the heterogeneity seen in the HTL data was a function of degree of hearing loss and gender in the decision to purchase binaural in-the-ear hearing aids. It is also possible that males who have a more handicapping hearing loss may perceive this as a threat to their employment status; and therefore, are more likely to try to hide the hearing loss rather than to purchase hearing aids. If gender is a factor under study, sampling strategies for this population should be random or based on equal percentages of hearing loss within each gender to reflect the population base of interest.
5.2 Conventional versus Uniformly-attenuating Earmuffs
The second hypothesis, that due to reduced audibility from hearing loss, hearing-impaired listeners would exhibit a significant difference in speech intelligibility between the two sets of earmuffs at a 90 dB SPL presentation level, was rejected.
While the trend of the data was consistent with the hypothesis and the results from the normal hearing group at 70 dB SPL, the 3-dB mean SNR50 difference between listening conditions was not statistically significant for this sample of 10 hearing-impaired subjects.
Often a small sample size can contribute to the lack of statistically significant findings since sample size influences the power of the test. However, the power of this
130 study’s test was excellent (0.997). This means that if a real difference between the earmuffs actually existed under these test conditions, 99.7 out of 100 groups of hearing- impaired listeners similar to the study group should show a significant difference with this test. In other words, less than one group out of 100 similar groups would fail to show the difference in speech intelligibility between the earmuffs if one existed.
A second possible explanation for this unexpected finding would be that the hearing loss characteristics of this hearing-impaired group produced a spurious sample rather than a representative sample of hearing-impaired listeners. The best hearing threshold predictor of QSIN performance in the earmuff conditions was the HTL at
2000 Hz in the better ear (r = 0.96 with 717 earmuffs; r = 0.89 with 817 earmuffs).
Figure 5.1 shows the QSIN scores in the unoccluded and earmuff conditions for the hearing-impaired subjects ordered by HTL at 2000 Hz. QSIN scores in the earmuff conditions were lower (indicating better speech intelligibility) for most subjects with lower HTLs and higher for the subjects with higher HTLs. However, this pattern was not as apparent in the unoccluded condition.
131 30
25 HI3
20
15
10
QSIN SNR50 (dB) 5 HI8
0
-5 25 35 35 45 45 50 60 60 65 65 HTL at 2 KHz in Better Ear (dB HL) Unoccluded 717 Muffs 817 Muffs
Figure 5.1: Individual speech intelligibility at 90 dB SPL with and without earmuffs as a function of HTL at 2 KHz.
Even though HTLs at 2000 Hz were strongly positively correlated with QSIN scores in the earmuff conditions, not all subjects showed this pattern. Two subjects,
HI3 and HI8, each had a 35 dB HL threshold at 2000 Hz, but performed very differently both in terms of overall QSIN score magnitude and specific earmuff performance. The complete audiogram for the better ear of each subject (Figure 5.2) shows that the higher scoring subject (HI3) had significantly more hearing loss in the low to mid frequencies
132 than the lower scoring subject (HI8). Conversely, subject HI3 had better thresholds at
3000 Hz and 4000 Hz than subject HI8. It may be possible that the greater low to mid frequency hearing loss adversely affected this subject’s ability to understand speech in noise under earmuffs. Additionally, the effects of the different attenuation characteristics of the earmuffs are manifest differently in these two subjects. Subject
HI3 performed better with the 717 earmuffs, which have less low frequency attenuation than the 817 earmuffs. Conversely, subject HI8, who had better low frequency hearing, performed better with the 817 earmuffs.
133 Frequency (Hz)
250 500 1000 2000 3000 4000 6000 8000 -10 0 10 20 30 40 50 60
HTL (dBHL) HTL 70 80 90 100 110 120
Better Ear HI3 Better Ear HI8
Figure 5.2: Better Ear Audiogram: Subject HI3 versus Subject HI8
Another possible explanation for the differences between subject HI3 and HI8’s
QSIN performances may be related to their central auditory processing abilities.
Unfortunately, this audiometric evaluation was not designed to account for central auditory processing capabilities of subjects. Subject HI3 volunteered that she has always had difficulty understanding the speech of unfamiliar people and speech in the presence of any other sounds. Interestingly, she is currently an experienced middle
134 school English teacher in the public schools, where understanding speech in noise is often a necessity. Although she has never been diagnosed with Central Auditory
Processing Disorder (CAPD), these symptoms would have warranted further testing for
CAPD if the subject had wanted to pursue a diagnosis.
A third possible explanation for not finding a significant difference between earmuffs was the large amount of variability inherent in the hearing-impaired group’s data. If the variability of the hearing-impaired group’s QSIN scores had been closer to the smaller, predicted value, the 3-dB effect size would have been statistically significant as well as clinically important.
Finally, the QSIN was administered at an equal SPL (90 dB SPL) rather than at a similar sensation level as recommended in the clinical protocol (Etymotic Research,
2001). The resulting inequalities in audibility between the hearing-impaired subjects may have confounded the data. Differences in audibility were present due to large differences in the HTLs and audiogram configurations of the subjects as well as in the transfer functions of the earmuffs. Confounding, due to multiple sources of audibility effects, may be better controlled in studies with groups of hearing-impaired subjects by more tightly constraining the HTLs as well as the audiometric configuration within each group.
5.3 Speech Intelligibility Predictions
The sound field NU-6 test administered at 70 dB SPL in the unoccluded condition was the best predictor of the experimental listening condition QSIN scores. If facilities are available for sound field testing, it would be better to compare different
135 HPDs for speech intelligibility performance in a given worker than to base decisions on one group of hearing-impaired listeners in one type of background noise at one level.
However, most hearing conservation audiometric test facilities are not large enough to implement sound field testing. If only headphone-administered testing is available, according to this research, the NU-6 test administered at the listener’s MCL in the worse ear would be the test of choice.
If speech testing is unavailable, the HTL at 2000 Hz in the better ear was found to be the best predictor among the HTL measures. However, individual audiometric configurations may affect speech intelligibility performance in unpredictable ways. For example, in this study the HTL at 2000 Hz in the better ear was correlated with the three most difficult experimental conditions. However, correlations were not significant for the more audible listening conditions.
This study found that the modified SII was a better predictor for protected listening conditions than the HTL at 2000 Hz in the better ear. For those hearing conservationists who have access to octave band or one-third octave band sound level survey data and cannot perform speech testing, the modified SII would be the best predictor to use.
5.4 Study Limitations
Results from this study should not be generalized to the hearing-impaired population at large since a random sampling technique was not employed. First, the convenience sample, which primarily came from the OSU Speech-Language-Hearing
Clinic, yielded a rather biased sample of hearing aids. Nearly all the hearing aids used
136 in the study were from one manufacturer with one type of signal processing. This may have reduced the variability in the aided listening conditions, which might have given a clearer picture of the contribution from the other effects on speech intelligibility. By essentially holding constant the type of amplification, the study failed to assess the effects of hearing aids in general.
Second, the fact that results from mild to moderately hearing-impaired subjects were less predictable than those of the more severely hearing-impaired subjects suggests that these two groups should have been analyzed separately. Unfortunately, there were not enough hearing-impaired subjects to subdivide into more tightly constrained hearing status groups. Additional diagnostic tests may have explained more of the variance within the hearing-impaired group since hearing impairment can be a result of a peripheral lesion, central lesion, or a lesion in both areas. This study’s test battery only differentiated between conductive and sensori-neural hearing losses.
Lastly, the earmuffs used in this study were chosen to represent a matched pair in all aspects except the attenuation transfer function. The Bilsom 817 earmuffs were the more uniformly-attenuating of the two earmuffs; however, results from this set of earmuffs cannot be generalized to all earmuffs described as being uniformly- attenuating. First, the 817 earmuffs are not perfectly flat in their attenuation transfer function. Secondly, other earmuffs may have more or less overall attenuation. Both of these factors can influence speech intelligibility in noise.
137 5.5 Conclusions
Normal hearing subjects did not experience handicapping amounts of decreased speech intelligibility as a result of wearing earmuffs in the experimental test conditions.
Hearing-impaired subjects clearly were handicapped in terms of speech intelligibility as a result of the combination of their hearing loss and wearing earmuffs in the experimental test conditions. Additionally, safe levels of amplification applied to the passively-attenuated sounds significantly improved speech intelligibility in the hearing- impaired subjects.
5.6 Future Research
This study has shown that speech intelligibility in noise in hearing-impaired listeners can be significantly degraded by both hearing loss and earmuff attenuation.
Also, the results from this study showed significant improvement in speech intelligibility with the use of hearing aids worn in combination with earmuffs. Given this information, future research should quantify the effects on speech intelligibility of
HPDs which incorporate amplification and/or radio transceivers.
Mild to moderately hearing-impaired subjects in this study showed greater variability than either the more severely hearing-impaired subjects or the normal hearing subjects. Future research should subdivide hearing-impaired listeners into two or more subcategories based on the degree of the hearing loss and configuration of the audiogram. Both of these factors may interact with the various attenuation transfer functions of the experimental HPDs. By more narrowly confining the audiometric
138 classification, the variability in the data should better reflect the effects of the HPD under experimentation.
To more accurately describe the etiology of the hearing loss, researchers would do well to incorporate some measure of central auditory processing. Do some hearing- impaired people show evidence of central auditory processing disorders which correlate with their reduced speech intelligibility in noise? Additionally, it would be interesting to explore whether the active mechanism in the inner ear is related to differences in speech intelligibility between listeners with similar degrees and configuration of hearing loss. As an indicator of the active mechanism in the inner ear, distortion product otoacoustic emissions (DPOAE) have been shown to be highly correlated with the presence or absence of normal hearing; however, DPOAE test results do not always separate a normal hearing ear from one with a mild hearing impairment (Dorn,
Piskorski, Gorga, Neely, & Keefe, 1999; Kummer, Janssen, & Arnold, 1998). Does the
DPOAE gram correlate with speech intelligibility in noise measures for mildly hearing- impaired listeners?
Although it is possible to compare HPDs in the laboratory in terms of speech intelligibility effects, very little has been done to validate the findings in the actual acoustic environment in which the HPDs are worn. One elusive piece of the puzzle is the speech-to-noise ratio at the worker’s ear. Field studies which quantify the speech- to-noise ratio for a given worker are needed. Speech levels at the listener’s ear will likely vary depending on talker distance and orientation. Therefore, calibrated digital recordings of an entire work shift should be made in order to obtain the most accurate representation of speech-to-noise ratios for a given worker. Is there a typical range of
139 speech-to-noise ratios that provide adequate communication for normal hearing workers in a given job? Does this range also accommodate the hearing-impaired worker in that job?
Some jobs are more communication-dependent than other jobs. A standardized way of evaluating the communication requirements for successful and safe completion of job duties needs to be developed. These ratings could then be used to justify the need for more technologically-advanced HPDs for workers in that job.
The effects of HPDs on speech intelligibility in noise for hearing-impaired listeners are significantly more handicapping than for normal hearing listeners. The effects of hearing loss on speech intelligibility in noise have been shown to vary among listeners with similar hearing losses. Future research and hearing conservation efforts with hearing-impaired workers need to obtain site-specific sound levels and octave- band measurements, listener-specific HTLs and speech intelligibility test data
(preferably in a sound field), and HPD-specific attenuation data in order to accurately predict speech intelligibility in noise for a given worker.
140
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148
APPENDIX A
TEXT OF DIRECT MAIL SUBJECT RECRUITMENT LETTER
Dear ______:
This letter is to let you know that according to our records, you may qualify for a study we are conducting with hearing aid clients.
The purpose of the study is to see if wearing hearing aids under certain earmuffs would allow a hearing-impaired worker to hear better at a noisy work site while not further damaging his or her hearing. If you elect to participate, you will be paid $20.00 for the 1 ½ - 2 hour test session. The study will take place at the Ohio State University Speech- Language-Hearing Clinic in Pressey Hall (141 Pressey 1070 Carmack Road Columbus, OH 43210).
Please call Shannon Hand at (614) 292 – 6251 (OSU Speech-Language-Hearing Clinic) to add your name to our list of subjects. One of our doctoral candidates will call you to set up an appointment and to answer any questions you may have.
We look forward to hearing from you!
Sincerely,
Lawrence L. Feth, Ph.D. Professor, OSU Speech & Hearing Science Department
Gail Whitelaw, Ph.D. Director, OSU Speech-Language-Hearing Clinic
149
APPENDIX B
SUBJECT RECRUITMENT NEWSPAPER ADVERTISEMENT (ENLARGED)
150
APPENDIX C
MEDICAL SCREENING QUESTIONS
1. Are you currently taking medication for an ear infection? YES NO
2. Do you have any ear pain or fullness today? YES NO
3. Have you noticed any change in your hearing since your
last hearing test? YES NO
151
APPENDIX D
SUBJECT CONSENT FORM
152 The Ohio State University Protocol #83B0171
Subject Consent Form
Department of Speech and Hearing Science
Project name: “Hearing aids worn in combination with earmuffs: Effects on speech intelligibility”
I, ______, hereby consent to participate in this speech intelligibility study being conducted by Babette L. Verbsky.
During this study I will receive a standard clinical audiometric evaluation and hearing aid check to assure that I currently meet the subject selection criteria for this study. In the experimental testing, I will be asked to listen to sentences in a background of speech and to repeat back what I hear. I will also be asked to wear non-electronic, commercially available earmuffs during some of the listening conditions.
There is minimum risk of physical harm to any subject. No deception is involved in the experiment, and there is nothing objectionable or questionable about any of the stimuli being used. There are no known risks or discomforts associated with any of the experimental procedures.
I understand that any inquiries that I may make concerning the procedure described above will be answered, and I understand that I am free to withdraw consent to participate in the project at any time without prejudicing myself.
I have read and fully understand the consent form. I have signed it freely and voluntarily and understand a copy will be made available to me upon request.
Date:______
Signed:______
I certify that I will comply with each of the stated requirements in this consent form and that no discomforts or risks are associated with the procedures to be conducted.
Signed:______
153
APPENDIX E
NORMAL HEARING SUBJECTS’ HEARING THRESHOLD LEVELS
FREQUENCY (HZ) ID EAR 250 500 1000 2000 3000 4000 6000 8000 R 0 05-550 5-5 SN1 L -5 0 5 5 10 0 5 0 R 0 0 0 0 5 10 20 -5 SN2 L 0 0 0 -5 5 10 15 5 R 0 0 5 -5 0 -5 10 5 SN3 L 5 0 0 0 -5 -10 10 5 R 5 10 5 10 0 10 5 0 SN4 L 10 10 0 -5 10 5 10 10 R -5 0 0 5 -5 -5 0 0 SN5 L -5 00550 1010 R 5 105505 55 SN6 L 10 10 5 10 5 10 0 10 R 5 0 10 0 10 10 5 5 SN7 L 0 0 5 10 0 10 10 10 R 10 5 0 5 -10 -10 5 -5 SN8 L 5 0 0 0 0 -5 15 0 R 0 5 5 5 0 10 -5 5 SN9 L 0 5 5 10 5 10 5 10 R 0 5 5 10 15 15 15 5 SN10 L 0 5 5 10 15 15 10 15
154
APPENDIX F
HEARING-IMPAIRED SUBJECTS’ HEARING THRESHOLD LEVELS
FREQUENCY (HZ) ID EAR 250 500 1000 2000 3000 4000 6000 8000 R 40 40 40 50 65 70 70 60 HI1 L 40 30 40 45 65 65 60 65 R 25 40 55 65 60 65 80 75 HI2 L 30 35 50 60 60 70 80 85 R 50 45 45 35 35 45 70 65 HI3 L 40 45 45 35 40 55 75 70 R 40 45 60 65 85 110 115 100 HI4 L 35 45 65 70 70 80 105 100 R 25 50 65 65 55 60 30 60 HI5 L 25 45 60 65 50 60 45 70 R 15 25 50 60 75 70 55 70 HI6 L 15 30 55 60 65 75 60 55 R 35 50 50 55 40 20 10 5 HI7 L 35 60 60 50 45 35 15 15 R 10 10 35 35 60 60 60 45 HI8 L 5 10 25 35 45 65 50 60 R 10 15 15 25 45 65 95 90 HI9 L 10 20 20 25 55 80 80 85 R 20 15 25 60 52 45 52 60 HI10 L 25 25 20 45 50 55 60 65
155
APPENDIX G
QUICK SIN TEST SCORES (SNR50 in dB) BY LISTENING CONDITION
156
Speech Level = 70 dB SPL Speech Level = 90 dB SPL ID 717 817 717 817 Unoccluded Unoccluded Earmuffs Earmuffs Earmuffs Earmuffs SN1 0.00 3.75 -1.00 -1.00 -2.00 -1.00 SN2 -2.00 1.00 -0.50 0.50 -1.00 -2.00 SN3 -1.00 3.50 0.50 0.50 -0.50 -1.50 SN4 0.50 3.00 -0.50 1.50 1.00 0.50 SN5 -1.50 3.00 3.50 -1.50 -1.50 -1.00 SN6 -1.50 3.00 2.00 0.50 -0.50 -1.50 SN7 -2.50 -1.00 0.25 -3.00 -3.00 -3.00 SN8 0.00 1.75 3.00 -0.50 2.00 0.50 SN9 -1.00 0.00 2.00 1.00 0.50 -0.50 SN10 -1.50 4.50 -1.00 1.00 -1.00 -2.50
Speech Level = 70 Speech Level = 90 dB SPL dB SPL ID 717 817 HA + HA + Unaided Aided Unoccluded Earmuffs Earmuffs 717s 817s HI1 4.75 5 7.50 20.00 8.50 18.00 12.00 HI2 13.5 4.5 5.00 21.50 22.50 19.00 9.50 HI3 3 2 3.50 17.00 23.50 8.00 8.00 HI4 25.25 8 11.50 25.50 25.50 18.50 13.00 HI5 23.75 5.5 8.50 24.50 25.50 22.50 20.50 HI6 11.75 3 1.00 24.00 21.50 12.50 4.00 HI7 19.25 9.5 2.50 20.50 20.00 14.50 11.00 HI8 0.5 0.5 1.50 3.50 2.00 3.50 4.00 HI9 4 1.5 3.50 6.50 2.50 5.00 5.50 HI10 4.25 2.5 1.50 15.50 7.00 5.50 2.50
157
APPENDIX H
HEARING-IMPAIRED SUBJECTS’ DATA USED FOR CORRELATION WITH
QSIN SCORES IN EXPERIMENTAL LISTENING CONDITIONS
Better Worse Speech Intelligibility Index Ear Ear 2000 NU-6 NU-6 ID HA + HA + Hz Head Sound Unoccluded 717s 817s 717s 817s HTL Phone Field (dBHL) (%) (%) No HI1 45 56 Test 0.2583 0.2322 0.1685 0.2372 0.2545 HI2 60 56 48 0.2453 0.1560 0.1191 0.26550.2830 HI3 35 84 74 0.2583 0.2429 0.2243 0.20840.2430 HI4 65 48 0 0.2315 0.1024 0.0973 0.25320.2520 HI5 65 58 8 0.2610 0.1166 0.1005 0.26410.2707 HI6 60 68 40 0.2482 0.1560 0.1191 0.25470.2836 HI7 50 60 34 0.2632 0.1988 0.1421 0.24330.2508 HI8 35 88 100 0.2610 0.2634 0.2445 0.28560.2751 CNT CNT HI9 25 92 92 0.2439 0.2634 0.2544 HA HA CNT CNT HI10 45 80 88 0.2583 0.2498 0.2087 HA HA
CNT HA = ‘Could Not Test Hearing Aids’ due to acoustic feedback with the real ear probe microphone in place.
158
APPENDIX I
DATA RECORDING FORM
159 SUBJECT ID ______
STUDY TITLE: ______Hearing Aids Worn in Combination with Earmuffs: Effects on Speech Intelligibility
P.I.: ______Babette L. Verbsky, M.S., CCC-A EXAMINER ______Babette L. Verbsky, M.S., CCC-A
_____1. Currently wears binaural ITE hearing aids HA Make/Model ______
fit between 1 and 5 years ago. Fitting Date ______2. Age 21 - 65 years. Date of Birth ______Age______3. Native English speaker.
_____4. Bilateral sensori-neural hearing loss. HTL @ 2 kHz = Right______Left______5. Hearing threshold > 25 dBHL at 2 kHz. Use gain at: 250 500 1k 2k 4k 8k _____6. No gain >: Right HA ______250 Hz 500 Hz 1 kHz 2 kHz 4 kHz 8 kHz 16.5 21.5 26.1 41.6 60.9 61.3 Left HA ______
CONTACT INFORMATION:
NAME ______
ADDRESS ______
CITY, STATE ZIP ______
APPOINTMENT INFORMATION: 1. Are you currently taking medication for an ear infection?
YES NO DATE OF CALL ______
2. Do you have any ear pain or fullness today? APPOINTMENT DATE ______
YES NO APPOINTMENT TIME ______
3. Have you noticed any change in your hearing since your PHONE NUMBER ______last hearing test? YES NO
160 APPENDIX I (continued)
SUBJECT ID ______
TEST DATE ______
TEST TIME ______
A) Otoscopic Exam:
Right Ear unremarkable excess cerumen other ______
Left Ear unremarkable excess cerumen other ______
B) Immittance Testing:
TYMPANOMETER ______PROBE SIZE ______
Tympanometry Acoustic Reflexes
Right Ear Left Ear Probe IPSI Reflex CONTRA Reflex Volume (ml) ______Ear 500 1K 500 1K Peak Pressure (daPa) ______R Compliance (ml) ______L
C) Pure Tone Audiometry: AUDIOMETER / HEADPHONE ______
Frequency (Hz) RELIABILITY: GOOD FAIR POOR 125 250 500 1000 2000 4000 8000 -20 -10 R L 0 10 HTL @ 2 kHz = ______(> 25 dBHL) 20 30 40 50 KEY 60 AIR MASKED Right Ear - O 70 Left Ear - X 80 BONEMASKED HearingThreshold (dBHL) 90 Right Ear - < 100 Left Ear - > 110 120
161 APPENDIX I (continued)
SUBJECT ID______
D) Speech Audiometry (Unaided): Headphones Soundfield CD 101R2: Cal. Tone - Track 1 QSIN CD: Cal. Tone - Track 1 NU-6 Word List NU-6 Word List Test SRT (Level = 70 dBSPL) Tracks 15, 16, 17, 18 (Set #7) PTA (Tracks 6 - 8 Lt) Ear .512 Track []2Lt List dBHL % List dBHL % Quick SIN Test SNR-50 R 66 (Level = 70 dBSPL)
L 66
A) Listening Check: B) 2 cc Coupler Tests: Right HA: clean distorted Check when tests completed. Attach data printout. Left HA: clean distorted 1. Use Volume
Notes______Right HA _____ Left HA _____ 2. Full On Volume ______Right HA _____ Left HA _____
C) Real Ear Analysis: D) Speech Audiometry (Aided): Check when tests completed. QSIN CD Attach data printout. Track # _____ Set # _____ 1. Use Volume Quick SIN Test R _____ L _____ SNR-50 2. Full On Volume (Level = 70 dBSPL) R _____ L _____
A) Speech Intelligibility Testing: Speech SPL = 90 dBSPL
Unoccluded CONDITION # _____ 717 + HAs CONDITION # _____ QSIN CD: Track # _____ Set # _____ QSIN CD: Track # _____ Set # _____ QSIN Score = ______QSIN Score = ______
717 Muffs CONDITION # _____ 817 + HAs CONDITION # _____ QSIN CD: Track # _____ Set # _____ QSIN CD: Track # _____ Set # _____ QSIN Score = ______QSIN Score = ______
817 Muffs CONDITION # _____ QSIN CD: Track # _____ Set # _____ QSIN Score = ______
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