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Measurements of Earplug Attenuation Under Supra-Aural and Circumaural Headphones

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Measurements of Earplug Attenuation Under Supra-Aural and Circumaural Headphones International Journal of Audiology 2012; 51: 730–738 Original Article Measurements of earplug attenuation under supra-aural and circumaural headphones Jennifer B. Tufts* , Jillian V. Palmer * & Lynne Marshall† *Department of Communication Sciences, University of Connecticut, Storrs, USA, and †Naval Submarine Medical Research Laboratory, Groton, Connecticut, USA Abstract Objective: Supra-aural audiometric headphones are generally not recommended for use in measuring the attenuation of earplugs, because contact between the headphone and pinna and/or earplug could alter the attenuation obtained, and because of concerns of non-comparability between modes of excitation from supra-aural headphones and the sound-fi eld procedure required by the standardized method. In this study, we compared measurements of earplug attenuation obtained under Telephonics TDH-50P supra-aural headphones with measurements obtained under circumaural headphones designed expressly for such testing. Design: The attenuation of three types of earplugs (foam, premolded quadruple-fl ange, and custom-molded) was measured in a repeated-measures design. Study sample: The study sample comprised 42 normal-hearing adults (21 females, 21 males). Results: With the foam earplugs, nearly all of the attenuation measurements under the supra-aural headphones fell within 10 dB of the measurements under the circumaural headphones. With the fl ange and custom earplugs, approximately 10% of individuals obtained spuriously high attenuation under the supra-aural headphones. Conclusions: We conclude that standard supra-aural audiometric headphones are suitable for measuring the attenuation provided by foam earplugs. However, supra-aural headphones should not be used to measure the attenuation of fl ange or custom-molded earplugs. The potential exists for substantial over-estimation of attenuation, especially of custom plugs. Key Words: Hearing protection; HPD; earplugs; attenuation; fi t-testing; supra-aural; circumaural; headphones; TDH For personal use only. The amount of protection one receives from hearing protection and attenuation needs. Fit-testing can also provide documentation of devices (HPDs) cannot be known with certainty unless the amount HPD adequacy in cases of possible noise-induced threshold shift, for of attenuation is measured while the HPDs are being worn. HPD defense against workers ’ compensation claims, for compliance with packaging is labeled with an attenuation rating, such as the Noise federal or state regulations, and for assessment of overall HPD effec- Reduction Rating (NRR; used in the U.S.A.) or the Single Number tiveness in a hearing conservation program. Methods of HPD fi t- Rating (SNR; used in the EU). However, it has long been known that testing include microphone-in-real-ear (MIRE) procedures, loudness these laboratory-based ratings do not correspond to the attenuation balancing procedures, and real-ear-attenuation-at-threshold (REAT) achieved by typical end-users in real-world environments. The NRR, procedures (Berger et al, 2008; Hager, 2011; Schulz, 2011). MIRE for example, notoriously overestimates the attenuation achieved by and loudness-balancing procedures are described elsewhere in the most end-users, especially with earplugs (Berger, 2000). For this literature and are not discussed in this paper. reason, both the U.S. Occupational Safety and Health Administration The REAT procedure is documented in past and current ANSI Int J Audiol Downloaded from informahealthcare.com by University of Connecticut on 10/23/12 and the U.S. National Institute for Occupational Safety and Health standards (ANSI 12.6) and is considered the “ gold standard ” for recommend that the NRR be derated when it is used to estimate measuring attenuation (Berger, 1986; ANSI S12.6-2008). Briefl y, in the adequacy of hearing protectors in particular noise environments. the REAT procedure, a listener ’ s hearing thresholds are measured Derating provides a more realistic estimate of the amount of attenu- in the sound fi eld with and without his/her ears occluded by HPDs. ation that an individual is likely to achieve, but it is not a substitute The difference between the “ open ” and “ occluded ” thresholds at a for individual fi t-testing. particular frequency is the amount of attenuation at that frequency. Individual HPD fi t-testing measures the attenuation achieved Attenuation per frequency can be reported, or the attenuation values by an individual wearing a particular HPD in a particular way at a at the different test frequencies can be used to compute a single- particular time. Fit-testing has several applications, as discussed by number measure of attenuation such as an NRR. When fi eld mea- Berger (2006), Berger et al (2008), Hager (2011), and Schulz (2011). surements are conducted on individuals, the data at the individual It can be used to help train employees to wear their HPDs correctly, test frequencies may be summarized in terms of an analogous single- to help trainers improve the fi t for an employee, and to guide the number value for the individual often called a personal attenuation assignment of HPDs to an individual based on his/her noise exposure rating (PAR). There is no standardized way to calculate a PAR; one Correspondence: Jennifer Tufts, Department of Communication Sciences, University of Connecticut, 850 Bolton Road, Unit 1085, Storrs, CT 06269-1085, USA. E-mail: jennifer. [email protected] (Received 2 September 2011 ; accepted 18 May 2012 ) ISSN 1499-2027 print/ISSN 1708-8186 online © 2012 British Society of Audiology, International Society of Audiology, and Nordic Audiological Society DOI: 10.3109/14992027.2012.696217 Earplug attenuation under headphones 731 fi ttings of four earplug types (foam, two types of premolded, and Abbreviations custom-molded). Differences between headphone and sound-fi eld HPD Hearing protection device measurements of 10 dB or less were considered acceptable, given NRR Noise reduction rating the variability inherent in hearing threshold level measurement. Of PAR Personal attenuation rating the 32 headphone-sound fi eld comparisons at 500 Hz (four sub- REAT Real-ear-attenuation-at-threshold jects ϫ four earplug types ϫ two fi ttings), 24 (i.e. 75%) met this SNR Single number rating criterion. In four of the remaining eight cases, the supra-aural head- TDH Telephonics Dynamic Headphone phone attenuation was more than 10 dB higher than the sound-fi eld attenuation. Those points were of concern, because they represented situations in which the supra-aural headphone REAT considerably overestimated the true REAT. They included two improper fi ts of published method is described later in this paper. The PAR is simi- one of the premolded plugs, an improper fi t of the foam plug, and lar in some respects to the well-known NRR. One major difference a proper fi t of the custom earplug. Berger (1984) stated that the between the two metrics is that the PAR represents the attenuation agreement between supra-aural headphone REAT and sound-fi eld achieved by an individual, whereas the NRR is an average across a REAT was better than might have been expected given the concerns group of test subjects, minus two standard deviations. listed above. Nevertheless, for some individuals, REAT testing with While REAT testing in the sound fi eld according to ANSI 12.6- supra-aural headphones may yield large overestimates of earplug 2008 is the gold standard, REAT can also be done under headphones. attenuation. Despite the promising results, the study was too small Testing under headphones is desirable when noise levels in the envi- to draw fi rm conclusions about the suitability of supra-aural head- ronment preclude testing with loudspeakers in a sound fi eld or when phones for use in REAT testing. a sound-fi eld set-up is not available or convenient. Such a scenario In the current study, we compared REAT under supra-aural head- might occur if REAT testing is done onsite as part of a workplace phones to REAT under circumaural headphones. In real-world fi t- hearing conservation program, for example. With headphone REAT, testing settings, REAT with circumaural headphones is accepted as of course, earplugs are the only type of HPD that can be evaluated. a proxy for sound-fi eld REAT. The comparison between supra-aural It should be noted that headphone REAT and sound-fi eld REAT yield and circumaural measurements is therefore a comparison between similar, though not identical, attenuation values. (1) a convenient and accessible headphone REAT paradigm that is Headphone REAT systems are commercially available (Hager, often discredited (supra-aural), and (2) an accepted headphone REAT 2011). The headphones used with these systems are circumaural, paradigm that is less widely available. We used identical threshold- i.e. designed to rest on areas of the skull surrounding the pinna, testing procedures across test conditions to isolate the effect of the with deep earcups to prevent contact with the earplug. Despite the structure and human interface of these two headphones. Our test availability of circumaural headphone REAT systems, their use is subjects were not highly trained and could be considered typical of not widespread at this time. Possible reasons could include the some- the general population of HPD users. For personal use only. what time-consuming nature of the REAT test or the expense of the Overall, the current study addressed the following questions: To fi t-testing system. what extent do measurements of earplug attenuation under supra- Nevertheless, audiologists
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