TRANSIENT EVOKED OTOACOUSTIC EMlSSlONS ELlClTED BY BONE-CONDUCTED ULTRASONIC STIMULI
Nicolae Schiopu
A thesis submitted in conformity with the requirements for the degree of Master of Applied Science Graduate Department of Electncal and Cornputer Engineering University of Toronto
O Copyright by Nicolae Schiopu (1997) National Library Bibliothèque nationale 1*1 of Canada du Canada Acquisitions and Acquisitions et Bibliographie Services services bibliographiques 395 Wellington Street 395. rue Wellington OttawaON KlAON4 Ottawa ON KIA ON4 Canada Canada
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The author retains ownership of the L'auteur conserve la propriété du copyright in this thesis. Neither the droit d'auteur qui protège cette thèse. thesis nor substantial extracts fkom it Ni la thèse ni des extraits substantiels may be printed or othexwise de celle-ci ne doivent être imprimés reproduced without the author's ou autrement reproduits sans son pemission. autorisation. TRANSIENT EVOKED OTOACOUSTIC EMISSlONS
ELlClTED BY BONE-CONDUCTED
ULTRASONIC STIMULI
Master of Applied Science 1997
Nicolae Schiopu
Graduate Oepartment of Electrical and Computer Engineering
and the lnstitute of Biomedical Engineering
University of Toronto
ABSTRACT
The ear's ability to generate sounds was first postulated 50 years ago. Twenty years ago
Otoacoustic Ernissions (OAEs), as these signals are now being called, were recorded using a sensitive microphone inserted in the ear canal. Due to their clinical investigation potential, growing attention has been given to Otoacoustic Emissions elicited by tones or clicks (TEOAEs).
The phenomenon of ultrasonic hearing is even more intriguing. When signais are presented via bone conduction, normal hearing subjects can detect tones as high in frequency as
100 kHz. Significant evidence indicate that the cochlea is the most probable candidate as an ultrasonic receiver.
We postulated that it was possible to elicit TEOAEs with ultrasonic stimulation. To study this, we developed new equipment and a modified investigation technique. We elicited and recorded TEOAEs using 40 kHz, 1 ms long stimuti and studied their main features and the rnethod's potential as a hearing investigation tool. Acknowledgments
I would first like to thank Dr. Kunov for the opportunity to work with him in the field of Biomedical Acoustics and enhance my academic and technical skills. I will always regard the time spent with the Institute of Biomedical Engineering as the most valuable learning experience of my life. Working with Prof. Poul B. Madsen and Poul Madsen Medical Devices was a privilege and a chance to gain knowledge and receive advice from one of the world's most experienced and qualified person in that field. Drs. Yuri and Lena Sokolov have helped me with refining my experimental method and shared with me their expertise in audiology. Many thanks to the other students in the lab: Nathanael, George, Tony, Peter and David have al1 been good colleagues and friends. They supported my work at one stage or another and helped me with preparing rny presentations and my thesis. I will miss their jokes and the family atmosphere they created in the lab. Thanks to the Professorial and Administrative staff at the IBME. I've corne to regard the lnstitute as rny home over the last two years. Franz Schuh's assistance on equiprnent development has been essential to rny thesis' successful completion. Last. but not least, a kind thought to my teachers from the University of Bucharest, Romania. Their professionalism and dedication provided me with the knowledge and skills required to succeed in such a competitive environment as the Canadian Biomedical Engineering Research. Nihil Sine Deo is my life's motto and 1 thank God for giving me the strength to overcome the inherent difficulties encountered in my quest for personal enhancement and for having such supportive farnily and friends.
iii Contents
.. ABSTRACT ...... 11
CONTENTS...... iv LIST OF FIGURES ...... vi LIST OF TERMS AND ABBREVIATIONS...... O...... vii
CHAPTER 1 INTRODUCTION ...... 1 1 .1 . TRANSIENT-EVOKEDOTOACOUST~C EMISSIONS AND THEIR CLINICALSIGNIFICANCE ...... I 1 .2. ULTRASONICHEARMG ...... 1 1.3. THEHYPOTHESES OF THE STUDY 'TEOAES ELICITEDBY BONE-CONDUCTEDULTRASONIC STIMULATION'...... -7 1.4. RESEARCHOBJECTIVES ...... ,7 CHAPTER 2 BACKGROUND ...... 3 2.1. HUMANEAR AND AUDITORYPHYSIOLOGY ...... 3 2.1.1. The Outer Ear ...... 4 2.1.2. The Middle Ear ...... 4 2.1.3. The Inner Ear ...... 4 2.2. AIRBORNE-ELICITEDTEOAE MECHANISMAND CHARACTERISTICS...... 6 2.2.1. The Outer Hait Cells as a Source for TEOAEs ...... 6 2.2.2. Clinical Significance of TEOAEs ...... 7 2.2.3. TEOAE Recording Technique ...... 7 2.2.4. Stimulus Versus TEOAE Frequency Spectrum ...... 8 2.1.5. Nonlinear Growth of the Amplitude of TEOAEs ...... 1 O 2.2.6. Latency of TEOAEs ...... 1O 2.2.7. Intersubject Variability and TestRetest Repeatability of TEOAE Recordings ...... 1 O 2.2.8. TEOAE Amplitude and Subject's Posture ...... I 1 2.3. TEOAES ELICITEDBY BONE-CONDUCTEDSTIMULI: PARTICULAR ASPECTS ...... 1 1 2.3.1. The Middle Ear's Role in Bone-Elicited TEOAE Measurement ...... 12 2.3 .2. Bone-Conducted Stimulus and TEOAE Characteristics ...... I 2 2.4. B~OLOG~CALEFFECTS OF ULTRASOUND...... 1 2 2.4.1. Airborne Ultrasound: Biological Effects and Exposure Standards...... 13 2.4.2. Contact Ultrasound: Bioiogical Effects and Exposure Limits ...... 14 2.5. ULTUSONIC HEARING...... 1 5 2.5.1. Bone-Conducted Ultrasonic Stimuli in Clinical Auditory Investigation ...... 15 2.5.2. Bone Conduction Sonic and Ultrasonic Hearing Thresholds...... 17 2.5.3. Signal Attenuation in the Bone and Tissue ...... 18 2.5.4. Hypotheses on the Mechanism of Ultrasonic Hearing; the Cochlea as an Ultrasonic Receiver ...... -...-..---...... 19 CHAPTER 3 TECHNICAL ASPECTS OF ULTRASOMC BONE-ELICITED TEOAEk AND EQUIPMENT DESIGN...... 25 3.1. THEPIEZOELECTMC BONECONDUCTION TFWNSDUCER: CONSTRUCTION AND FREQUENCY RESPONSE...... ,...... 25 3.1.1. Construction ...... ---...... 25 3.1.2. Frequency Response ...... 26 3.2. HEADBAND...... 28 3.3. ACCELERATIONLEVELS FOR B.C. HEARMGiN THE ULTRASONICRANGE: PIEZOELECTRIC BONECONDUCTOR OUTPUT LEVEL...... 39 3.4. CALCULATIONOF ULTRASOUNDEXPOSURE LIMIT ...... 32 3 .5 . EQUIPMENTDESIGN ...... 33 3.5.1. Signal Conditioning Board ...... 33 3.5.2. High Voltage Amplifiers for Piezoelectric Bone Conductors ...... 34 3 .5.3. Bridge Amplifier ~...... 36 3.5.4. High Voitage Piezoelectric Driver ...... ,...... 38 CHAPTER 4 EXPERIMENTAL RESULTS ...... 40 4.1 . EXPERIMENTAL SETUP ...... ~...... 40 4.1.1. Equipment List ...... ,...... 40 4.2. TESTPROCEDURE ...... -...... 4 1 4.3. SUBJECTS...... 43 4.4. TEOAE MEASUREMENTRESULTS ...... 43 4.4.1 . Skuil Vibratory Pattern ...... 44 4.4.2. Bone Conductor Positioning and Contact Force Influence on TEOAE Detection ...... 45 4.4.3. Components of an Ultrasonic Bone-Conduction Stimutated TEOAE Recording ...... 46 4.4.4. Ultrasonic Bone-Stimulated TEOAE Versus Stimulus Amplitude ...... 47 4.4.5 . UItrasonic Bone- Stimulated TEOAE Frequency Spectrum ...... 49 4.4.6. Left / Right Ear Variations of TEOAE Recordings ...... 49 4.4.7. Intersubject Variation of UItrasonic Bone-Conduction Stimulated TEOAEs ...... 51 4.4.8. Subject Posture Effect on TEOAE Measurement...... 51 4.5. STIMULUSARTIFACT ...... 54 CHAPTEX 5 CONCLUSIONS ...... 57 5.1. FEATURESOF ULTRASONICBONE-CONDUCTION STIMULATED TEOAES ...... 57 5.2. PROSPECTSFOR THE BONE-CONDUCTIONSTIMULATED TEOAE METHODAS AN AUDIOLOGICAL ASSESSMENT TOOL...... 58 List of Figures
Fig . 2.1 The Organ of Hearing...... 3 Fig . 2.2 Cross-section of the Cochlea and The Organ of Corti ...... 5 Fig. 2.3 TEOAE Investigation Instrument. the AAS9000 ...... 8 Fig. 2.4 Recording of an Air Conduction Click-Stimulated TEOAE ...... 9 Fig. 2.5 Limits for Human Exposure to High Frequency Airborne Sound ...... 14 Fig. 2.6 Recommended Maximum Permissible Contact Exposure Levels for Ultrasound...... 15 Fig. 2.7 Threshold of Hearing for Sonic and Ultrasonic Bone-Conducted Signals...... -18 Fig . 2.8 Human Skull Attenuation for Sonic and Ultrasonic Signals ...... 19 Fig. 2.9 Outer Hair Cell Non-linear Model...... 23 Fig. 3.1 Piezoelectric transducer structure and electrical wiring ...... 26 Fig . 3.2 Bone Conductor Frequency Response Measurement Setup ...... -27 Fig. 3.3 Frequency Response of Piezoelectric Bone Conductors ...... 28 Fig. 3.4 Headband with Piezoelectric Bone Conductor ...... 29 Fig. 3.5 Signal Conditioning Board ...... -35 Fig . 3.6 Bridge Amplifier and Output Monitor Circuit...... 37 Fig. 3.7 High Voltage Piezoelectric Driver ...... 39 Fig. 4.1 Experirnental Setup ...... -42 Fig . 4.2 Skull Vibratory Pattern to a 4OkHz, 1 ms Stimulus ...... 45 Fig. 4.3 Components of a B.C. Ultrasonic Stimulus TEOAE Recording...... 46 Fig . 4.4 Stimulus Amplitude Influence on TEOAE ...... -48 Fig. 4.5 Left I Right Ear TEOAE Variation in a Subject ...... 50 Fig . 4.6 TEOAE lntersubject Variation ...... -52 Fig . 4.7 Posture Influence on TEOAE ...... 53 Fig . 4.8 Airborne Click and Ultrasonic Bone-conducted Stimulus Artifact ...... -55 Fig. 4.9 Typical B.C. Ultrasonic Stimulus TEOAE Recording ...... 55 List of Terms and Abbreviations
ACTEOAE Air Conduction Transient Evoked Emission AK Audiometric Keyboard B&K Bruel 8 Kjaer BC Bone Conductor or Bone Conducted BCOAE Bone Conduction Transient Evoked Emission dbc Direct Bone Conduction DIFF. OUT Differential Output FFT Fourier Frequency Transform HL Hearing Level IHC lnner Hair Cell NRC National Research Council! (of Canada) OAE Otoacoustic Emission OHC Outer Hair Cell PRU Patient Roorn Unit Prr PiezoeIectric Transducer RETAL Reference Equivalent Threshold Acceleration Level RETFL Reference Eq uivalent Th res hold Force Level SPL Sound Pressure Level TEOAE Transient Evoked Otoacoustic Emission WIPI Word lntelligibility by Picture Identification
vii Chapter 1
Introduction
1A. Transient-Evoked Otoacoustic Emissions and their Clinical Significance Otoacoustic Ernissions are a fom of energy leakage from the cochlea through the middle ear to the ear canal during the active process of stimulus processing by the outer hair cells of the cochlea (Kemp, 1986). Transient Evoked Otoacoustic Emissions are cochlear responses recorded when the subject is presented with short clicks or tone bursts of moderate intensity (Kemp, 1978). The frequency spectrum of the response is dispersed, presenting a peak centered on the frequency of the stimulus in the case of tone burst excitation. In the case of click-evoked emissions, high frequency components dominate at the start of the response, about 3-5 ms post- stimulus time, followed by lower-frequency components at later stages (2urek, 1985). Strong Otoacoustic Emissions are considered to be indicative of a healthy cochlea. Some studies (Rossi et al, 1988; Collet, 1989) have proven the validity of bone conduction stimulated Otoacoustic Emissions. They present general characteristics similar to air-elicited OAEs with an occasional longer stimulus artifact and increased duration of the OAEs.
1.2. Ultrasonic Hearing When presented with bone-conducted signals in the near-ultrasonic range (20-100 kHz), human subjects report perceiving a tone with a pitch similar to a 10-13 kHz airborne sound, of varying intensrty as we change the frequency and/or the ampllude of the stimulus. Ultrasonic thresholds of heanng are higher in patients with sensonneural heanng loss than in normal hearing people (Abramovich, 1978). The phenornenon of ultrasonic hearing has not been elucidated so far but reasonabie evidence indicates a cochlear decoding rnechanisrn (Abrarnovich, 1978; Lenhardt. 1991; Fritze, 1996).
1.3. The Hypotheses of the Study 'TEOAEs Elicited by BoneConducted Ultrasonic Stimulation' Based on the two auditory phenornena previously mentioned (Otoacoustic Emissions and Ultrasonic Hearing) we suggested the following hypotheses: 1) Assuming that uitrasonic perception is a cochlear process. ultrasonic stimuli should elicit OAEs too. 2) Recording TEOAEs elicited by bone-conducted short ultrasonic tone bursts is possible. 3) These cochlear responses should present similar characteristics to audio-range boneconducted TEOAEs regarding the stimulus artifact. amplitude and duration of the response.
1.4. Research Objectives In order to investigate the hypotheses stated, two main research objectives have been pursued: 1) To design and build suitable equipment for stimulating and recording TEOAEs elicited by bone-conducted ultrasound. 2) To record ultrasonic bone-condudon stimulated TEOAEs and to observe their main features and distinctive properties. Chapter 2
Background
2.1. Human Ear and Auditory Physiology The sounds in the environment are acoustically collected by the outer ear, mechanically filtered and arnplified by the middle ear and transfomed into neural pulses in the inner ear (Fig. 2.1).
Outer Ear Middle Ear lnner Ear I Ti 1 I Temporal m banc External Semicircular canals \ cyai 1 A hlwindow
\ Meatus
Fig. 2.1 The Organ of Hearing 2.1 -1. The Outer Ear The outer ear consists of the pinna with the concha and the external auditory meatus and the external auditory canal (Fig. 2.1). The pinna, formed prirnarily of cartilage, aids in localking sounds by directing them toward the external auditory canal and protects the middle and inner ear (Yost. 1994). The outer ear amplifies signals in the frequency range of 1.5 to 7 kHz by about 10 to 20 dB, mainly due to the resonant frequencies of the concha (5 kHz) and the ear canal (2.5 kHz). It also protects the eardrum against foreign bodies and changes in humidity and temperature.
2.1.2. The Middle Ear The middle ear (Fig. 2.1) consists of the ossicular chain (malleus, incus and stapes) and its ligaments and muscles (the tensor tympani and the stapedial muscles) and the eustachian tube which connects the middle ear cavity to the nasophavnx (the nose cavity) for pressure equalization and ventilation. When excited by sounds traveling through the outer ear, the eardrurn vibrates and sets in motion the ossicular chain. Due to its physical structure, the ossicular chain has an amplification factor of up to 27 dB depending on the frequency and the intensity of the signal. The stapes is embedded in the oval window membrane and delivers piston-like amplified and filtered signals to the inner ear.
2.1.3. The lnner Ear The inner ear (Fig. 2.1) consists of the semicircular canals, the vestibule and the cochlea, which are al1 located in the temporal bone of the skull. The semicircular canals and the vestibule are parts of the balance control system of the body. The cochlea, a spiral-coiled organ. is the site where acoustic signals are transfonned from mechanical oscillations into neural pulses. A cross-section of the cochlea (Fig. 2.2) reveals the three fluid-filled cochlear ducts: the scala vestibuii, scala media and scala tympani. The organ of Corti lies on the basilar membrane in the scala media.
R«ssn«'s membrane
Tedorial Retiwler membrane Tedonal \ tamina
Fig. 2.2 Cross-section of the Cochlea and The Organ of Corti
It consists of inner (1 HC) and outer hair cells (OHC),suppotting cells. and the tectorial membrane. The basilar membrane is wider and under no tension at the apical end. The basal end is narrower and stiffer and may be under a small amount of tension. The oscillations received from the stapes through the oval window propagate along the basilar membrane's entire length in the fon of a traveling wave. The natural frequency of maximum displacement of the basilar membrane decreases from the base to the apex. A complex signal will be decomposed into different points of maximal displacement; the basilar membrane processes signals like a series of bandpass filters. The shear movement between the free-standing inner hair cells and the tectorial membrane gives rise to the IHCs' excitation which further elicits neural pulses of the neurons of the cochlear nerve. Then neural signals are processed by the central auditory nervous system including brainstem structures and the auditory cortex. The outer hair cells embedded in the tectorial membrane are actively changing their length and mechanical characteristics (Norton, 1992) for enhanced signal detection and sharp frequency discrimination by potentiating the sensitivity of IHCs.
2.2. Airborne-Elicited TEOAE Mechanism and Characteristics The conventional method of eliciting TEOAEs using airborne stimuli was used as a benchmark against which our results were compared. Therefore, a thorough description of this method, the underlying mechanisms and the main characteristics of the responses will be given in the following subsections.
2.2.1. The Outer Hair Cells as a Source for TEOAEs The remarkable fine-tuning frequency resolution of the organ of hearing is not based solely on the resonant characteristics of the basilar membrane. Therefore, the hypothesis of a 'second filter' (Evans, 1970) was considered. An active intracochlear mechanical process (Davis, 1981) would further increase the 'resonance' characteristics of the oscillations produced by sound in the basilar membrane. Gold (1948) was the first theorist who has hypothesized the existence of such an active intracochlear process as well as sound generation in the ear canal due to subsequent energy leakage. The outer hair cells have been shown to posses stimulus-induced contractile properties (Brownell et al, 1985) resembling excitation-contraction coupling in muscle cells. This may be due to the fact that OHC stereocilia contain actin and fibrin (Flock, 1980) and the OHC cellular membrane contains actin and myosin which are protein substances involved in the muscle contraction rnechanism (Rossi et al, 1988). Since the longest OHC stereocilia are ernbedded in the tectorial membrane, they can actively modify the mechanical properties of the tectorial membrane during acoustic stimulation (Zenner, 1988). Recently it was found that OHCs feature electromotility, the capacity to change shape in response to electrical stimulation (Dallos et al, 1993). Therefore, it seems reasonable to assume that energy leakage from the cochlea giving rise to acoustic signais in the ear canal may be due to the OHC contractile properties.
2.2.2. Clinical Significance of TEOAEs
TEOAE detection is considered to be a fast screening method for peripheral hearing loss detection. Over 98% of normal hearing subjects exhibit some degree of TEOAEs (Probst et al, 1991; Lichtenstein et al, 1996). TEOAEs can play an important role in clinical investigation of the status of the cochlea, but cannot measure low levels of hearing loss. TEOAEs are usually absent in subjects with mild (above 30 dB HL) or greater hearing loss (Kemp, 1978). New studies (Lichtenstein, 1996) show that frequency-band analysis of TEOAE recordings can detect hearing loss at 500, 1000, 2000 and 4000 Hz at levels up to 30 dB SPL. This screening method can be used for fast peripheral hearing loss detection in a large number of subject groups such as newborns, school children, high-risk factory workers, the military, and train conductors.
2.2.3. TEOAE Recording Technique
We used a cornputer-based Audiological Assessment System (AAS9000) currently under development in the lnstitute as an investigation tool (Kunov et al, 1997). Using a probe, clicks or tone bursts are generated in the subject's ear canal and OAEs are recorded using a sensitive microphone in the probe and stored in memory buffers for signal processing. The AAS9000 instrument (Fig. 2.3) physically consists of three main parts: a Pentium computer with a rnonitor, an Audiometric Keyboard (AK) and a Patient Room Unit (PRU). The core of the instrument is the AAS9000 LabVIEW-based software package which controls the hardware, generates stimuli, and records and analyze ear's responses. An ear probe containing a miniature speaker and a microphone is inserted in the subject's ear canal using sofi tips to obtain an air-tight seal. A good seal of the ear canal is critical for quality recordings.
EAR PROBE
Fig. 2.3 TEOAE Investigation Instrument, the AAS9000
OAEs are usually of the same order of magnitude as the physiologie noise normally present in the ear canal. To cope with this noise, a synchronous time- averaging rnethod is employed for signal detection. A large number of sweeps (more than 400) are stored in memory buffers and averaged, and the FFT of the signal and noise is displayed on the computer screen. The noise Roor of the recording is reduced by dN (where N is the number of sweeps) during synchronous time-averaging. 2.2.4. Stimulus Venus TEOAE Frequency Spectrum Studies of TEOAEs in normal ears show that the frequency spectrum (Fig. 2.4 b) strongly depends on the frequency content of the stimulus (Grandori, 1985). High frequency tones elicit TEOAEs with an elevated spectrum in the high frequency bands, but they have shorter duration than low frequency responses (Kemp et al, 1990).
Recording of an Ait Conduction Click-Stirnulated TEOAE lime rns ,,,,,a,o2zzEZ&~~~~~soN~g~g 5 I
a) Time-domain recording of an air-conduction click stirnulated TEOAE
F re q uency @Hz)
b) FFT of an air-conduction click stimulated TEOAE Fig. 2.4 Recording of an Air Conduction Click-Stimulated TEOAE
TEOAEs can also be recorded in ears with normal sensitivity at some frequencies combined with hearing loss at other frequencies (Kemp et al, 1986). Until recently, the frequency spectrum of TEOAEs was considered not to accurately reflect the frequency aspect of the hearing loss. A new study (Lichtenstein and Stapells, 1996) contradicts this general belief. The authors proposed octave or half-octave band analysis of click-evoked TEOAEs to detect mild hearing loss at 1000, 2000 and 4000 Hz. They recommend 500 Hz tones to identify low frequency cochlear dysfunction.
2.2.5. Nonlinear Growth of the Amplitude of TEOAEs The magnitude of the cochlear emissions increases with increasing the intensity of the stimulus. However this relationship is not proportional (Kemp, 1978). At very low stimulus levels the amplitude of TEOAEs increases almost linearly; at moderate levels of stimulation the response saturates, growing by approximately 1 dB for each 3 dB increase in stimulus amplitude (Kemp and Chum, 1980). The maximum sound pressure level of the evoked emission does not usually exceed 20 dB SPL.
2.2.6. Latency of TEOAEs The latency of the emissions depends on their frequency. High frequency components dominate during the first part (3-5 ms post-stimulus time) of the click-evoked cochlear emissions, followed by lower frequency cornponents over the next 10-15 ms (Kemp, 1978). This latency dependence on frequency correlates with the sound wave travel time to the cochlear place of its characteristic frequency. However, the emission latency is 2-3 times higher, indicating additional active processing in the cochlea (Kemp and Chum, 1980).
2.2.7. lntersubject Variability and TesuRetest Repeatability of TEOAE Recordings Cochlear responses differ significantly from ear to ear, even in the same subject. This is not unexpected, since small audiometric differences as well as anatomical asymmetries at the cochlear level are to be expected even in nomally heanng subjects. On the other hand, the reproducibility of the waveforrn in the same ear is rernarkable (Kemp, 1978) even with a long time between tests. Crosscorrelation factors of 80-90% are usually achieved during properly conducted investigation sessions (Kemp, 1986). The variance of the responses from a healthy ear from session to session can be high due to measurement errors difficult to control: ear probe insertion depth. probe-rneatus sealing and meatus or probe occlusion by debris or cerumen.
2.2.8. TEOAE Amplitude and Subject's Posture Cochlear emission amplitude and latency depends on the subject's head position relative to the whole body (Antonelli and Grandori, 1986). TEOAEs were recorded in subjects who first sat on a chair and then lay on a reclinable bed tilted at angles of 0, -20 and -40 degrees with respect to the horizontal plane. The subjects were held in one particular position for at least five minutes prior to a TEOAE recording session. The authors found that TEOAEs amplitude and latency had significantly decreased when subjects moved from vertical to supine posture. Wlson (1984) suggested that pressure variations of the cochlear fiuids could cause mechanical bias on the stapes, thus rnodifying the transfer characteristic of the middle ear. Pressure changes of the cochlear fluids could also influence the generation of emissions directly (Antonelli and Grandori, 1986). The major aspect of the recordings remained largely unchanged though.
2.3. TEOAEs Elicited by Bone-Conducted Stimuli: Parücular Aspects TEOAEs can be elicited by bone-conducted stimuli too. In this case, a bone conductor usually placed on the subject's mastoid process replaces the speaker as a stimulus source. These cochlear responses retain most of the airborne-elicited TEOAE characteristics regarding frequency spectrum dependence on the stimulus frequency, response nonlinear growth, latency, and testfretest repeatability. Due to the fact that the middle ear is being bypassed during stimulus delivery, bone conduction stimulated TEOAEs are also most likely to be recorded in ears with middle ear disorders, such as otosclerosis.
2.3.1. The Middle Ear's Role in Bone-Elicited TEOAE Measurement Bone-elicited TEOAEs (BCOAEs) could be recorded in otosclerotic patients because the stimulus bypasses the dysfunctional ossicular chain, while no air conduction stimulated responses (ACTEOAEs) were detected. After reconstructive surgery to mobilize the ossicular chain, the amplitude of BCOAEs increased. ACTEOAEs could also be recorded postoperatively. Therefore, BCOAE measurement can help in identifying the cause for hearing loss, i.e. cochlear or middle ear impainent.
2.3.2. Bone-Conducted Stimulus and TEOAE Characteristics BCTEOAEs' spectral characteristics strongly depend on the frequency characteristics of the stimulus (Rossi et al, 1988). Due to vibration of the bony part of the extemal ear canal, the stimulus artifact can be longer than when using airborne stimuli. Similar to ACTEOAEs, bone-elicited TEOAEs show a nonlinear relationship between stimulus and response amplitude (Grandori, 1985). The BCTEOAE threshold is about 10 dB HL higher than the subjective tonal threshold obtained using the same stimulation modality and stimulus. However, boneelicited responses have a higher average amplitude than air-elicited TEOAEs for stimulus intensities of 20 and 30 dB HL above their threshold level (Rossi et ai, 1988).
2.4. Biological Effects of Ultrasound Since the goal in this project is to elicit TEOAEs using bone-conducted ultrasonic stimuli. However, the bone conductor will unavoidably generate a significant amount of airborne ultrasonic signal as well. Hence, aspects of ultrasonic sound exposure protection, both airborne and bone-conducted, will be considered .
2.4.1. Airborne Ultrasound: Biological Effects and Exposure Standards Industrial devices or processes employing ultrasound sources can generate noise in the audio range. possibly as subhamionics of the fundamental frequency (Acton, 1983). This type of noise can cause temporary hearing threshold shifts, permanent hearing loss and subjective effects such as fatigue, nausea, tinnitus and headaches in humans (Acton, 1983; Grezsik and Pluta, 1986). Fatal body temperature increase occurred in srnaIl furry animals (rats. guinea pigs and rabbits) exposed to intense (144-165 dB SPL) signals in the 20- 30 kHz frequency range. Due to the much lower absorption coefficient of the human skin, the calculated whole-body lethal exposure level is above 180 dB SPL at 20 kHz. The lntemational Non-lonizing Radiation Committee of the lntemational Radiation Protection Association adopted in 1984 the lnterim Guidelines on Limits of Human Exposure to Airbome Ultrasound (Fig. 2.5).
Limits for Continuous Occupational Exposure to Airborne Sound 120
Fig. 2.5 Limits for Human Exposure to High Frequency Airbome Sound Adapted from Acton (1967) and IRPA Guidelines (1984) Wth minor differences. these Guidelines are accepted by most of the industrialized countries as protection limits to high frequency airbome sound.
2.4.2. Contact Ultrasound: Biological Effects and Exposure Limits There are many documented adverse effects of strong contact ultrasound on biological tissue (Wiernicki and Karoly, 1985). Damage can occur due to a thermal effect, mechanical disruption or cavitation. Exposure of mice to intense contact ultrasound have reportedly produced lesions of the nervous system (Stolzenberg et al. 1980). the ear (Barnett. 1980). testes (O'Brien et al. 1978). as well as congenital malformations (Lele. 1979). Genetic and cellular irregularities have also been reported (Liebeskind et al. 1981). The American lnstitute of Ultrasound in Medicine (October, 1978) has concluded that: 'In the low megahertz frequency range there have been (as of this date) no independently confirmed significant biological effects in mammalian tissues exposed to intensities below 100 mW/cm2. Furthermore, for ultrasonic exposure times less than 500 seconds and greater than 1 second. such effects have not been demonstrated even at higher intensities. when the product of intensity and exposure times is less than 50 joules/cm2 (Bendwell and Repacholi. 1980). The recommended maximum permissible contact levels for ultrasound are presented in Fig. 2.6 (Nyborg, 1978). A: Fatal WeigM Reduction 8: Postpartum Mortal-Ry C: Wound Healing D: Altered Miofic Rate (Variable ResuRs) E Genetic Oamage (Hegatiue) I I i I r I I I F: Fatal Abnormalities 100 1000 10,000 ('?ostulate") Time, sec
Fig. 2.6 Recommended Maximum Permissible Contact Exposure Levels for Ultrasound (Nyborg, 1978)
2.5. Ultrasonic Hearing
Normal hearing human subjects can perceive airborne signals in the range of 20 Hz to 20 kHz. However, the range of hearing extends up to 100 kHz for signals presented through the bone using a transducer usually placed on the subject's skull (Corso, 1963).
2.5.1. Bone-Conducted Ultrasonic Stimuli in Clinical Auditory Investigation
It is well known that patients with sorne types of hearing loss (mainly of sensorineural origin) show increased ultrasonic hearing thres holds. Hence, our rnethod shows potential as a supplementary investigation tool on the causes and the type of hearÏng impainent. Early observations found that patients with cochlear deafness could not hear an ultrasound tone at 62.5 kHz (Belluci and Schneider, 1962). Patients with sensorineural hearing loss had increased ultrasonic hearing thresholds although patients with conductive hearing loss (otosclerosis) showed normal perception of ultrasound (Sagalovich and Pokryvalova, 1966). A more thorough investigation on the clinical usefulness of bone- conducted ultrasonic stimuli in distinguishing between various hearing disorders was conducted by Abramovich (1978). All the patients with conductive hearing loss exhibited normal ultrasonic hearing thresholds as well as those with hearing loss due to noise exposure or in the early stages of the Meniere disease (when no extensive hair cell damage occurred). Some of the subjects with mixed (conductive and sensorineural) hearing loss showed increased ultrasonic thresholds. The picture had dramatically changed in the case of the patients with sensorineural hearing loss likely associated with hair cell damage: 81% of these patients showed increased ultrasonic thresholds. The increased ultrasonic threshold seemed to correlate well with the severity of hearing loss in the audiometric range (125 to 8000 Hz). Likewise, al1 patients with severe hearing loss due to ototoxic drug exposure. which also associated with destruction of hair cells, showed increased ultrasonic thresholds. A new ultrasonic hearing testing approach has been devised by Lenhardt et al (1992). Speech signals rnodulated into the ultrasonic range using two carrier frequencies (28 or 40 kHz) were presented to normal and hearing irnpaired subjects. The sig nals were clearly perceived as speech. Furthermore, in a WlPl (Word Intelligibility by Picture Identification) test, the rate of correct identification was 81% by normal hearing subjects, 51% by patients with moderate sensorineural hearing loss and 40% in patients with profound acquired sensorineural hearing loss. Thus, al1 of the above-mentioned observations show a significant potential for bone-conducted ultrasonic stimuli as an investigation (Abramovich, 1978) or even an alternative rehabilitation tool (Lenhardt et al, 1992). 2.5.2. Bone Conduction Sonic and Ultrasonic Hearing Thresholds The frequency range of hearing for ultrasonic bone-conducted signals was first investigated by Pumphrey (1950) on three normal hearing subjects, using a transducer placed on their rnastoid process. Although the subjects' upper frequency hearing limit for airborne signal was 16.5 kHz, they could perceive bone-conducted signals up to 100 kHz. The threshold of hearing for ultrasound decreases with increasing the area of contact of the transducer with the subject's body. Deatherage et al (1954) recorded a threshold value of 140 dB re 0.0002 dyn/cm2 at 50 kHz when the jawbone of a subject was brought in contact with the surface of the water in a bucket holding the transducer. When the entire body of the subject was submerged in a tank containing the sound field, the threshold value decreased to 134 dB. The threshold of hearing for sonic and ultrasonic bone-conducted signals at 12 frequencies from 5 to 100 kHz was measured by Corso (1963) in young, normal hearing adults. A custom-made crystal transducer was placed on the subjects' right or lefi mastoid process and the stimulus was presented using a modified method of limits. One ascending and one descending series of trials was presented to 53 male and 50 fernale coliege students on one ear. The resultant average threshold of hearing is shown in Fig. 2.7. Ultrasonic signals above 64 kHz pass with less attenuation across the cranial vault, while audio range stimuli propagate better through the more dense base of the skull (Dunlap et al, 1988). Fig. 2.7 Threshold of Hearing for Sonic and Ultrasonic Bone-Conducted Signals (Adapted from Corso, 1963)
2.5.3. Signal Attenuation in the Bone and Tissue It is likely that ultrasonic bone-conducted perception relies on the inner ear only since the middle ear cannot vibrate at such high frequencies due to the physical characteristics of the ossicles (Tonndorf, 1972). Therefore, bone- conducted signals above 20 kHz propagate along the osseous route only, not the osteotympanic one. The attenuation of the signal propagating through the bone depends on frequency: it increases at a rate of approxirnately 12 dBloctave (Fig. 2.8) from 400 Hz to 64 kHz (Dunlap et al, 1988). The skin and soft tissue attenuation in the audio range varies frorn 10 to 20 dB depending on the frequency of the stimulus and on the contact pressure between the transducer and the skull (Hakansson et al, 1985). No data on skin and soft tissue attenuation of ultrasonic signals is currently available but it also seems to depend largely on the contact pressure between the head and the transducer. +Intact -+ Sectioned -Intact on Foam +Sectioned on Foam -42 dB Slope
I 0.4 2.0 8.0 32.0 640 Frequency, kHz
Fig. 2.8 Human Skull Attenuation for Sonic and Ultrasonic Signals (Dunlap et al, 1988)
2.5.4. Hypotheses on the Mechanism of Ultrasonic Hearing; the Cochlea as an Ultrasonic Receiver To date, the phenornenon of ultrasonic hearing has not been fully explained, however a few hypotheses have been considered: a) demodulation at the surface of the skin, in the soft tissue or in the cochlear fluid; b) piezoelectric effect of the bone; c) the existence of a yet unidentified ultrasonic receptor residing in the vestibulo-cochlear system; d) cochlear reception outright. Each of these will now be discussed. 2.5.4.1. Ultrasound Demodulation A slight, even-order (rectifying) nonlinearity in the conduction pathway from the transducer to the cochlea (Dobie and Wiederhold, 1992) would result in demodulation of the ultrasonic stimulus. The resulting audio-range signals will undoubtedly produce auditory effects. Amplitude-modulated oscillations can be demodulated in water due to acoustical pressure (Altenburg and Kastner, 1952). The radiation pressure S is related to the oscillation displacement A by the expression S=1/2p02A2 (2.1 in which p is the density of the medium and o = 2 IT f. If the amplitude of the ultrasonic signal of frequency f is modulated according to the law Z~SCOSQ~ (2-2) where m = modulation factor and n = 2 K FIF = modulating frequency then the ultrasonic wave will acquire an additional low-frequency alternating pressure component (m212) S cos R t (2-3) Alternatively, demodulation could take place by interaction between waves with frequencies f and fzF (half wave rectification), where F is the demodulation frequency, F = R / 2 z (Gavrilov et al, 1977).
2.5.4.2. Piezoelectric Effect of the Bone The piezo-electric properties of bone have been thoroughly investigated. When compressed, a piezo-electric material generates an electromagnetic field which can be detected and analyzed using suitable instrumentation. More importantly, the reverse phenornenon is also true: within its characteristic limits, a piezo-electric material excited by an electromagnetic field will vibrate to the tune of the frequency and intensity of the driving field. Dynamic rnethods. such as wave propagation and vibration of bones have been used as noninvasive methods for measuring in-vivo properties of bone and for therapeutic purposes. Wave propagation in long bones can be used to monitor the rate of fracture healing (Guzelsu and Saha, 1984). The mechanical wave propagation in bone can be monitored using a sensitive magnetic detector capable of picking up the induced electro-motive force due to magnetic induction outside the bone (Saha and Lakes. 1977). Ultrasonic wave propagation in human bone is influenced by its piezo- electric properties (Yoon and Katz. 1976) although to a lesser extent than in the case of Quartz structures because of bone's weaker electromagnetic properties. Consequently, vibrations of the human skull in the ultrasonic range can generate low intensity electrornagnetic fields which can directly excite nerve fibers.
2.5.4.3. The Otolith Organs In subjects with residual hearing. the saccule. an otolithic organ usually involved in posture and balance control, is also believed to be able to transduce sound after destruction of the cochlea (Cazals et al, 1980). A nerve branch from the saccule innervates the base (the high frequency area) of the cochlea and a branch of the cochlear nerve innervates the saccule. suggesting a reciprocal interdependence between these two organs. In addition, the freestanding hair cells with short cilia in the human striola region of the saccule can be set into resonance by ultrasound from 20 to 100 kHz (Freeman and Weiss, 1990). Therefore, bone-conducted ultrasound could be coded into neural pulses at the level of the saccule and the information passed along to the cochlear nerve bundle.
2.5.4.4. The Cochlea as an Ultrasonic Receiver At frequencies above 14 kHz the outer and middle ear make no contribution to sound conduction enhancernent due to impedance mismatch (Tonndorf, 1972). High frequency hearing must rely solely on inner ear signal processing. Bone-conducted ultrasound could be received and coded in the basal turn of the cochlea in the same fashion as 10 to 13 kHz signals. Such an assurnption is supported by the physical dimensions of the outer hair cells and their electro-mechanical and physiologic properties. When excited, a non-linear system such as the outer hair cell tends to vibrate at 2 frequencies: the exciting frequency and its intrinsic mechano- electrical frequency which could very well be in the ultrasonic range (Fritze. 1996). In quiet. the outer hair cells are in perpetual movement due to Brownian motion. Clusters of cells will vibrate at different phase and frequency, continually building up and replacing one another. The incoming energy at the threshold of hearing is considered to be too small to vibrate the outer hair cells, but it can lead to a low-energy self-organization process of the neighboring clusters (Foerster, 1960). Lower frequency osciilating clusters will gain energy at the expense of high frequency neighboring clusters. much like a chain of coupled pendulums of progressively increasing lengths. Very recent reports show that outer hair cells are interconnected, with only the longest OHC's tips embedded in the basilar membrane. The three rows of outer hair cells could. therefore, act as energy concentrators at the threshold of hearing. This process of low-energy organization could also take place in the presence of ultrasonic stimuli, giving rise to a decodable signal in the audio range, most likely in the basal turn of the cochlea. This would explain the reportedly perceived 10 to 13 kHz tone during bone-conducted ultrasonic hearing experiments. A recent non-linear model (Fig. 2.9) of the outer hair cell's high frequency receptor potential characteristics shows that, under certain physiologic conditions, the cutoff frequency of the OHC response can extend way above the audio range (Hassan, 1997). The model will now be described. The OHCs are embedded in the Organ of Corti with their apex facing the endolymph of the scala media and their basolateral membrane facing the perilyrnph within the Organ of Corti. The apical part of the OHC is rnodelled by an apical conductance at rest. g,, an apical capacitance, Cc,and a parallel shunting conductance g,, whose value is proportional to the stimulus intensity.
Fig. 2.9 Outer Hair Cell Non-linear Model
The basolateral part of the OHC is modeled as a conductance g,, and a capacitance composed of a linear part Cm and a voltage-dependent part Cu, whose value is dependent on the membrane potential U,. The voltage sources V, and V, represent the endocochlear and the potassium potential, respectively. Analysis of the mode1 shows that the non-linear voltage-dependent capacitance can theoretically enhance the ability of the OHC to sustain a membrane potential at high frequencies. This enhancernent effect occurs for resting potentials less than -56 mV and strongly depends on the value of the resting potential and on the ratio of the nonlinear and linear capacitances of the OHC. At a resting potential of -90 mV and capacitance ratio of about 1.65 the cutoff frequency of the OHC response extends way beyond the audio range. The actual resting potential of the OHC was found to be approximately -70 mV (Cody and Russell, 1987). Stretching of the OHC membrane activates potassium channels which generate a hyperpolarization on the order of 10 mV (Iwasa et al, 1991). This OHC resting potential shift allows iower capacitance ratios to extend the frequency response of the OHC. In conclusion, the electro-mechanical and physiologie characteristics of the Outer Hair Cells support the hypothesis of ultrasonic signal detection at the level of the cochtea. Chapter 3
Technical Aspects of Ultrasonic Bone-Elicited TEOAEs
and Equipment Design
Eliciting ultrasonic bone-conduction TEOAEs is a challenging task. This chapter will present the main technical difficulties and the solutions to obtain reliable measurements for ultrasonic bone-elicited TEOAEs.
3.1. The Piezoelectric Bone Conduction Transducer: Construction and Frequency Response
Due to the specific equipment requirements for our study (wide frequency response and reasonably high output power) no comrnercially available bone transducer could be used. Electrornagnetic bone conductors used for audiornetric investigation purposes have a frequency response upper lirnit around 6 kHz. The only option seemed to be the use of a custom-made piezoelectric device. The following sections present the structure of the bone conductor developed in the lnstitute and its frequency response.
3.1 .l. Construction A piezoelectric transducer supplied by NRC (Wang, G.S.K., 1995) has been modified to suit Our needs. The device consists of a stack of four PZT elements supplied by Sensor Technology Limited of Collingwood, Ontario, and a brass plate (Fig. 3.1). The ceramic elernents are connected in such a way so that the outermost plates are connected to ground. Additional protection against high voltage (up to 200 Vp,) is provided by the two thin ceramic disks glued on the transducer and by the soft rubber (Dow Corning Sylgard 184 Silicone Elastomere) housing. The housing also attenuates airbome signal emissions frorn the transducer. The brass plate provides the inertial mass necessary to directing the output signal towards the front (open) end of the transducer.
Fig. 3.1 Piezoelectric transducer structure and electrical wiring
3.1.2. Frequency Response To measure the frequency response of the transducer we used the experimental setup presented in Fig. 3.2. An HP 3326A Signal Synthesizer was used as a signal generator. It generated frequency sweeps of 6 second duration from 1 kHz to 60 kHz. We adjusted the amplitude of the generator's output signal to obtain a 100 V, signal at the output of the bone conductor amplifier. The shape and amplitude of the input signal to the bone vibrator was monitored on the oscilloscope. The bone vibrator was placed on the B8K type 4930 Artificial Mastoid adjusted for a contact force of 650 grams in order to sirnulate the mechanical Joad of human mastoid. A piezoelectric film transducer supplied by NRC was inserted between the artficial skin layer of the artificial mastoid and the bone conductor and used as a vibration pick-up sensor. We used the BlANK Z output of the signal generator as a synchronization signal for the Stanford SR770 FFT Network Analyzer.
Signal Generator Amplifier
Sync 1 Generator Output Amplifier Output
Oscilloscope
Fig. 3.2 Bone Conductoi Frequency Response Measurement Setup
The output of the piezoelectric film sensor was connected to the A input of the FFT Analyzer and 400 sweeps were averaged. The frequency response of five bone conductors, labeled BC2 to BC6 is shown in Fig. 3.3. With one exception, BC3, the bone conductors' frequency response were remarkably consistent. The peak of the response around 38 kHz corresponds to the resonant frequency of the vibrators. Throughout our experiments, BC2 was used as a stimulus source due to its smooth frequency response. In order to increase the output power, we delivered stimuli at 40 kHz, slightly above BCZ's resonance frequency.
Bone Conductor BC2, BC3, BC4, BC5 and BC6 frequency Response
Frequency kHz
Fig. 3.3 Frequency Response of Piezoelectric Bone Conductors
The piezoelectric transducers' capacitance was measured using a Capacitance Bridge and was found to average 1.8 nF at 1 kHz.
3.2. Headband The Headband (Fig. 3.4) was made of two pieces of Blue Tampered and Polished Spring Steel (# C1095) which were 3/8" wide and 0.035" thick and covered with black rubber tubing. The universal joint alignment holder on one end attaches to the piezoelectric bone conductor and allows for adjustment of the transducer with the subject's skull for optimal contact. Fig. 3.4 Headband with Piezoelectric Bone Conductor.
3.3. Acceleration Levels for B.C. Hearing in the Ultrasonic Range: Piezoelectric Bone Conductor Output Level
The acceleration levels of the bone conductor at the threshold of hearing in normally hearing subjects were found to range between 62 to 92 dB re 1 cm/s2 for signals in the 20 to 100 kHz range (Lenhardt et al, 1991), depending on stimulus frequency. A stimuius 30 dB above threshold was perceived as loud and unpleasant. The measurements were taken using an accelerometer encased in the bone conductor as well as externally attached sensors. The authors did not specify the exact positioning of the accelerometer(s) relative to the bone vibrator. Both the bone vibrator and the accelerometer were firmly pressed against the forehead or the mastoid process. No skin penetration method was employed. To compare the consistency of the data available in the sonic and ultrasonic range, we considered the threshold acceleration level for direct bone conduction at 1 kHz as measured by Hakansson (Table 3.1) and estimated the threshold acceleration level at 64 kHz: - At 1 kHz the direct bone conduction (dbc) threshold acceleration level is 2.5 dB re 1 cm/s2(Table 3.1); - From 500 Hz to 64 kHz we have 80 dB of attenuation (Fig. 2.8). Therefore, the dbc threshold acceleration level can be estimated to be 82.5 dB re 1 cmis2; - To find the bone conduction acceleration level at threshold of hearing, the signal attenuation in skin and soft tissue should be also accounted for. There is no information available about skin and soft tissue attenuation in the ultrasonic range. Estimating an attenuation level of 12 dB, a bone conduction acceleration threshold of 95 dB is found. This is in good agreement with rneasured threshold levels (Lenhardt et al, 1991).
Conventional Bone Conduction Direct Bone Conduction Mechanical RETAL Correction, RETAL Correction, RETFL Irnpedance. I&i2 Freq. [Hz] dB re dB re d8 dB re dB dB re 1 ph 1 Nslm 1 crnls2 1 cm/s2 250 44.5 6.5 16.5 -1 0.0 -5.5 72-5 375 42.5 7.0 16.0 -9.0 -3.5 65.5 500 40.5 7.5 19.0 -1 1.5 0.5 57.5 750 38.0 4.0 20.5 -1 6.5 3.0 45.5 1O00 36.0 2.5 27.0 -24.5 9.0 33.5 1500 33.0 3.0 25.5 -22.5 4.5 32.0 2000 39.5 2.5 27.5 -25.0 4.5 26.5 3000 27.5 8.0 20.0 -12.0 -2.5 32.5 4000 29 .O 14.5 15.5 -1 .O -3.0 38.5 6000 34.0 17.5 20.0 -2.5 9.0 31 .O
Table 3.1 Reference threshold data in conventional bone conduction and the fint proposal of reference threshold data in direct bone conduction (from Hakansson et al, 1985) In order to deliver stronger ultrasonic signals we can increase the output power of the transducer the following ways: 1) lncrease the amplitude of the input signal to the transducer. There is a severe limitation due to the signal amplifier's maximum output voltage swing (200 V,, at most), as well as safety concerns. 2) Use a stimulus close to the resonant frequency of the transducer (36 kHz in this case) to further increase the output power. Throughout the experiment, a 40 kHz stimulus was used. 3) Increase the contact pressure between the transducer and the skull. A new headband, providing 800 grams contact pressure was designed and built in the Institute.
To determine the output level of our bone conductor coupled to the high voltage amplifier we used the following estimates and assumptions: - The Bone Conductor has a 57 dB SPL output level at 1 kHz, 116 V,, input signal (Purcell ef al, 1996); - The threshold of hearing in acceleration units at 1 kHz is 10 dB re 1cm/sec2 (Dirks et al. 1976) according to the British Standard 2497. Hence. the 57 dB SPL translates to 67 dB re 1 cm/sec2. - We doubled the input signal to the bone conductor. which leads to an output level of 67+ 6 = 73 dB re 1 cm/sec2. - For the same amplitude of the input signal to the bone conductor, its output increases by about 10 dB in the frequency range of 1 to 40 kHz (Fig. 3.3). This amounts to 83 dB re 1 crn/sec2 at 40 kHz. - The skull attenuation from 1 to 40 kHz is about 60 dB (Fig. 2.8). Therefore, the effective output level of the bone conductor is 23 dB re 1 crn/sec2 above the threshold of hearing in the ultrasonic range. This level does not account for the additional signal attenuation in the skin and soft tissue. Nonetheless, it shows that our system's output level is suitable for investigation purposes in normal hearing subjects. Lenhardt et al (1992) has reported a dynamic range for ultrasonic hearing of about 30 dB, from threshold of hearing to unpleasantly loud. Our output level estimate is in good agreement with his report; the signal perceived by al1 our subjects was reported as being reasonably loud, but without causing discornfort.
3.4. Calculation of Ultrasound Exposure Limit The 50 joules/cm2 contact ultrasound exposure limit (Benwell and Repacholi, 1980) was considered the maximum pennissible level during our experirnents and the actual exposure levels was cornputed and compared against it. Since we cannot directly measure the output power of our bone conductor. we can estimate it from the input power to the transducer and assume a 50% efficiency of the device, which is actually a very optimistic figure. Throughout Our study, we used tti,= 1 ms long stimuli during a T = 40 ms long recording sweep and averaged a maximum of N = 4000 sweeps. The maximum output voltage of Our High Voltage Amplifier is 200 V,, (approxirnately 140 Vms)and its maximum output current is 10 mA. Hence. the maximum input power to the bone conductor is: P = 140 V * 0.01 A = 1.4 W (3.1) The output radiated energy for 50% transducer efficiency is: E = 0.5 * P * f,, 1 T = 0.5 * 1.4 W * 1/40 ms = 0.0175 mJ (34 during one sweep The total energy radiated during a 4000 sweep recording session is: E, E, = E '4000 = 0.0175 mJ * 4000 = 0.7 J (3.3) The piezoelectric transducer we used has a radius of about 1 cm. Its radiating area is: S = x * (1 cm)' = 3.14 cm2 (3-4) The estimated exposure level is, therefore, 0.7 / 3.14 = 0.22 Joule / cm2 (3-5) which is well below the maximum permissible level. The actual exposure level is considerably lower, for various reasons. Firstly. our bone conductor's efficiency is not 50% and secondly the contact area between the transducer and the subject's head is less than 3.14 cm2 due to the dome shape of the mastoid p rocess.
3.5. Equipment Design The electronic circuits we designed and built will be described during the following subchapters. The Signal Conditioning Board and the High Voltage Piezoelectric Driver were mounted in the same metal housing and physically represent one single piece of equipment. Two Bridge Amplifiers enclosed in another metal case make another distinct piece of equiprnent.
3.5.1. Signal Conditioning Board The Signal Conditioning Board provides synchronization signais for the various pieces of equipment used throughout the experiment and controls the shape of the stimulus. The electrical diagram of the Signal Conditioning Board is shown in Fig. 3.5. The click, usually routed to the miniature speaker in the AAS9000 Ear Probe, is now amplified by UlOl (OP27) and converted to a TTL signal by the Schmidt Trigger gates U102A and U102B. The falling edge of this signal activates two monostable vibrators. U1O3A and U1O3B. which generate the 'STIMULUS WIDTH' and 'TRIG OUT' signals respectively. The stimulus duration can be adjusted to 1 ms or 0.5 ms using the switch SWlOl which controls U103A1s output pulse width. The rise/fall slope times of the stimulus can be adjusted to 0.1 or 0.2 ms with the switch SWIOZ. The ultrasonic signal is generated by an external signal source activated by the 'TRIG OUT" signai and connected to the noninverting input of the programmable operational amplifier U105 (MC1776). The amplification and output shape and duration of the ultrasonic signal can be controlled by the programming signal at pin 8, obtained from operational amplifier U104 output. The amplitude of the output stimulus can be controlled by adjusting the external signal 'SIGNAL IN' and should not exceed 5 V,,. The 'STIMULUS OUT signal is connected to the Bone Conductor Signal Amplifier and delivered to the piezoelectric transducer as will be described in the next sections of this chapter.
3.5.2. High Voltage Amplifiers for Piezoelectric Bone Conductors As described earlier, the piezoelectric bone conductor offers a wide band frequency response with low distortion content, but requires a high amplitude excitation signal in order to increase the output power. Two amplifier configurations have been tested during our study and their electrical diagrams, circuit description, benefits and limitations will be described in the following sections. Fig. 3.5 Signal Conditioning Board 3.5.3. Bridge Amplifier The bridge amplifier configuration (Fig. 3.6) offers the advantage of increased output voltage swing using inexpensive electronic components. Two Power Operational Amplifiers (LM675. National Semiconductor) are connected in an inverting (U201) and a noninverting (U202) configuration respectively and the Bone Conductor is connected between the two outputs 'DIFF. OUTI' and 'DIFF. OUT2'. The 'STIMULUS OUT' signal from the Signal Conditioning Board is connected to the 'STIMULUS IN' input of the bridge amplifier and arnplified by 10 and -10 in the two ICs. This way the voltage across the output toad doubles while still using low voltage components. A disadvantage of using such a configuration is the differential output. A microphone wire with 2 shielded cables is needed to connect the Bone Conductor to this signal amplifier. In addition, the shape of the output signal can be displayed on an oscilloscope's screen only if it has a differential input. An instrument such as this was not avaiiable, so we built an instrumentation amplifier (Fig. 3.6, inside the dotted line) to convert and to scale down the differential output signal. R207, 208 and 209 form a signal divider to attenuate the signal by a factor of 30, while the rest of the instrumentation amplifier has a voltage gain of 3, leading to an overall inputIoutput signal attenuation of 10. The 'MONITOR' output of the circuit can be connected to the input of a oscilloscope or other instrument to view the bone conductor input signal. Fig. 3.6 Bridge Amplifier and Output Monitor Circuit 3.5.4. High Voltage Piezoelectric Driver The use of a specialized piezoelectric driver as a signal amplifier has proven to be the best choice for our experiment. A High Voltage, High Speed Operational Amplifier type 3584 (Burr-Brown) was used as a noninverting amplifier with a voltage gain of 20 (Fig. 3.7). This device has a wide-band frequency response (50 MHz gain-bandwidth product), high speed (150 V/ms slew rate) and can drive capacitive loads up to 10 nF. The load can be connected to its output using a standard coaxial cable and the shape of the signal can be monitored directly using an oscilloscope. The disadvantage of this circuit is the need for a high voltage power supply (up to 51 50 Vcc). In addition, its limited output cuvent capability (215 mA) prevents us from setting the frequency of the stimulus to the resonant frequency of the Bone Conductor. In order to maximize the output power of the transducer the frequency of the stimulus was adjusted around the resonant frequency (36 kHz) to obtain a clean, undistorted output wavefom (monitored on an oscilloscope's screen) and the sharpest frequency spectrum (monitored on the FFT Analyzer's screen using the piezoelectric film transducer as a vibration sensor). Throughout the study, a 40 kHz signal of adjustable amplitude, duration and rise/fall dope was used as an excitation stimulus. Fig. 3.7 High Voltage Piezoelectric Driver Chapter 4
Experimental Results
Having solved the technical difficulties incurred in our attempt to elicit TEOAEs using bone conducted ultrasonic stimuli. a series of experiments were conducted on normal hearing young adults to record cochlear emissions and to detennine their main characteristics. This chapter will present the experimental setup, the test parameters and procedure and the results obtained.
4.1. Experimental Setup
The experimental setup (Fig. 4.1) makes use of the AAS9000 Audiornetric Assessment System's features. Due to the nature of our study, additional equipment was developed as described in the previous chapter.
4.1 A. Equipment List
We used the following pieces of equipment: - AAS 9000 Audiometric Assessment System, PMMD - Patient Room Unit (PRU), PMMD - Ear Probe, PMMD - 24 dBloctave Filter, Krohn-Hite 3700 - Signal Conditioning Board, own design, see section 3.5.1. - High Voltage Bone Conductor Amplifier, own design, see section 3.5.4. - Piezoelectric Bone Conductor, own modification of NRC's BC2, see section 3.1.1. - Piezoelectric Film Transducer, see section 3.1.1. - FFT Network Analyzer, Stanford Research Systems SR770 - 15 MHz Synthesized Function Generator, Stanford Research Systems DS340 - Oscill~scope,Tektronix 51 11 A
4.2. Test Procedure The AAS9000, a LabView-based audiometric instrument is set on TEOAE Measurernent mode. The stimulus used was a 2 ms, 80 dB SPL click. The Patient Room Unit (PRU) is the hardware system which generates stimulus signals and filters and amplifies cochlear responses using the miniature speaker and microphone residing in the Ear Probe. Since we need a stimulus in the ultrasonic range. which exceeds the system's maximum output frequency, the click stimulus is disconnected from the speaker and rerouted to the Signal Conditioning Circuit which generates the synchronization signal (TRIG OUT) for the Signal Generator, FFT Analyzer and Oscilloscope. This circuit also receives the ultrasonic signal from the Signal Generator and shapes the stimulus to be arnplified by the Bone Conductor Driver and delivered to the subject's skull by means of the Bone Conductor. The frequency and amplitude of the stimulus can be adjusted using the corresponding controls of the Signal Generator. The duration of the stimulus and its riselfaIl slopes can be adjusted using the switches on the Signal Conditioning Circuit's front panel. The shape of the Bone Conductor Driver and Piezoelectric Film Transducer output can be monitored on the oscilloscope's screen. Their frequency spectrum can be analyzed using the FFT Analyzer. The ear responses are detected by the Ear Probe's microphone, amplified in the PRU's microphone preamplifier stage, band-pass filtered from 500 Hz to 10 kHz by the Kron-Hite filter and delivered to the cornputer's 16 bit Data Acquisition Card. 1 Signal Generator
Trig. IN 4 L \
Fm The TEOAE amplitude is on the order of magnitude of the physiological noise usually present in the ear canal. To improve the signal to noise ratio of the measurements and enhance cochlear response detection, a synchronous time- averaging technique is employed by the AAS9000 software. The number of sweeps averaged during a recording session can be controlled from the instrument's specialized keyboard. At the same tirne, the noise level is being computed and subtracted from the recording. The AAS9000 displays the time- domain waveform of the cochlear response and its frequency spectrum. as well as the frequency spectrum of the noise. TEOAEs from both ears can be simultaneously displayed in their corresponding windows on the computer's screen for audiornetric investigation purposes.
4.3. Subjects The subjects were three male adults, 23, 24 and 32 years old, without any disease of ear, nose and throat and no cornplaints on hearing problems. Conventional pure-tone audiometry performed at Mount Sinai Hospital's Audiology Clinic s howed both air- and bone-conducted hearing threshold levels better than 15 dB HL in the frequency range of 125 to 8000 Hz, with bone-air gaps better than 5 dB. Thus, the subjects were considered as normally hearing listeners.
4.4. TEOAE Measurement Results We conducted a series of tests to derive ultrasonic bone-elicited TEOAEs and to compare them with the conventional TEOAEs. Features of ultrasonic bone-stimulated cochlear emissions are presented in the following subsections. 4.4.1. Skull Vibratory Pattern The primary concern during our investigation was with the amount of audio-range contaminating signals generated by the Bone Conductor or arising as a byproduct of human skull ultrasonic excitation. Hence, the vibratory pattern of the subject's skull was monitored during a typical investigation session. The piezoelectric film transducer was inserted between the bone conductor and the subject's skull and its output was examined on an oscilloscope's screen as well as with the FFT Analyzer (Fig. 4.1). The amplitude of a 40 kHz stimulus, 1 ms long, with 0.2 ms riselfall slopes was adjusted to 100, 150 and 200 Vpp.The amplitude of the skull vibration has increased monotonicaliy and closely resembled the shape of the stimulus. The frequency spectrum of the skull vibration was recorded by the FFT Analyzer and can be seen in Fig. 4.2. We can see a peak at the stimulus frequency of 40 kHz which increases with the amplitude of the excitation signal. A crucial result is that there is no significant amount of frequency components in the audio range from O to 20 kHz. The peak we can see at very low frequencies is due to d.c. bias. This measurement also shows that, in spite of the large input signal to the bone conductor (maximum 200 V,), the transducer's output has not reached saturation. When changing the riselfall dope of the stimulus from 0.2 to 0.1 ms, the peak component of the skull vibration pattern had a decreased amplitude and wider sidebands, as expected. Reducing the stimulus duration from 1 ms to 0.5 ms only decreased the amplitude of the frequency spectrum components. with no change in its general shape. As a result of these observations, we decided to use a 40 kHz, 1 ms long stimulus with 0.2 ms riselfall time in our study since it delivers the strongest ultrasonic excitation with no significant amount of audio-range contamination. Frequency Spectrurn of Head Vibration to a 40 kHz, 1 ms Stimulus
------.- - -100Vpp Output of B.C. Driver - - - - 150 Vpp Output of B.C. Driver -200 Vpp Output of B.C. Driver -
Fig. 4.2 Skull Vibratory Pattern to a 40 kHz, 1 ms Stimulus
4.4.2. Bone Conductor Positioning and Contact Force Influence on TEOAE Detection
Throughout the study, the correct positioning of the bone conductor on the subject's mastoid process was found to be very important for TEOAE elicitation. A slight misplacement of the transducer led to poor recordings or no cochiear response detection at all. The headband contact pressure played an important role too. Although the standard audiometric headband delivers a 450 gram contact force on the average human skull, it was found to be unsuitable for Our investigation. This observation lead to the design of the customized headband described in section 3.2. which almost doubles the contact force to 800 gram. At the beginning of each measuremcnt the subject was asked to listen to the ultrasonic stimulus and to adjust the Bone Conductor position to maximize the loudness of the perceived sound. The transducer was held in that position during the test session. 4.4.3. Components of an Ultrasonic Bone-Condoction Stimulated TEOAE Recording A TEOAE recording consists of two main components: - The Stimulus Artifact, which consists of refiections of the stimulus signal from the structures of ear canal and middle ear and post-stimulus vibrations of the skull. It has a high amplitude (Fig. 4.3a.) and in our experiments lasts for about 7 ms after stimulus onset.
Stimulus Artifact
Cochlear Response / a) B.C. Ultrasoni7 Stimulus TEOAE Recording, Subject 'C', Right Ear
b) Response Part of a B.C. Ultrasonic Stimulus TEOAE Recording, Subject 'C', Right Ear Fig. 4.3 Components of a B.C. Ultrasonic Stimulus TEOAE Recording - The Transient Evoked Otoacoustic Emission (cochlear response), which is the active response of the cochlea to the stimulus. Its amplitude is much lower (Fig. 19b.) than the artifact's amplitude and slowly decays until being overlapped by the physiological noise in the ear canal. The early part of a TEOAE cannot be detected due to the presence of the much higher stimulus artifact. We can see the presence of an audio range signal in the 'Stimulus Artifact' part of the recording. This signal can arise due to ultrasonic stimulation by way of signal demodulation at the surface of the skin or in the soft tissue, mechanical vibration of elements of the outer and rniddle ear or due to the residual audio range output signal of the bone conductor. as can be seen in Fig.4.2. Throughout Our measurernents, cochlear emissions were found to last for about 15 ms post-stimulus time. The tirne-domain waveform from 7 to 15 ms was examined to detect cochlear responses and their frequency spectra. Unless otherwise specified, this part of a recording as well as its FFT will be presented as the TEOAE recording throughout the following sections of this chapter.
4.4.4. Ultrasonic Bone-Stimulated TEOAE Versus Stimulus Amplitude A well known phenornenon is the monotonic growth of air-elicited TEOAE amplitude with stimulus level for low to mild intensity stimuli in the audio range. The same dependency was found to be true for ultrasonic bone-elicited TEOAEs. Fig. 4.4 shows the results of a measurement on one subject using a 40 kHz, 1 rns long, 0.2 ms riselfal1 time, 150 and 200 V,, amplitude ultrasonic stimuli. The figures show the results after averaging 1000 sweeps recorded at a rate of 25 sweepskec. Fig 4.4a. shows the time domain waveforms from 7 to 15 ms (the Response part of the recording). The correlation between the two cochlear responses is very good at the starting point of the plot and deteriorates as the TEOAE elicited by the lower amplitude stimulus decreases faster and disappears into the noise fioor. TEOAE Recording, Subject 'A', Rig ht Ear Time ms
------1SOVpp Input to B.C. --- 2OOVpp Input to B.C. ------
a) Time-domain wavefortn
b) FFT O~TEOAERecording and of Noise. 15CFVi Input to ~one~onductor
1 2 4 5 7 8 Frequency kHz 1 c) FFT of TEOAE Recording and of Noise. 200 V,, lnput to Bone Conductor
Fig. 4.4 Stimulus Amplitude Influence on TEOAE 4.4.5. Ultrasonic Bone- Stimulated TEOAE Frequency Spectrum
Figures 4.4b. and 4.4~.show the frequency components of the two recordings and of the corresponding noise levels. Unlike click- or audio range tone-elicited TEOAEs, ultrasonic stimulated responses show a broader band distribution of their frequency-domain components. The most significant feature is the extension of the upper frequency range, from 5 to 8 kHz. As we can see, contrary to airborne stimulated TEOAEs, high frequency responses elicited by bone-conducted ultrasonic stimuli can be easily detected because they last longer are not overlapped by the stimulus artifact (typical frequency limit is 4 or 5 kHz).
4.4.6. LeR 1 Right Ear Variations of TEOAE Recordings Conventionally-elicited TEOAEs show significant differences in shape, duration and frequency content between different ean, even when recorded in both ears of a single subject. This is not unexpected. The hearing ability is not identical in the two ears of a normally hearing person. Since conventional TEOAEs are considered tu reflect one's hearing ability, TEOAEs could differ from ear to ear. To reveal the leWright ear variation of responses to ultrasonic bone- conducted stimuli, we tested both ears of the three subjects. We found differences from ear to ear in each one of them. Fig. 4.5 shows recordings of right and left ear TEOAE responses detected in subject 'T using a 40 kHz, 1 ms long, 0.2 ms riselfall time, 200 V,, amplitude stimulus. There is a large variation in the amplitude, shape and duration of the emissions as well as in their frequency content, consistent with pure tone audiometry measurernents. Left and Right Ear TEOAE of Subject 'A' rime ms
--- Right Ear
a) Tirnedomain wavefom
\Frequency kHz b) FFT of Left Ear TEOAE and of Noise
4 5 7 Frequency kHz
c) FFT of Right Ear TEOAE and of Noise
Fig. 4.5 Left 1 Right Ear TEOAE Variation in Subject 'A' 4.4.7. lntersubject Variation of Ultrasonic Bone-Conduction Stimulated TEOAEs Conventional TEOAEs are as distinct as fingetprints; each individual ear generates a cochlear response of unique shape, amplitude, duration and frequency spectrum (Ryan and Kemp. 1996). Fig.4.6a. shows the differences in the tirnedomain waveforms of TEOAEs recorded in one subject's right ear and in another subject's left ear. Significant differences can also be observed in their FFT plots (Fig. 4.6b. and c). The same stimulus (40 kHz, 1 rns long, 0.2 ms rise/fall time, 200 V,, amplitude) as in Our previous experiment was delivered to subject 'T' left ear and subject 'G' right ear. These results are consistent with intersubject variability of TEOAE recording with airborne stimuli (Kemp et al, 1990).
4.4.8. Subject Posture Effect on TEOAE Measurement. During the first investigation session we recorded ultrasonic bone-elicited emissions in the subjects when sitting on an examination bed (vertical posture). The bed was readjusted during the second session so the subjects would lay in the supine position. We took special care not to change the placement of the bone conductor on the subject's mastoid process or the probe position in the ear canal during the test sessions. There was a 5-minute interval between test sessions. We found significant differences between the two recordings. As it can be seen from Fig. 4.7, the overall aspect of the emissions has changed. The amplitude of the low frequency components has decreased, meanwhile the amplitude of the medium and high frequency components has increased when the subject's position was changed from vertical to supine, as can be seen on the FFT plots of the recordings. Similar results have been obtained on al1 three test subjects. TEOAEs of Subject 'A' Left Ear and Subject 'B' Right Ear Time ms ~C;~~~;~$~"~==~>~~,,,,,Nv?@??qYcYu?~?$% 2.0 1.5
--- Subject 'A' Left Ear Subjed 'B' RigM Ear I
a) Time-âornain waveform
b) FFT of Subject 'A' Left Ear TEOAE and of Noise
'"\Y7quency kHz c) FFT of Subject 'B' Right Ear TEOAE and of Noise
Fig. 4.6 TEOAE lntersubject Variation Posture Influence on TEOAE, Subject 'B' Right Ear Tirne ms
a) Time-domain Waveform
a 5 Frequency kHz
2\ 3\ b) FFT of TEOAE and of Noise. Subject 'B' Vertical Posture
c) FFT of TEOAE and of Noise, Subject 'B' Supine Posture
Fig. 4.7 Posture Influence on TEOAE This finding contradicts the results reported on previous studies on air- elicited TEOAEs and subject posture. Conventionally elicited TEOAE's amplitude decreases when changing the subject's posture frorn vertical to supine; ultrasonic bone-stimulated TEOAEs show a growth in the amplitude of the high frequency components. Supine posture may in fact enhance ultrasonic signal propagation to the inner ear.
4.5. Stimulus Artifact
The origin of the bone-conducted ultrasonic stimulus artifact is unclear; vibration of the probe in the ear canal, oscillations of the ossicular chain and eardrum, piezoelectric effect of the bone and demodulation in the cochlear fluid or soft tissue can al1 be considered to be underlying factors. A strong audio range artifact can elicit TEOAEs. We conducted a test to measure its amplitude. The common method for determining an airborne tone or click stimulus artifact is to insert the Ear Probe in a standard 2 cc cavity and to measure the amplitude of the signal detected by the probe microphone. This method cannot be used for ultrasonic bone-conducted signals for obvious reasons: there is no head simulator for bone conduction. To determine the amplitude of the signal present in the ear canal due to ultrasonic stimulation we designed Our own method. One subject's ear was plugged with vaseline-impregnated cotton to simulate a 'deaf ear, emitting no TEOAE. The ear probe was inserted into the ear canal and the amount of audio- range signal generated by the ultrasonic stimulus (40 kHz, 1 ms long, 0.2 ms riselfall time, 200 V,, amplitude) was recorded. For comparison, a 1 ms. 5 dB SPL airborne audio-range click was also generated and its corresponding artifact was recorded. Fig. 4.8 shows the plots of the two recordings over the first 10 ms after stimulus onset. Airborne and Bone-conducted Stimulus Artifact Comparison Time ms
- - - - Airborne Click -"Ut " 1, -Bonecanducted Utrasound 1
Fig. 4.8 Airborne Click and Ultrasonic Boneconducted Stimulus Artifact Measured in Subject's 'C' Lefi Ear
TEOAE Recording Time ms
Fig. 4.9 Typical B.C. Ultrasonic Stimulus TEOAE Recording, Subject 'C' Right Ear
Due to the time delay in the additional equipment used for ultrasonic stimulus generation, there is approximately a 1.5 ms lag between the two signals' starting point. Nonetheless, both stimulus artifacts al1 but disappear after about 7 ms. As seen, the amount of airborrte signal generated as a byproduct during ultrasonic bone-conducted stimulation is very low: its amplitude is about three times smaller than the 5 dB SPL audio-range click's amplitude. The 5 dB SPL airborne click is considered too low to elicit detectable TEOAEs. Furthenore, even regular TEOAE recordings show a stimulus artifact of comparable amplitude to that of the 5 dB SPL airborne click (Fig. 4.8 and 4.9). Chapter 5
Conclusions
We proved the validity of our research hypotheses: 1) Ultrasonic bone-conducted stimuli can elicit TEOAEs. 2) We recorded TEOAEs elicited by bone-conducted 40 kHz, 1 ms long stimuli. 3) Ultrasonic bone-elicited TEOAEs present characteristics similar to conventional TEOAEs with respect to stimulus artifact, amplitude and duration of the response. In the process, a new bone conductor with a wideband frequency response and a suitable signal amplifier, as well as a stimulus generation board have been built and tested.
5.1. Features of Ultrasonic Bone-Conduction Stimulated TEOAEs Ultrasonic bone-conduction stimulated TEOAEs present characteristics cornmon to audio tone- or click-elicited TEOAEs with respect to amplitude, duration, ear specificity and dependency on stimulus amplitude. The BC ultrasonic stimulation TEOAE frequency spectrum is wider, with stronger medium and high frequency components. This could lead to a major improvement in the TEOAE investigation method. Airborne stimuli's inability to elicit strong high frequency cochlear emissions could be overcome by presenting ultrasonic bone- conducted stimuli using our new wideband transducer. The stimulus artifact, although still present, has a much lower amplitude than the corresponding audio-range click evoked TEOAEs (Fig. 4.9). It is about 50 dB smaller than the stimulus artifact recorded during audio-range, airborne or bone-conducted, stimulation. This significantly reduces the required dynamic range of the microphone preamplifier and can potentially improve on recording accuracy and signal to noise ratio.
5.2. Prospects for the Bone-Conduction Stimulated TEOAE Method as an Audiological Assessrnent Tool
During our study, we made no atternpt to directly link the amplitude and frequency spectrum of the emissions we recorded to the hearing characteristics of Our subjects. Clinical investigation on subjects with various hearing disorders should be undertaken to fully explore this method's potential as an audiometric assessrnent tool. One major application could be rapid screening of personnel at risk for occupational hearing loss (Lucertini et al, 1996). Another contribution can be made on monitoring hearing loss in patients with ototoxic drug exposure; the outer hair cell damage first takes place in the high frequency area and are difFicult to detect at early stages using conventional pure-tone audiometry and conventional TEOAE recording. Reports on ultrasonic threshold of hearing suggest that this category of subjects can be monitored using bone-conducted ultrasound (Abramovich, 1978). References
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