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Nov 1990

Volume 88, Issue S1, pp. S1-S200

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back to top Session 2NS: Noise and Psychological and Physiological Acoustics: Recent Developments in Hearing Conservation Programs and Hearing Protection Devices
Invited Papers
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Developments in using the boundary element method (BEM) to investigate the dynamic characteristics for foam earplug‐earcanal and earmuff‐earcanal systems (A)

Larry H. Royster, Kassem Mourad, Ke Jun Xie, and Robert Ciskowski

J. Acoust. Soc. Am. Volume 88, Issue S1, pp. S9-S9 (1990); (1 page)

Online Publication Date: 14 Aug 2005

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The boundary element method (BEM) has been utilized to formulate a three‐dimensional math model for the foam earplug‐earcanal and earmuff‐earcanal systems. The BEM technique consists of transforming the partial differential equation describing the behavior of the unknowns, inside and on the boundary of the domain, into an integral equation relating only boundary values. In representing the viscoelastic properties of the foam earplug, both integer differential operator and fractional operator constitutive equations were utilized. The BEM model was then utilized to study the steady‐state and transient responses (including the prediction of each system's insertion loss) for different earplug‐earcanal and earmuff‐earcanal configurations. The basic BEM model has demonstrated the ability to predict the internal resonances for a production earmuff shell‐earcanal system and the measured insertion loss of a foam earplug‐earcanal system subjected to an impulse. Together with these findings a general discussion of the need for improvements in the ability to model a variety of earplug‐ and earmuff‐earcanal configurations will be presented.
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Speech intelligibility measures on level‐dependent hearing protectors (A)

Sharon M. Abel

J. Acoust. Soc. Am. Volume 88, Issue S1, pp. S9-S10 (1990); (2 pages)

Online Publication Date: 14 Aug 2005

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This research compares signal detection and speech perception in subjects wearing either conventional level‐independent muff‐style hearing protectors (E‐A‐R 3000 and BILSOM 2315) or their level‐dependent counterparts (E‐A‐R 9000 and BILSOM 2390). Three groups of 20 subjects, two with normal hearing, aged 20 to 35 yr and 40 to 60 yr, and one with moderate bilateral sensorineural hearing loss, in the older age category, are being assessed. The measurements include detection thresholds for one‐third octave bands within the audible frequency range, consonant discrimination, and the recognition of words in sentences, in quiet and in a background sound pressure level of 75 dB cable swager noise. Speech items are presented at 80 dB SPL. The results obtained thus far for normal subjects indicate that, except for the BILSOM 2390 at 4000 Hz, detection in noise remains the same or is slightly enhanced in the occluded relative to the unoccluded condition. While speech intelligibility decreases precipitously in noise, the wearing of three of the devices, the E‐A‐R muffs and BILSOM 2315, mitigates the effect. Aging, without concomitant hearing loss, is not a statistically significant factor. [Work supported by Ont. Min. Labour.]
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Assessment of insert earphones for the use in occupational hearing conservation program monitoring audiometry (A)

Julia Doswell Royster, Elliott H. Berger, and Larry H. Royster

J. Acoust. Soc. Am. Volume 88, Issue S1, pp. S10-S10 (1990); (1 page)

Online Publication Date: 14 Aug 2005

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The application of insert earphones for annual audiometry was evaluated in an on‐going occupational hearing conservation program. For four consecutive years employees were given an extra left‐ear audiogram using a 3A‐type insert earphone immediately after completing their standard annual audiograms using TDH‐49 earphones in supra‐aural cushions. For the final 3 yr an additional left‐ear audiogram using the TDH‐49 earphone was given last, to control for order effects. Sixty employees participated all 4 yr. Analysis of thresholds obtained each year using each earphone indicated that small threshold correction factors for the insert earphone are necessary at each frequency (not just at 6 and 8 kHz) to allow results to be compared directly to supra‐aural earphone thresholds. The manufacturer's suggested 10‐dB correction factor at 8 kHz appeared too small for the particular combination of TDH‐49 and insert earphones used. Standard deviations of thresholds between individuals were similar for both earphones. Year‐to‐year repeatability of thresholds within individuals was equivalent with both earphones. As long as absolute calibration is accounted for, insert earphones are a viable alternative transducer for audiometry in hearing conservation.
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Determination of occupational noise exposure limits for very high intensity impulses when hearing protection is used (A)

James H. Patterson, Jr. and Daniel L. Johnson

J. Acoust. Soc. Am. Volume 88, Issue S1, pp. S10-S10 (1990); (1 page)

Online Publication Date: 14 Aug 2005

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The U.S. Army needs validation of safe limits for exposure to impulse noise produced by heavy weapons. Current impulse noise limits are based on data from small arms. Recent studies indicate these standards may be overly conservative. In order to define new limits, a systematic study of the effects of high‐intensity impulse noise on human volunteers is underway. The number of impulses, the peak pressure level, and spectral distribution of energy are being varied systematically. Four groups of at least 60 volunteers will be given a series of exposures to one of four impulse types. The impulse spectrum is varied by changing the distance between the volunteer and an explosive detonation. The peak pressure level is varied in 3‐dB steps by changing the weight of the explosive charge. The number of impulses is 6, 12, 25, 50, or 100. Volunteers wear hearing protection for all exposures. After each exposure, TTS is determined. Each volunteer starts with an exposure of six impulses at the lowest intensity. If the TTS is less than 15 dB, the subject receives six impulses at the next higher level the next day. This continues through all intensities. Then the number of impulses is increased using the maximum intensity permitted by nonauditory injury limits. The first group, using an earmuff as a protective device, has completed all exposures at the first impulse detonation distance of 5 m. The peak sound pressure levels varied from 172 to 192 dB with A durations of approximately 3 ms and B durations of approximately 20 m. No TTS in excess of 15 dB was observed for any condition. The percentages of volunteers unwilling to undergo the higher intensity exposures also will be presented.
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Results of a pilot interlaboratory comparison of REAT measurements of hearing protectors, coordinated by ANSI S12/WG11 (A)

Elliott H. Berger, Julia D. Royster, John G. Casali, Carol J. Merry, Ben T. Mozo, and Larry H. Royster

J. Acoust. Soc. Am. Volume 88, Issue S1, pp. S11-S11 (1990); (1 page)

Online Publication Date: 14 Aug 2005

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ANSI working group S12/WG11 has been tasked with developing laboratory and/or field procedures that yield useful estimates of the real‐world attenuation of hearing protection devices. The first goal selected was the development of a laboratory‐based real‐car attenuation at threshold (REAT) protocol that would estimate the protection that can be, or is being, obtained in the top 10%–20% of today's hearing conservation programs. A protocol was developed based upon ANSI S 12.6‐1984, but with modified subject selection, fitting, and training procedures that were described explicitly and in substantially greater detail than in the standard. Pilot testing of two earplugs in four laboratories with ten naive subjects per facility showed subject‐fit attenuation results comparable to available real‐world studies. Although subjects achieved significantly greater attenuation after the experimenter demonstrated how to insert the devices (informed user fit), the interlaboratory reproducibility of the group data was not significantly improved. The indications are that a subject‐fit protocol, in which experimenter involvement is minimized, yields the best estimates of real‐world attenuation, and still provides acceptable reproducibility. The pilot results suggested refinements in the design of a full‐scale interlaboratory comparison which should begin in the later part of 1990.
Contributed Papers
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Significant threshold shifts and follow‐up audiograms in the Army Hearing Conservation Program (A)

Amy M. Donahue and Eileen K. Resta

J. Acoust. Soc. Am. Volume 88, Issue S1, pp. S11-S11 (1990); (1 page)

Online Publication Date: 14 Aug 2005

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This paper examines the Department of the Army's (DA) audiometric follow‐up program subsequent to identifying standard threshold shift (STS). The Occupational Safety and Health Administration's current policy has provisions for one optional follow‐up audiometric test to be completed within 30 days of identifying an STS. Current DA policy requires two follow‐up audiometric tests to be conducted within 60 days of identifying the STS. Successes and failures of the DA policy are presented and discussed using 387 000 annual and follow‐up audiograms obtained on noise‐exposed civilians and soldiers and entered into the Hearing Evaluations Automated Registry System (HEARS) audiometric database. Program participation rates, elapsed time for completion of the follow‐up audiograms, and noted changes in STS identified in the follow‐up will be presented. The results have implications for hearing conservation policy decisions on follow‐up audiometric requirements.
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Simulation of the effects of high‐frequency hearing impairment in noise and estimation of the effects of hearing aids incorporating filtering of low‐frequency components in the presence of background noise (A)

Vishakha W. Rawool

J. Acoust. Soc. Am. Volume 88, Issue S1, pp. S11-S11 (1990); (1 page)

Online Publication Date: 14 Aug 2005

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An attempt was made to simulate the effects of high‐frequency hearing impairment in “noisy” situations. It was assumed that the difficulties experienced by individuals with high‐frequency hearing impairment in noise can be simulated, if in addition to the spectrum shaping of speech, the noise was also filtered. Such filtered noise, containing mainly low‐frequency elements was expected to effectively mask the only low‐frequency cues available after filtering the high‐frequency elements from the speech spectrum. The speech discrimination ability of normal individuals was also assessed in a condition where both the speech and noise were low pass filtered at 500 Hz. This condition was presented to simulate the effects of hearing aids incorporating noise‐suppressor switches that filter out both the noise and the low‐frequency speed cues in the presence of background noise. Detailed analyses of the results will be discussed.
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Sound attenuation evaluation of four prototype helmet‐integrated ANR systems (A)

C. E. Williams, D. W. Maxwell, and G. B. Thomas

J. Acoust. Soc. Am. Volume 88, Issue S1, pp. S11-S11 (1990); (1 page)

Online Publication Date: 14 Aug 2005

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Current Navy flight helmets do not provide sufficient passive sound attenuation of the high‐intensity, low‐frequency noise present in some naval aircraft. As part of a program to provide improved hearing protection for aviators and aircrew personnel, the Naval Aerospace Medical Research Laboratory conducted sound attenuation evaluations of four helmet‐integrated active noise reduction (ANR) systems. Sound attenuation measurements (passive attenuation and combined passive/active attenuation) were obtained on Navy and Marine Corps student aviators wearing the prototype helmets in a laboratory noise environment. A Knowles miniature microphone was used to obtain sound levels at the subject's ear. Three sets of measurements were obtained on each subject for each of the prototype systems. A comparison of the combined passive/active attenuation measurements with the passive attenuation measurements reveal increased sound attenuation at 125,250, and 500 Hz and decreased attenuation at 1000 and 2000 Hz. Speech intelligibility measurements obtained on one of the prototypes demonstrated improved speech intelligibility at high sound pressure levels (105 dB and 115 dB) in the ANR “on” mode. [Work supported by the Naval Air Development Center.]
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