Journal of Occupational and Environmental Hygiene, Mar 6, 2017
The National Institute for Occupational Safety and Health (NIOSH) sponsored tests of three earplu... more The National Institute for Occupational Safety and Health (NIOSH) sponsored tests of three earplug fittest systems (NIOSH HPD Well-Fit™, Michael & Associates FitCheck, and Honeywell Safety Products VeriPRO ®). Each system was compared to laboratory-based real-ear attenuation at threshold (REAT) measurements in a sound field according to ANSI/ASA S12.6-2008 at the NIOSH, Honeywell Safety Products, and Michael & Associates testing laboratories. An identical study was conducted independently at the U.S. Army Aeromedical Research Laboratory (USAARL), which provided their data for inclusion in this report. The Howard Leight Airsoft premolded earplug was tested with twenty subjects at each of the four participating laboratories. The occluded fit of the earplug was maintained during testing with a soundfield-based laboratory REAT system as well as all three headphone-based fittest systems. The Michael & Associates lab had highest average A-weighted attenuations and smallest standard deviations. The NIOSH lab had the lowest average attenuations and the largest standard deviations. Differences in octave-band attenuations between each fittest system and the American National Standards Institute (ANSI) sound field method were calculated (Atten fittest -Atten ANSI). A-weighted attenuations measured with FitCheck and HPD Well-Fit systems demonstrated approximately ±2 dB agreement with the ANSI sound field method, but A-weighted attenuations measured with the VeriPRO system underestimated the ANSI laboratory attenuations. For each of the fittest systems, the average Aweighted attenuation across the four laboratories was not significantly greater than the average of
Exposure of the auditory system to either continuous or interrupted nontraumatic noises, often co... more Exposure of the auditory system to either continuous or interrupted nontraumatic noises, often collectively referred to as priming exposures, has been shown, in a number of experimental paradigms, to reduce the susceptibility of the auditory system to noiseinduced hearing and sensory cell loss from a subsequent traumatic exposure. Using auditory evoked potentials to obtain pure-tone thresholds and cochleograms to quantify sensory cell losses, the issue of priming-induced protective effects was examined in the chinchilla. Priming was accomplished with either a continuous noise or with a continuous noise followed by an interrupted noise. Trauma was induced by exposure to high-level impacts over a 5-day period that resulted in an asymptotic threshold shift. A comparison of the two groups of primed subjects with an unprimed control group showed that there were some statistically significant reductions in the asymptotic response of the primed groups to the traumatic exposure but no differences in permanent changes in thresholds among the three groups 30 days following the traumatic exposure. There were, however, some statistically significant, frequency-specific, reductions in outer hair cell loss in the primed groups. When conditioning was followed by the interrupted exposure that produced a threshold shift toughening effect, the conditioning protocol had no effect on the response of subjects to the interrupted exposure. There were also no differences in thresholds or sensory cell loss between the two primed groups 30 days post-trauma. Priming protocols may have different effects on the development of noise-induced trauma that are dependent on the nature of the traumatic stimulus, that is, long-term high-level impact noise exposure versus acute continuous noise exposure.
A computer-automated, threshold-tracking procedure was developed for measuring evoked response th... more A computer-automated, threshold-tracking procedure was developed for measuring evoked response thresholds on-line. The procedure utilizes an objective detection criterion (correlation) to evaluate whether a response is absent or present. Sound intensity is adjusted according to a modified PEST (parameter estimation by sequential testing) procedure in order to estimate threshold. The computer-automated procedures were used to obtain objective estimates of evoked response thresholds in normal and hearing-impaired chinchillas. The evoked response waveforms stored in the computer were also used to obtain visual judgments of threshold. The objective thresholds determined by the computer were virtually identical to the visual detection thresholds. Thus, the computer-automated procedure provides a reliable, objective, and efficient method of estimating evoked response thresholds.
Journal of the Acoustical Society of America, Jul 1, 1991
Permanent threshold shifts obtained from 242 chinchillas that were exposed to various impulse noi... more Permanent threshold shifts obtained from 242 chinchillas that were exposed to various impulse noise paradigms have been related to the energy spectra of the impulses. The impulses were generated by three different shock tubes that produced impulse noise spectra whose A-weighted energies showed peaks at 0.25, 1, and 2 kHz. The results show that there is an increasing susceptibility to NIPTS as the audiometric test frequency increases from 0.5 to 16 kHz. This increase in susceptibility to NIPTS is further accentuated by approximately 5 to 10 dB for impulses whose spectra peak at 2 kHz.
Journal of the Acoustical Society of America, Mar 1, 2018
Anticipatory middle ear muscle contractions (MEMC) have been implemented as protective components... more Anticipatory middle ear muscle contractions (MEMC) have been implemented as protective components of Damage-Risk Criteria (DRC) for impulsive noises. However, no studies have shown that anticipatory MEMC are pervasive among humans. This presentation describes a series of studies of the viability of assumed anticipatory MEMC obtained either through classical conditioning or while operating a model gun. Participants were adults with normal hearing, and the conditioning tasks varied on sensory modality and attention. Both between- and within-subjects designs were used. A conditioned response was defined as an MEMC occurring prior to the unconditioned stimulus and when only the conditioning stimulus was presented. These studies do not suggest that anticipatory MEMC should be included in DRC for impulsive noises.
Journal of the Acoustical Society of America, Nov 1, 1988
Acoustic emissions were detected using the fast Fourier analysis of sounds in an ear canal during... more Acoustic emissions were detected using the fast Fourier analysis of sounds in an ear canal during injection of alternating currents at different frequencies into the scala media of guinea pigs anaesthetized with urethane (1.2 g/kg i.p.). The amplitude of the distortion products relative to the fundamentals in the emitted sound was about ten times as large as that in the cochlear current, which was already slightly distorted, probably due to nonlinearity at the metal electrode surface. This discrepancy implies a larger additional nonlinearity in the cochlea. During COCB stimulation or after moderately intense sound exposure, the emitted distortion products were suppressed reversibly, in contrast to almost no change in the fundamental emissions. All the emissions were suppressed reversibly after temporary anoxia or furosemide administration, and they disappeared completely after severe acoustic trauma or death. These results suggest that the normal metabolic activity of the cochlea is required for generation of the emissions, and that the electromechanical transduction is less vulnerable than the cochlear nonlinearity.
Journal of the Acoustical Society of America, Oct 25, 2002
Tracked-vehicle crewmembers operate in an acoustical environment that is one of the most inhospit... more Tracked-vehicle crewmembers operate in an acoustical environment that is one of the most inhospitable encountered in the U.S. Army. Overall noise levels in the M1A2 Abrams tank and M2A2 Bradley Fighting Vehicle can be well over 100 dBA and can exceed 130 dB in some octave bands. Due to the nature of the crewmembers’ duties, speech communication is essential for effective operations and this capability is seriously degraded because of the nature of the noise environment. The Communications Earplug (CEP), consisting of miniature receivers attached to replaceable foam earplugs, is used in Army aviation to provide superior noise attenuation with improved speech communication. The noise attenuation of the DH-132A Combat Vehicle Crewmans helmet (CVC) when used in conjunction with CEP (measured in accordance with ANSI S12.6-1997) is reported with an NRR of 27 dB vs 15 dB for the CVC alone. CEPs were installed in the helmets of 77 tracked-vehicle crewmembers. Following a 6-month trial period, crewmembers (n=36) reported increased noise attenuation, improved speech communication, and reduced stress when using the CVC-CEP integration. The CEP is a cost-effective alternative to expensive active noise reduction helmets currently being fielded for use by tracked-vehicle crews. [Work supported in part by USACHPPM.]
Journal of the Acoustical Society of America, 2009
͓͑2002͒. American National Standard Methods for the Measuring Real-Ear Attenuation of Hearing Pro... more ͓͑2002͒. American National Standard Methods for the Measuring Real-Ear Attenuation of Hearing Protectors, American National Standards Institute, New York͔. Six hearing protection devices ͑two earmuffs, foam, premolded, custom-molded earplugs, and canal-caps͒ were tested in six laboratories using the experimenter-supervised, Method A, and ͑naïve͒ subject-fit, Method B, protocols with 24 subjects per laboratory. Within-subject, between-subject, and between-laboratory standard deviations were determined for individual frequencies and A-weighted attenuations. The differences for the within-subject standard deviations were not statistically significant between Methods A and B. Using between-subject standard deviations from Method A, 3-12 subjects would be required to identify 6-dB differences between attenuation distributions. Whereas using between-subject standard deviations from Method B, 5-19 subjects would be required to identify 6-dB differences in attenuation distributions of a product tested within the same laboratory. However, the between-laboratory standard deviations for Method B were −0.1 to 3.0 dB less than the Method A results. These differences resulted in considerably more subjects being required to identify statistically significant differences between laboratories for Method A ͑12-132 subjects͒ than for Method B ͑9-28 subjects͒.
The auditory system, toughened by an interrupted noise exposure, has been shown in several report... more The auditory system, toughened by an interrupted noise exposure, has been shown in several reports to be less affected by (or protected from) a subsequent high-level noise exposure. Exposure to 115 dB peak SPL, 1 kHz narrow band (400 Hz) transients presented 1/s, 6 h/day, to four groups of chinchillas produced a 10^28 dB toughening effect across the 0.5^8.0 kHz test frequency range. Following either a 30 day or an 18 h recovery period the animals were exposed to the same impulses but presented at 121 or 127 dB peak SPL for five uninterrupted days, thus producing an asymptotic threshold shift (ATS) condition. Comparisons between toughened and untoughened control subjects showed: (1) During the 121 dB exposure there was a statistically significant reduction of 10^25 dB in ATS across the entire test frequency range. Thirty days following the 121 dB exposure there were no significant differences in the postexposure permanent effects on thresholds and sensory cell loss. (2) During the 127 dB exposure only the group with the 30 day interval between the toughening and traumatic exposures showed a small (V10 dB), statistically significant, frequency-specific (8 kHz), reduction in ATS. Thirty days following the 127 dB exposure a statistically significant protective effect on threshold was measured only at 16.0 kHz. However, both toughened groups showed less inner hair cell loss at and above 1.0 kHz, while only the group with the 18 h interval between the toughening and traumatic exposures showed less outer hair cell loss at and above 1.0 kHz. There were no systematic differences in the response of the toughened animals that could be attributed to the 30 day or 18 h post-toughening interval.
Objective: To determine whether acoustic reflexes are pervasive (i.e., known with 95 % confidence... more Objective: To determine whether acoustic reflexes are pervasive (i.e., known with 95 % confidence to be observed in at least 95 % of people) by examining the frequency of occurrence using a friction-fit diagnostic middle ear analyzer. Design: A group of 285 adult participants with very good hearing sensitivity underwent audiometric and middle ear testing. Acoustic reflexes were tested ipsilaterally and contralaterally in both ears across a range of elicitor frequencies. Two automated methods were used to detect the presence of an acoustic reflex. Results: There were no conditions in which the proportion of participants exhibiting acoustic reflexes was high enough to be deemed pervasive. Ipsilateral reflexes were more likely to be observed than contralateral reflexes and reflexes were more common at .5 and 1 kHz elicitor frequencies as compared to 2 and 4 kHz elicitor frequencies. Conclusions: Acoustic reflexes are common among individuals with good hearing. However, acoustic reflexes cannot be considered pervasive and should not be included in damage risk criteria and health hazard assessments for impulsive noise.
Abstract Objective: To determine whether acoustic reflexes are pervasive (i.e. sufficiently preva... more Abstract Objective: To determine whether acoustic reflexes are pervasive (i.e. sufficiently prevalent to provide 95% confidence of at least 95% prevalence) and might be invoked in damage-risk criteria (DRC) and health hazard assessments (HHA) for impulsive noise. Design: Cross-sectional analyses of a nationally-representative study. Study sample: National Health and Nutrition Examination Survey (NHANES) data collected between 1999 and 2012 were used. Over 60 thousand reflex traces obtained from 15,106 NHANES participants were used in the study, along with demographic, audiometric, health and exposure variables obtained in that study. Results: Acoustic reflexes were not sufficiently prevalent to be deemed pervasive by any detection method or in any subgroup defined by age or audiometric characteristics. The odds of observing acoustic reflexes were greater for women, young adults, and people with better hearing sensitivity. Abnormally high tympanometric admittance and “Other” race/ethnicity (i.e. people who do not self-identify as exclusively Non-Hispanic White, Non-Hispanic Black, Mexican–American, or Hispanic) were associated with lower odds. Conclusions: Acoustic reflexes are not sufficiently prevalent to be included in DRC and HHA for impulsive noise.
Journal of the Acoustical Society of America, Aug 1, 1996
A normative study of the cubic distortion product emissions from 104 monaural and binaural chinch... more A normative study of the cubic distortion product emissions from 104 monaural and binaural chinchillas was undertaken to establish criteria upon which noise exposed animals could be evaluated. From this normative group, 47 randomly selected chinchillas were exposed to various high level ͑150-, 155-, and 160-dB peak SPL͒ impulse noises. Auditory evoked potentials and cubic distortion product otoacoustic emissions were measured on each animal pre-and post-exposure and related to the sensory cell populations 30 days post-exposure. Both group mean and individual animal data indicated that the distortion product emissions were more sensitive, frequency-specific indices of noise-induced cochlear effects than pure-tone threshold measures. This was particularly evident near the threshold for noise-induced damage to the outer hair cell system.
On the basis of experimental data obtained from 420 noise-exposed animals (chinchilla), the amoun... more On the basis of experimental data obtained from 420 noise-exposed animals (chinchilla), the amount of sensory cell loss has been quantitatively related to the amount of permanent threshold shift at eight audiometric test frequencies between 0.125 and 16 kHz. The noise exposures, which varied extensively in spectrum, intensity and duration, produced permanent threshold shifts that ranged from 0 to 70 dB across a broad range of test frequencies. These data show: (1) consistent outer hair cell losses with less than 5 dB permanent threshold shifts (PTS) across all the test frequencies; (2) the first approximately 30 dB of PTS is established by losses of primarily outer hair cells; (3) in regions of the cochlea that transduce frequencies higher than or equal to 2 kHz, the three rows of outer hair cells show the same degree of loss for a given PTS, while in the 0.5 to 1.0 kHz region of the cochlea, the third row of outer hair cells (OHC) consistently shows less loss than do rows one and two; (4) appreciable inner hair cell (IHC) loss does not begin to appear until PTS exceeds approximately 30 dB; (5) in the virtual absence of OHC, hearing thresholds are least sensitive to IHC loss in the octave band centered at 4 kHz, i.e., the 4 kHz region can be as functional as other areas of the cochlea in spite of a greater amount of damage. The quantitative relation between cell loss and PTS varies as a function of test frequency in an orderly fashion.
Journal of the Acoustical Society of America, 2002
Threshold shifts ͑TS͒ were measured at various times following a wide variety of noise exposures ... more Threshold shifts ͑TS͒ were measured at various times following a wide variety of noise exposures on over 900 chinchillas. An analysis of postexposure TS measures and noise-induced permanent threshold shift ͑PTS͒ showed that, across audiometric test frequency, there was a consistent relation between these variables of the form PTS (dB)ϭ␣(e TS/ Ϫ1), where, for a given test frequency, ␣ ͑dB͒ and  ͑dB͒ are constants. TSs were measured immediately following exposure (TS 0), 24 h after exposure (TS 24), and at several intermediate times in order to estimate the maximum TS (TS max). Correlation between TS and PTS at the various test frequencies was highest for TS 24. An analysis of the 90th-percentile PTS showed a linear growth of PTS with TS 24 of approximately 0.7 dB PTS/dB TS 24. These data provide some support, in the chinchilla model, for a variation of the three postulates originally presented by Kryter et al. ͓J. Acoust. Soc. Am. 39, 451 ͑1966͔͒. Specifically: ͑i͒ TS 24 is a consistent measure of the effects of a traumatic noise exposure. ͑ii͒ All exposures that produce a given TS 24 will be equally hazardous. ͑iii͒ Noise-induced PTS in the most susceptible animals, following many years of exposure, is approximately equal to (0.7)TS 24 measured after an 8-h exposure to the same noise.
A variety of interrupted noise exposure paradigms will produce a toughening effect in the mammali... more A variety of interrupted noise exposure paradigms will produce a toughening effect in the mammalian auditory system. That is, the threshold shift will gradually become smaller with each successive daily exposure. The ability of the system to be toughened has not been explored in subjects with a pre-existing noise-induced hearing loss. Using the chinchilla as the experimental animal, evoked potential audiometry to obtain thresholds, and surface preparation histology to quantify the sensory cell population, the issue of toughening was examined in the noise-damaged auditory system. Toughening was produced by a 1.0 kHz, narrow-band impact at 115 dB peak SPL for 10 days, 6 h/day, and trauma was produced by a 1.0 kHz, narrow-band impact at 121 dB peak SPL for 5 days, 24 h/day. Four groups of animals were used. Group 1: traumatic exposure followed 30 days later by the toughening exposure. Group 2: toughening exposure followed 30 days later by the traumatic exposure. Group 3: a trauma-only control. Group 4: a tougheningonly control. Group 2 that received the toughening exposure 30 days prior to the traumatic exposure showed a 10 to more than 20 dB toughening effect between the 0.5 and 4.0 kHz test frequencies, while Group 1 that received the traumatic exposure followed 30 days later by the toughening exposure showed no toughening. The permanent changes in the evoked response audiograms and sensory cell populations were the same in Groups 1, 2 and 3 that were exposed to the traumatic noise, regardless of whether or not the animals were ever subjected to the toughening noise or whether the toughening noise preceded or followed the traumatic noise.
Journal of the Acoustical Society of America, May 1, 1993
Existing criteria for safe exposure to impulse noise do not consider the frequency spectrum of an... more Existing criteria for safe exposure to impulse noise do not consider the frequency spectrum of an impulse as a variable in the evaluation of the hazards to the auditory system. This study was designed to determine the relative potential that impulsive energy concentrated at different frequencies has in causing auditory system trauma. One hundred and thirty (130) chinchillas, divided into 22 groups of 5 to 7 animals, were used. Pre- and postexposure audiograms were measured on each animal using avoidance conditioning procedures. Quantitative histology was used to determine the extent and pattern of the sensory cell damage (cochleograms). Noise exposure employed seven different computer-generated narrow-band impulses (approximately 400-Hz bandwidth) having center frequencies located at 0.260, 0.775, 1.025, 1.350, 2.075, 2.450, and 3.550 kHz, presented at two to four different intensities. An isohazard weighting function derived from the audiometric and histological data demonstrates that equivalent amounts of impulsive energy concentrated at different frequencies are not equally hazardous to the auditory system. Comparison of the derived weighting function with the A-weighting curve indicates that low-frequency impulses are less hazardous than predicted by A-weighted sound exposure level.
Journal of Occupational and Environmental Hygiene, Mar 6, 2017
The National Institute for Occupational Safety and Health (NIOSH) sponsored tests of three earplu... more The National Institute for Occupational Safety and Health (NIOSH) sponsored tests of three earplug fittest systems (NIOSH HPD Well-Fit™, Michael & Associates FitCheck, and Honeywell Safety Products VeriPRO ®). Each system was compared to laboratory-based real-ear attenuation at threshold (REAT) measurements in a sound field according to ANSI/ASA S12.6-2008 at the NIOSH, Honeywell Safety Products, and Michael & Associates testing laboratories. An identical study was conducted independently at the U.S. Army Aeromedical Research Laboratory (USAARL), which provided their data for inclusion in this report. The Howard Leight Airsoft premolded earplug was tested with twenty subjects at each of the four participating laboratories. The occluded fit of the earplug was maintained during testing with a soundfield-based laboratory REAT system as well as all three headphone-based fittest systems. The Michael & Associates lab had highest average A-weighted attenuations and smallest standard deviations. The NIOSH lab had the lowest average attenuations and the largest standard deviations. Differences in octave-band attenuations between each fittest system and the American National Standards Institute (ANSI) sound field method were calculated (Atten fittest -Atten ANSI). A-weighted attenuations measured with FitCheck and HPD Well-Fit systems demonstrated approximately ±2 dB agreement with the ANSI sound field method, but A-weighted attenuations measured with the VeriPRO system underestimated the ANSI laboratory attenuations. For each of the fittest systems, the average Aweighted attenuation across the four laboratories was not significantly greater than the average of
Exposure of the auditory system to either continuous or interrupted nontraumatic noises, often co... more Exposure of the auditory system to either continuous or interrupted nontraumatic noises, often collectively referred to as priming exposures, has been shown, in a number of experimental paradigms, to reduce the susceptibility of the auditory system to noiseinduced hearing and sensory cell loss from a subsequent traumatic exposure. Using auditory evoked potentials to obtain pure-tone thresholds and cochleograms to quantify sensory cell losses, the issue of priming-induced protective effects was examined in the chinchilla. Priming was accomplished with either a continuous noise or with a continuous noise followed by an interrupted noise. Trauma was induced by exposure to high-level impacts over a 5-day period that resulted in an asymptotic threshold shift. A comparison of the two groups of primed subjects with an unprimed control group showed that there were some statistically significant reductions in the asymptotic response of the primed groups to the traumatic exposure but no differences in permanent changes in thresholds among the three groups 30 days following the traumatic exposure. There were, however, some statistically significant, frequency-specific, reductions in outer hair cell loss in the primed groups. When conditioning was followed by the interrupted exposure that produced a threshold shift toughening effect, the conditioning protocol had no effect on the response of subjects to the interrupted exposure. There were also no differences in thresholds or sensory cell loss between the two primed groups 30 days post-trauma. Priming protocols may have different effects on the development of noise-induced trauma that are dependent on the nature of the traumatic stimulus, that is, long-term high-level impact noise exposure versus acute continuous noise exposure.
A computer-automated, threshold-tracking procedure was developed for measuring evoked response th... more A computer-automated, threshold-tracking procedure was developed for measuring evoked response thresholds on-line. The procedure utilizes an objective detection criterion (correlation) to evaluate whether a response is absent or present. Sound intensity is adjusted according to a modified PEST (parameter estimation by sequential testing) procedure in order to estimate threshold. The computer-automated procedures were used to obtain objective estimates of evoked response thresholds in normal and hearing-impaired chinchillas. The evoked response waveforms stored in the computer were also used to obtain visual judgments of threshold. The objective thresholds determined by the computer were virtually identical to the visual detection thresholds. Thus, the computer-automated procedure provides a reliable, objective, and efficient method of estimating evoked response thresholds.
Journal of the Acoustical Society of America, Jul 1, 1991
Permanent threshold shifts obtained from 242 chinchillas that were exposed to various impulse noi... more Permanent threshold shifts obtained from 242 chinchillas that were exposed to various impulse noise paradigms have been related to the energy spectra of the impulses. The impulses were generated by three different shock tubes that produced impulse noise spectra whose A-weighted energies showed peaks at 0.25, 1, and 2 kHz. The results show that there is an increasing susceptibility to NIPTS as the audiometric test frequency increases from 0.5 to 16 kHz. This increase in susceptibility to NIPTS is further accentuated by approximately 5 to 10 dB for impulses whose spectra peak at 2 kHz.
Journal of the Acoustical Society of America, Mar 1, 2018
Anticipatory middle ear muscle contractions (MEMC) have been implemented as protective components... more Anticipatory middle ear muscle contractions (MEMC) have been implemented as protective components of Damage-Risk Criteria (DRC) for impulsive noises. However, no studies have shown that anticipatory MEMC are pervasive among humans. This presentation describes a series of studies of the viability of assumed anticipatory MEMC obtained either through classical conditioning or while operating a model gun. Participants were adults with normal hearing, and the conditioning tasks varied on sensory modality and attention. Both between- and within-subjects designs were used. A conditioned response was defined as an MEMC occurring prior to the unconditioned stimulus and when only the conditioning stimulus was presented. These studies do not suggest that anticipatory MEMC should be included in DRC for impulsive noises.
Journal of the Acoustical Society of America, Nov 1, 1988
Acoustic emissions were detected using the fast Fourier analysis of sounds in an ear canal during... more Acoustic emissions were detected using the fast Fourier analysis of sounds in an ear canal during injection of alternating currents at different frequencies into the scala media of guinea pigs anaesthetized with urethane (1.2 g/kg i.p.). The amplitude of the distortion products relative to the fundamentals in the emitted sound was about ten times as large as that in the cochlear current, which was already slightly distorted, probably due to nonlinearity at the metal electrode surface. This discrepancy implies a larger additional nonlinearity in the cochlea. During COCB stimulation or after moderately intense sound exposure, the emitted distortion products were suppressed reversibly, in contrast to almost no change in the fundamental emissions. All the emissions were suppressed reversibly after temporary anoxia or furosemide administration, and they disappeared completely after severe acoustic trauma or death. These results suggest that the normal metabolic activity of the cochlea is required for generation of the emissions, and that the electromechanical transduction is less vulnerable than the cochlear nonlinearity.
Journal of the Acoustical Society of America, Oct 25, 2002
Tracked-vehicle crewmembers operate in an acoustical environment that is one of the most inhospit... more Tracked-vehicle crewmembers operate in an acoustical environment that is one of the most inhospitable encountered in the U.S. Army. Overall noise levels in the M1A2 Abrams tank and M2A2 Bradley Fighting Vehicle can be well over 100 dBA and can exceed 130 dB in some octave bands. Due to the nature of the crewmembers’ duties, speech communication is essential for effective operations and this capability is seriously degraded because of the nature of the noise environment. The Communications Earplug (CEP), consisting of miniature receivers attached to replaceable foam earplugs, is used in Army aviation to provide superior noise attenuation with improved speech communication. The noise attenuation of the DH-132A Combat Vehicle Crewmans helmet (CVC) when used in conjunction with CEP (measured in accordance with ANSI S12.6-1997) is reported with an NRR of 27 dB vs 15 dB for the CVC alone. CEPs were installed in the helmets of 77 tracked-vehicle crewmembers. Following a 6-month trial period, crewmembers (n=36) reported increased noise attenuation, improved speech communication, and reduced stress when using the CVC-CEP integration. The CEP is a cost-effective alternative to expensive active noise reduction helmets currently being fielded for use by tracked-vehicle crews. [Work supported in part by USACHPPM.]
Journal of the Acoustical Society of America, 2009
͓͑2002͒. American National Standard Methods for the Measuring Real-Ear Attenuation of Hearing Pro... more ͓͑2002͒. American National Standard Methods for the Measuring Real-Ear Attenuation of Hearing Protectors, American National Standards Institute, New York͔. Six hearing protection devices ͑two earmuffs, foam, premolded, custom-molded earplugs, and canal-caps͒ were tested in six laboratories using the experimenter-supervised, Method A, and ͑naïve͒ subject-fit, Method B, protocols with 24 subjects per laboratory. Within-subject, between-subject, and between-laboratory standard deviations were determined for individual frequencies and A-weighted attenuations. The differences for the within-subject standard deviations were not statistically significant between Methods A and B. Using between-subject standard deviations from Method A, 3-12 subjects would be required to identify 6-dB differences between attenuation distributions. Whereas using between-subject standard deviations from Method B, 5-19 subjects would be required to identify 6-dB differences in attenuation distributions of a product tested within the same laboratory. However, the between-laboratory standard deviations for Method B were −0.1 to 3.0 dB less than the Method A results. These differences resulted in considerably more subjects being required to identify statistically significant differences between laboratories for Method A ͑12-132 subjects͒ than for Method B ͑9-28 subjects͒.
The auditory system, toughened by an interrupted noise exposure, has been shown in several report... more The auditory system, toughened by an interrupted noise exposure, has been shown in several reports to be less affected by (or protected from) a subsequent high-level noise exposure. Exposure to 115 dB peak SPL, 1 kHz narrow band (400 Hz) transients presented 1/s, 6 h/day, to four groups of chinchillas produced a 10^28 dB toughening effect across the 0.5^8.0 kHz test frequency range. Following either a 30 day or an 18 h recovery period the animals were exposed to the same impulses but presented at 121 or 127 dB peak SPL for five uninterrupted days, thus producing an asymptotic threshold shift (ATS) condition. Comparisons between toughened and untoughened control subjects showed: (1) During the 121 dB exposure there was a statistically significant reduction of 10^25 dB in ATS across the entire test frequency range. Thirty days following the 121 dB exposure there were no significant differences in the postexposure permanent effects on thresholds and sensory cell loss. (2) During the 127 dB exposure only the group with the 30 day interval between the toughening and traumatic exposures showed a small (V10 dB), statistically significant, frequency-specific (8 kHz), reduction in ATS. Thirty days following the 127 dB exposure a statistically significant protective effect on threshold was measured only at 16.0 kHz. However, both toughened groups showed less inner hair cell loss at and above 1.0 kHz, while only the group with the 18 h interval between the toughening and traumatic exposures showed less outer hair cell loss at and above 1.0 kHz. There were no systematic differences in the response of the toughened animals that could be attributed to the 30 day or 18 h post-toughening interval.
Objective: To determine whether acoustic reflexes are pervasive (i.e., known with 95 % confidence... more Objective: To determine whether acoustic reflexes are pervasive (i.e., known with 95 % confidence to be observed in at least 95 % of people) by examining the frequency of occurrence using a friction-fit diagnostic middle ear analyzer. Design: A group of 285 adult participants with very good hearing sensitivity underwent audiometric and middle ear testing. Acoustic reflexes were tested ipsilaterally and contralaterally in both ears across a range of elicitor frequencies. Two automated methods were used to detect the presence of an acoustic reflex. Results: There were no conditions in which the proportion of participants exhibiting acoustic reflexes was high enough to be deemed pervasive. Ipsilateral reflexes were more likely to be observed than contralateral reflexes and reflexes were more common at .5 and 1 kHz elicitor frequencies as compared to 2 and 4 kHz elicitor frequencies. Conclusions: Acoustic reflexes are common among individuals with good hearing. However, acoustic reflexes cannot be considered pervasive and should not be included in damage risk criteria and health hazard assessments for impulsive noise.
Abstract Objective: To determine whether acoustic reflexes are pervasive (i.e. sufficiently preva... more Abstract Objective: To determine whether acoustic reflexes are pervasive (i.e. sufficiently prevalent to provide 95% confidence of at least 95% prevalence) and might be invoked in damage-risk criteria (DRC) and health hazard assessments (HHA) for impulsive noise. Design: Cross-sectional analyses of a nationally-representative study. Study sample: National Health and Nutrition Examination Survey (NHANES) data collected between 1999 and 2012 were used. Over 60 thousand reflex traces obtained from 15,106 NHANES participants were used in the study, along with demographic, audiometric, health and exposure variables obtained in that study. Results: Acoustic reflexes were not sufficiently prevalent to be deemed pervasive by any detection method or in any subgroup defined by age or audiometric characteristics. The odds of observing acoustic reflexes were greater for women, young adults, and people with better hearing sensitivity. Abnormally high tympanometric admittance and “Other” race/ethnicity (i.e. people who do not self-identify as exclusively Non-Hispanic White, Non-Hispanic Black, Mexican–American, or Hispanic) were associated with lower odds. Conclusions: Acoustic reflexes are not sufficiently prevalent to be included in DRC and HHA for impulsive noise.
Journal of the Acoustical Society of America, Aug 1, 1996
A normative study of the cubic distortion product emissions from 104 monaural and binaural chinch... more A normative study of the cubic distortion product emissions from 104 monaural and binaural chinchillas was undertaken to establish criteria upon which noise exposed animals could be evaluated. From this normative group, 47 randomly selected chinchillas were exposed to various high level ͑150-, 155-, and 160-dB peak SPL͒ impulse noises. Auditory evoked potentials and cubic distortion product otoacoustic emissions were measured on each animal pre-and post-exposure and related to the sensory cell populations 30 days post-exposure. Both group mean and individual animal data indicated that the distortion product emissions were more sensitive, frequency-specific indices of noise-induced cochlear effects than pure-tone threshold measures. This was particularly evident near the threshold for noise-induced damage to the outer hair cell system.
On the basis of experimental data obtained from 420 noise-exposed animals (chinchilla), the amoun... more On the basis of experimental data obtained from 420 noise-exposed animals (chinchilla), the amount of sensory cell loss has been quantitatively related to the amount of permanent threshold shift at eight audiometric test frequencies between 0.125 and 16 kHz. The noise exposures, which varied extensively in spectrum, intensity and duration, produced permanent threshold shifts that ranged from 0 to 70 dB across a broad range of test frequencies. These data show: (1) consistent outer hair cell losses with less than 5 dB permanent threshold shifts (PTS) across all the test frequencies; (2) the first approximately 30 dB of PTS is established by losses of primarily outer hair cells; (3) in regions of the cochlea that transduce frequencies higher than or equal to 2 kHz, the three rows of outer hair cells show the same degree of loss for a given PTS, while in the 0.5 to 1.0 kHz region of the cochlea, the third row of outer hair cells (OHC) consistently shows less loss than do rows one and two; (4) appreciable inner hair cell (IHC) loss does not begin to appear until PTS exceeds approximately 30 dB; (5) in the virtual absence of OHC, hearing thresholds are least sensitive to IHC loss in the octave band centered at 4 kHz, i.e., the 4 kHz region can be as functional as other areas of the cochlea in spite of a greater amount of damage. The quantitative relation between cell loss and PTS varies as a function of test frequency in an orderly fashion.
Journal of the Acoustical Society of America, 2002
Threshold shifts ͑TS͒ were measured at various times following a wide variety of noise exposures ... more Threshold shifts ͑TS͒ were measured at various times following a wide variety of noise exposures on over 900 chinchillas. An analysis of postexposure TS measures and noise-induced permanent threshold shift ͑PTS͒ showed that, across audiometric test frequency, there was a consistent relation between these variables of the form PTS (dB)ϭ␣(e TS/ Ϫ1), where, for a given test frequency, ␣ ͑dB͒ and  ͑dB͒ are constants. TSs were measured immediately following exposure (TS 0), 24 h after exposure (TS 24), and at several intermediate times in order to estimate the maximum TS (TS max). Correlation between TS and PTS at the various test frequencies was highest for TS 24. An analysis of the 90th-percentile PTS showed a linear growth of PTS with TS 24 of approximately 0.7 dB PTS/dB TS 24. These data provide some support, in the chinchilla model, for a variation of the three postulates originally presented by Kryter et al. ͓J. Acoust. Soc. Am. 39, 451 ͑1966͔͒. Specifically: ͑i͒ TS 24 is a consistent measure of the effects of a traumatic noise exposure. ͑ii͒ All exposures that produce a given TS 24 will be equally hazardous. ͑iii͒ Noise-induced PTS in the most susceptible animals, following many years of exposure, is approximately equal to (0.7)TS 24 measured after an 8-h exposure to the same noise.
A variety of interrupted noise exposure paradigms will produce a toughening effect in the mammali... more A variety of interrupted noise exposure paradigms will produce a toughening effect in the mammalian auditory system. That is, the threshold shift will gradually become smaller with each successive daily exposure. The ability of the system to be toughened has not been explored in subjects with a pre-existing noise-induced hearing loss. Using the chinchilla as the experimental animal, evoked potential audiometry to obtain thresholds, and surface preparation histology to quantify the sensory cell population, the issue of toughening was examined in the noise-damaged auditory system. Toughening was produced by a 1.0 kHz, narrow-band impact at 115 dB peak SPL for 10 days, 6 h/day, and trauma was produced by a 1.0 kHz, narrow-band impact at 121 dB peak SPL for 5 days, 24 h/day. Four groups of animals were used. Group 1: traumatic exposure followed 30 days later by the toughening exposure. Group 2: toughening exposure followed 30 days later by the traumatic exposure. Group 3: a trauma-only control. Group 4: a tougheningonly control. Group 2 that received the toughening exposure 30 days prior to the traumatic exposure showed a 10 to more than 20 dB toughening effect between the 0.5 and 4.0 kHz test frequencies, while Group 1 that received the traumatic exposure followed 30 days later by the toughening exposure showed no toughening. The permanent changes in the evoked response audiograms and sensory cell populations were the same in Groups 1, 2 and 3 that were exposed to the traumatic noise, regardless of whether or not the animals were ever subjected to the toughening noise or whether the toughening noise preceded or followed the traumatic noise.
Journal of the Acoustical Society of America, May 1, 1993
Existing criteria for safe exposure to impulse noise do not consider the frequency spectrum of an... more Existing criteria for safe exposure to impulse noise do not consider the frequency spectrum of an impulse as a variable in the evaluation of the hazards to the auditory system. This study was designed to determine the relative potential that impulsive energy concentrated at different frequencies has in causing auditory system trauma. One hundred and thirty (130) chinchillas, divided into 22 groups of 5 to 7 animals, were used. Pre- and postexposure audiograms were measured on each animal using avoidance conditioning procedures. Quantitative histology was used to determine the extent and pattern of the sensory cell damage (cochleograms). Noise exposure employed seven different computer-generated narrow-band impulses (approximately 400-Hz bandwidth) having center frequencies located at 0.260, 0.775, 1.025, 1.350, 2.075, 2.450, and 3.550 kHz, presented at two to four different intensities. An isohazard weighting function derived from the audiometric and histological data demonstrates that equivalent amounts of impulsive energy concentrated at different frequencies are not equally hazardous to the auditory system. Comparison of the derived weighting function with the A-weighting curve indicates that low-frequency impulses are less hazardous than predicted by A-weighted sound exposure level.
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Papers by William Ahroon