Hearing loss from gun and railroad noise--relations with ISO standard 1999.

Pure-tone hearing thresholds and anamnestic data pertaining to nosocusis and exposure to gun noise were analyzed for 9778 male railroad train-crew workers. A major portion of losses in hearing sensitivity due to railroad noise are obscured in comparisons of hearing levels of trainmen with the hearing levels of the unscreened samples of United States males given in Annex B, ISO 1999 [ISO 1999 (1990), "Acoustics--Determination of occupational noise exposure and estimation of noise-induced hearing impairment" (International Organization for Standardization, Geneva)]. Comparisons of the hearing levels, adjusted for nosocusis, of trainmen who had used no guns, with the hearing levels of otologically and noise screened males (Annex A, ISO 1999) reveal significant losses due to railroad noise. Additional losses were found at high frequencies in trainmen who had used guns. It appears that the effective Leq8h exposure level of trainmen to railroad noise is about 92 dBA, and 87-89 dBA to gun noise. These results are in general agreement with those of study of railway workers by Prosser et al. [Br. J. Audiol. 22, 85-91 (1988)]. Asymmetries in losses between the two ears, effects of ear protection, losses from nosocusis, and losses from sport, as compared to military, gun noise exposures, are examined.     

Evaluation of the risk of noise-induced hearing loss among unscreened male industrial workers

Variability in background risk and distribution of various risk factors for hearing loss may explain some of the diversity in excess risk of noise-induced hearing loss (NIHL). This paper examines the impact of various risk factors on excess risk estimates of NIHL using data from the 1968-1972 NIOSH Occupational Noise and Hearing Survey (ONHS). Previous analyses of a subset of these data focused on 1172 highly "screened" workers. In the current analysis, an additional 894 white males (609 noise-exposed and 285 controls), who were excluded for various reasons (i.e., nonoccupational noise exposure, otologic or medical conditions affecting hearing, prior occupational noise exposure) have been added 2066) to assess excess risk of noise-induced material impairment in an unscreened population. Data are analyzed by age, duration of exposure, and sound level (8-h TWA) for four different definitions of noise-induced hearing impairment, defined as the binaural pure-tone average (PTA) hearing threshold level greater than 25 dB for the following frequencies: (a) 1-4 kHz (PTA1234), (b) 1-3 kHz (PTA123), (c) 0.5, 1, and 2 kHz (PTA512), and (d) 3, 4, and 6 kHz (PTA346). Results indicate that populations with higher background risks of hearing loss may show lower excess risks attributable to noise relative to highly screened populations. Estimates of lifetime excess risk of hearing impairment were found to be significantly different between screened and unscreened population for noise levels greater than 90 dBA. Predicted age-related risk of material hearing impairment in the ONHS unscreened population was similar to that predicted from Annex B and C of ANSI S3.44 for ages less than 60 years. Results underscore the importance of understanding differential risk patterns for hearing loss and the use of appropriate reference (control) populations when evaluating risk of noise-induced hearing impairment among contemporary industrial populations.     

Sound exposures and hearing thresholds of symphony orchestra musicians

To assess the risk of noise-induced hearing loss among musicians in the Chicago Symphony Orchestra, personal dosimeters set to the 3-dB exchange rate were used to obtain 68 noise exposure measurements during rehearsals and concerts. The musicians' Leq values ranged from 79-99 dB A-weighted sound pressure level [dB(A)], with a mean of 89.9 dB(A). Based on 15 h of on-the-job exposure per week, the corresponding 8-h daily Leq (excluding off-the-job practice and playing) ranged from 75-95 dB(A) with a mean of 85.5 dB(A). Mean hearing threshold levels (HTLs) for 59 musicians were better than those for an unscreened nonindustral noise-exposed population (NINEP), and only slightly worse than the 0.50 fractile data for the ISO 7029 (1984) screened presbycusis population. However, 52.5% of individual musicians showed notched audiograms consistent with noise-induced hearing damage. Violinists and violists showed significantly poorer thresholds at 3-6 kHz in the left ear than in the right ear, consistent with the left ear's greater exposure from their instruments. After HTLs were corrected for age and sex, HTLs were found to be significantly better for both ears of musicians playing bass, cello, harp, or piano and for the right ears of violinists and violists than for their left ears or for both ears of other musicians. For 32 musicians for whom both HTLs and Leq were obtained, HTLs at 3-6 kHz were found to be correlated with the Leq measured.     

Distribution of risk factors for hearing loss: implications for evaluating risk of occupational noise-induced hearing loss

This paper presents an analysis of hearing threshold levels among 2066 white male workers employed in various U.S. industries studied in the 1968-72 NIOSH Occupational Noise and Hearing Survey (ONHS). The distribution of hearing threshold levels (HTL) is examined in relation to various risk factors (age, prior occupational noise, medical conditions) for hearing loss among a population of noise exposed and control (low noise-exposed) industrial workers. Previous analyses of a subset of these data from the ONHS focused on 1172 highly "screened" workers. An additional 894 male workers (609 noise-exposed and 285 controls), who were excluded for various reasons (i.e., nonoccupational noise exposure, otologic or medical conditions affecting hearing, prior occupational noise exposure) have been added to examine hearing loss in an unscreened population. Data are analyzed by age, duration of exposure, and sound level (8-h TWA) by individual test frequency. Results indicate that hearing threshold levels are higher among unscreened noise-exposed and control workers relative to screened workers. Analysis of risk factors such as nonoccupational noise exposure, medical conditions, and type of industry among unscreened controls indicated that these factors were not significantly associated with increased mean HTLs or risk of material impairment over and above what is expected due to age. Age-specific mean hearing threshold levels (and percentiles of the distribution) among the unscreened ONHS control population may be used as a comparison population of low-noise exposed white male industrial workers for evaluating the effectiveness of hearing conservation programs for workers less than 55 years of age. To make valid inferences regarding occupational noise-induced hearing loss, it is important to use hearing data from reference (control) populations that are similar with respect to the degree of subject screening, type of work force (blue vs white collar), and the distribution of other risk factors for hearing loss.     

Noise levels and hearing thresholds in the drop forging industry

A-weighted equivalent continuous noise levels for hammer and press operations in a drop-forging industry were determined using both tape recordings of the noise and personal noise dosimeters. The results indicated average A-weighted Leq values of 108 dB for hammer operators and 99 dB for press operators. Comparison of hearing level statistics for 716 hammer and press operators and 293 control subjects indicated the severe hazard to hearing of impact noise exposures. For mean exposure times of less than 10 years, hearing levels for the press (99 dB) and hammer (108 dB) operator age groups are nearly identical, and in the latter case are less than those predicted for exposure to equivalent continuous noise. For long-term exposures of 10 years or more, the results of this study indicate that hearing losses resulting from impact noise in the drop-forging industry are as great or greater than those resulting from continuous noise.     

Conditioning-induced protection from impulse noise in female and male chinchillas

Sound conditioning (pre-exposure to a moderate-level acoustic stimulus) can induce resistance to hearing loss from a subsequent traumatic exposure. Most sound conditioning experiments have utilized long-duration tones and noise at levels below 110 dB SPL as traumatic stimuli. It is important to know if sound conditioning can also provide protection from brief, high-level stimuli such as impulses produced by gunfire, and whether there are differences between females and males in the response of the ear to noise. In the present study, chinchillas were exposed to 95 dB SPL octave band noise centered at 0.5 kHz for 6 h/day for 5 days. After 5 days of recovery, they were exposed to simulated M16 rifle fire at a level of 150 dB peak SPL. Animals that were sound conditioned showed less hearing loss and smaller hair cell lesions than controls. Females showed significantly less hearing loss than males at low frequencies, but more hearing loss at 16 kHz. Cochleograms showed slightly less hair cell loss in females than in males. The results show that significant protection from impulse noise can be achieved with a 5-day conditioning regimen, and that there are consistent differences between female and male chinchillas in the response of the cochlea to impulse noise.     

Speech recognition in noise by individuals with mild hearing impairments

The study was designed to test the validity of the American Academy of Ophthalmology and Otolaryngology's (AAOO) 26-dB average hearing threshold level at 500, 1000, and 2000 Hz as a predictor of hearing handicap. To investigate this criterion the performance of a normal-hearing group was compared with that of two groups, categorized according to the AAOO [Trans. Am. Acad. Ophthal. Otolaryng. 63, 236-238 (1959)] guidelines as having no handicap. The latter groups, however, had significant hearing losses in the frequencies above 2000 Hz. Mean hearing threshold levels for 3000, 4000, and 6000 Hz were 54 dB for group II and 63 dB for group III. Two kinds of speech stimuli were presented at an A-weighted sound level of 60 dB in quiet and in three different levels of noise. The resulting speech recognition scores were significantly lower for the hearing-impaired groups than for the normal-hearing group on both kinds of speech materials and in all three noise conditions. Mean scores for group III were significantly lower than those of the normal-hearing group, even in the quiet condition. Speech recognition scores showed significantly better correlation with hearing levels for frequency combinations including frequencies above 2000 Hz than for the 500-, 1000-, and 2000-Hz combination. On the basis of these results the author recommends that the 26-dB fence should be somewhat lower, and that frequencies above 2000 Hz should be included in any scheme for evaluating hearing handicap.     

Underwater temporary threshold shift in pinnipeds: effects of noise level and duration

Behavioral psychophysical techniques were used to evaluate the residual effects of underwater noise on the hearing sensitivity of three pinnipeds: a California sea lion (Zalophus californianus), a harbor seal (Phoca vitulina), and a northern elephant seal (Mirounga angustirostris). Temporary threshold shift (TTS), defined as the difference between auditory thresholds obtained before and after noise exposure, was assessed. The subjects were exposed to octave-band noise centered at 2500 Hz at two sound pressure levels: 80 and 95 dB SL (re: auditory threshold at 2500 Hz). Noise exposure durations were 22, 25, and 50 min. Threshold shifts were assessed at 2500 and 3530 Hz. Mean threshold shifts ranged from 2.9-12.2 dB. Full recovery of auditory sensitivity occurred within 24 h of noise exposure. Control sequences, comprising sham noise exposures, did not result in significant mean threshold shifts for any subject. Threshold shift magnitudes increased with increasing noise sound exposure level (SEL) for two of the three subjects. The results underscore the importance of including sound exposure metrics (incorporating sound pressure level and exposure duration) in order to fully assess the effects of noise on marine mammal hearing.     

Temporary threshold shift in bottlenose dolphins (Tursiops truncatus) exposed to mid-frequency tones

A behavioral response paradigm was used to measure hearing thresholds in bottlenose dolphins before and after exposure to 3 kHz tones with sound exposure levels (SELs) from 100 to 203 dB re 1 microPa2 s. Experiments were conducted in a relatively quiet pool with ambient noise levels below 55 dB re 1 microPa2/Hz at frequencies above 1 kHz. Experiments 1 and 2 featured 1-s exposures with hearing tested at 4.5 and 3 kHz, respectively. Experiment 3 featured 2-, 4-, and 8-s exposures with hearing tested at 4.5 kHz. For experiment 2, there were no significant differences between control and exposure sessions. For experiments 1 and 3, exposures with SEL=197 dB re 1 microPa2 s and SEL > or = 195 dB re 1 microPa2 s, respectively, resulted in significantly higher TTS4 than control sessions. For experiment 3 at SEL= 195 dB re 1 microPa2 s, the mean TTS4 was 2.8 dB. These data are consistent with prior studies of TTS in dolphins exposed to pure tones and octave band noise and suggest that a SEL of 195 dB re 1 microPa2 s is a reasonable threshold for the onset of TTS in dolphins and white whales exposed to midfrequency tones.     

Speech-reception threshold in noise with one and two hearing aids

The binaural free-field speech-reception threshold (SRT) in 70-dBA noise was measured with conversational sentences for 24 hearing-impaired subjects without hearing aids, with a hearing aid left, right, and left plus right, respectively. The sentences were always presented in front of the listener and the interfering noise, with a spectrum equal to the long-term average spectrum of the sentences, was presented either frontally, from the right, or from the left side. For subjects with only moderate hearing loss, PTA (average air-conduction hearing level at 500, 1000, and 2000 Hz) less than 50 dB, the SRT in 70-dBA noise in both ears is determined by the signal-to-noise ratio even if only one hearing aid is used. For larger hearing losses the SRT appears to be partly determined by the absolute threshold. In conditions with a high noise level relative to the absolute threshold, in which case for both ears the SRT is determined by the signal-to-noise ratio, a second hearing aid, just as a monaural hearing aid, generally does not improve the SRT. However, in the case of a high hearing level, or a low noise level, in which a monaural hearing aid is profitable, the use of two hearing aids is even more profitable. In a separate experiment, acoustic head shadow was measured at the entrance of the ear canal and at the microphone location of a hearing aid. It appeared that, for a lateral noise source and speech frontal, the microphone position of behind-the-ear hearing aids has a negative effect on the signal-to-noise ratio of 2-3 dB.     

Use of the sound levels of noise for assessing the adequacy of hearing protectors

The dB(A) sound level of a noise is accepted as a measure of the damage risk to unprotected ears but often it is not a reliable guide to the risk to ears fitted with hearing protectors. For any dB(A) level inside a protector, normally there will be substantially higher sound levels outside that protector. This paper shows how, from sequential frequency attenuation bands of the protector, and sound level weightings, external sound levels can be calculated, below which the noise inside the protector does not exceed a chosen dB(A) level. Further valuable information may be obtained by mapping external dB(A) and dB(C) levels to cover all possible noise spectra that give the chosen dB(A) level inside the protector. Thus, from a pair of measured sound levels, use of the method indicates whether the protector is sufficient or not, or whether more detailed measurment of the noise is required. This knowledge enhances the scope of the sound level meter and reduces the need for frequency analysis of industrial noise. Its application should be a helpful addition to the data provided by suppliers of hearing protectors.     

Normal hearing threshold levels in the low-frequency range determined by an insert earphone.

Hearing threshold levels have been determined in the low-frequency range (20-500 Hz) on a group of 30 young normal-hearing subjects using monaural stimulus presentation through an insert earphone (Etymotic Research ER-3A). A retest was performed on half of the group to provide data on test-retest reliability. The mean hearing threshold levels obtained agree closely with the Minimum Audible Field data of ISO 226, however, with some deviation at the very lowest frequencies below 40 Hz. The test-retest difference results yielded mean values that averaged 1.15 dB with an average standard deviation across test frequencies of 3.9 dB. The results show that low-frequency hearing thresholds for pure tones of frequencies from 40 Hz and up can be determined with acceptable validity and reliability by the use of this type of insert earphone.     

Underwater temporary threshold shift induced by octave-band noise in three species of pinniped.

Pure-tone sound detection thresholds were obtained in water for one harbor seal (Phoca vitulina), two California sea lions (Zalophus californianus), and one northern elephant seal (Mirounga angustirostris) before and immediately following exposure to octave-band noise. Additional thresholds were obtained following a 24-h recovery period. Test frequencies ranged from 100 Hz to 2000 Hz and octave-band exposure levels were approximately 60-75 dB SL (sensation level at center frequency). Each subject was trained to dive into a noise field and remain stationed underwater during a noise-exposure period that lasted a total of 20-22 min. Following exposure, three of the subjects showed threshold shifts averaging 4.8 dB (Phoca), 4.9 dB (Zalophus), and 4.6 dB (Mirounga). Recovery to baseline threshold levels was observed in test sessions conducted within 24 h of noise exposure. Control sessions in which the subjects completed a simulated noise exposure produced shifts that were significantly smaller than those observed following noise exposure. These results indicate that noise of moderate intensity and duration is sufficient to induce TTS under water in these pinniped species.     

Amplitude modulation thresholds in chinchillas with high-frequency hearing loss

Estimates of auditory temporal resolution were obtained from normal chinchillas using sinusoidally amplitude modulated noise. Afterwards, the animals were exposed to noise whose bandwidth was progressively increased toward the low frequencies in octave steps. The first exposure was to an octave band of noise centered at 8 kHz. Three additional octave bands of noise were subsequently added to the original exposure in order to progressively increase the extent of the high-frequency hearing loss. The first exposure produced a temporary hearing loss of 50 to 60 dB near 8 kHz and elevated the amplitude modulation thresholds primarily at intermediate (128 Hz) modulation frequencies. Successive noise exposures extended the temporary hearing loss toward lower frequencies, but there was little further deterioration in the amplitude modulation function until the last exposure when the hearing loss spread to 1 kHz. The degradation in the amplitude modulation function observed after the last exposure, however, was due to a reduction in the sensation level of the test signal rather than to a decrease in the hearing bandwidth. The results of this study suggest that the high-frequency regions of the cochlea may be important for temporal resolution.     

Speech-reception threshold for sentences as a function of age and noise level.

For 140 male subjects (20 per decade between the ages 20 and 89) and 72 female subjects (20 per decade between 60 and 89, and 12 for the age interval 90-96), the monaural speech-reception threshold (SRT) for sentences was investigated in quiet and at four noise levels (22.2, 37.5, 52.5, and 67.5 dBA noise with long-term average speech spectra). The median SRT as well as the quartiles are given as a function of age. The data are described in terms of a model published earlier [J. Acoust. Soc. Am. 63, 533-549 (1978)]. According to this model every hearing loss for speech (SHL) is interpreted as the sum of a loss class A (attenuation), characterized by a reduction of the levels of both speech signal and noise, and a loss class D (distortion), comparable with a decrease in signal-to-noise ratio. Both SHLA+D (hearing loss in quiet) and SHLD (hearing loss at high noise levels) increase progressively above the age of 50 (reaching typical values of 30 and 6 dB, respectively, at age 85). The spread of SHLD as a function of SHLA+D for the individual ears is so large (sigma = 2.7 dB) that subjects with the same hearing loss for speech in quiet may differ considerably in their ability to understand speech in noise. The data confirm that the hearing handicap of many elderly subjects manifests itself primarily in a noisy environment. Acceptable noise levels in rooms used by the aged must be 5 to 10 dB lower than those for normal-hearing subjects.     

Age changes in pure-tone hearing thresholds in a longitudinal study of normal human aging

Hearing thresholds were obtained on 813 adult males (20-95 years) measured at 11 frequencies ranging from 0.125-8 kHz from pure-tone audiograms collected over a 20-year period from 1968 to 1987. Audiograms taken at two to six different ages spanning a maximum observation period of 15 years were obtained for each male belonging to one of seven different age groups (20,30,...,80 years) based on the age of initial observation. The males were participants in the Baltimore Longitudinal study of Aging (BLSA), a multidisciplinary community-based study of normal human aging. Changes in hearing thresholds occurred in all age groups during the 15-year follow-up period. For example, at 0.5 and 8 kHz for combined left and right ears there was an average longitudinal loss of 5.7-7.6 and 5.1-21.1 dB, respectively, for 20-year-olds, 10.0-12.7 and 35.2-53.0 dB for 50-year-olds, and 22.9-48.5 and 69.0-84.5 dB for 80-year-olds. As in results from previous cross-sectional studies, hearing loss in the males 70 years and older is greatest at the highest frequencies. The rate of change for these older males is faster in the speech-range frequencies 0.5-2 kHz than in the higher frequencies, since their hearing has already diminished at the high frequencies.     

Speech reception in quiet and in noisy conditions by individuals with noise-induced hearing loss in relation to their tone audiogram.

Tone thresholds and speech-reception thresholds were measured in 200 individuals (400 ears) with noise-induced hearing loss. The speech-reception thresholds were measured in a quiet condition and in noise with a speech spectrum at levels of 35, 50, 65, and 80 dBA. The tone audiograms could be described by three principal components: hearing loss in the regions above 3 kHz, from 1 to 3 kHz and below 1 kHz; the speech thresholds could be described by two components: speech reception in quiet and speech reception in noise at 50-80 dBA. Hearing loss above 1 kHz was related to speech reception in noise; hearing loss at and below 1 kHz to speech reception in quiet. The correlation between the speech thresholds in quiet and in noise was only R = 0.45. An adequate predictor of the speech threshold in noise, the primary factor in the hearing handicap, was the pure-tone average at 2 and 4 kHz (PTA2,4, R = 0.72). The minimum value of the prediction error for any tone-audiometric predictor of this speech threshold was 1.2 dB (standard deviation). The prediction could not be improved by taking into account the critical ratio for low-frequency noise nor by its upward spread of masking. The prediction error is due to measurement error and to a factor common to both ears. The latter factor is ascribed to cognitive skill in speech reception. Hearing loss above 10 to 15 dB HL (hearing level) already shows an effect on the speech threshold in noise, a noticeable handicap is found at PTA2,4 = 30 dB HL.     

Isolating the auditory system from acoustic noise during functional magnetic resonance imaging: examination of noise conduction through the ear canal, head, and body

Approaches were examined for reducing acoustic noise levels heard by subjects during functional magnetic resonance imaging (fMRI), a technique for localizing brain activation in humans. Specifically, it was examined whether a device for isolating the head and ear canal from sound (a "helmet") could add to the isolation provided by conventional hearing protection devices (i.e., earmuffs and earplugs). Both subjective attenuation (the difference in hearing threshold with versus without isolation devices in place) and objective attenuation (difference in ear-canal sound pressure) were measured. In the frequency range of the most intense fMRI noise (1-1.4 kHz), a helmet, earmuffs, and earplugs used together attenuated perceived sound by 55-63 dB, whereas the attenuation provided by the conventional devices alone was substantially less: 30-37 dB for earmuffs, 25-28 dB for earplugs, and 39-41 dB for earmuffs and earplugs used together. The data enabled the clarification of the relative importance of ear canal, head, and body conduction routes to the cochlea under different conditions: At low frequencies (< or =500 Hz), the ear canal was the dominant route of sound conduction to the cochlea for all of the device combinations considered. At higher frequencies (>500 Hz), the ear canal was the dominant route when either earmuffs or earplugs were worn. However, the dominant route of sound conduction was through the head when both earmuffs and earplugs were worn, through both ear canal and body when a helmet and earmuffs were worn, and through the body when a helmet, earmuffs, and earplugs were worn. It is estimated that a helmet, earmuffs, and earplugs together will reduce the most intense fMRI noise levels experienced by a subject to 60-65 dB SPL. Even greater reductions in noise should be achievable by isolating the body from the surrounding noise field.     

Ambient habitat noise and vibration at the Georgia Aquarium

Underwater and in-air noise evaluations were completed in performance pool systems at Georgia Aquarium under normal operating conditions and with performance sound tracks playing. Ambient sound pressure levels at in-pool locations, with corresponding vibration measures from life support system (LSS) pumps, were measured in operating configurations, from shut down to full operation. Results indicate noise levels in the low frequency ranges below 100?Hz were the highest produced by the LSS relative to species hearing thresholds. The LSS had an acoustic impact of about 10?dB at frequencies up to 700?Hz, with a 20?dB re 1?μPa impact above 1000?Hz.     

Temporary threshold shift in human subject exposed to noise

Using an audiometer,the effect of the noise level upon temporarythreshold shift(TTS)for five trained normal subjects(left ear only)was studied.The measurements were carried out after 6 min exposure(in third octave band)for different sound pressure levels ranging between 75-105 dB at three test fre-quencies 2,3,and 4 kHz.The results indicated that at exposure to noise of soundpressure level(SPL)above 85 dB,TTS increases linearly with ths SPL for all thetest frequencies.The work had extended to study the recovery curves for the sameears.The results indicated that the reduction in TTS on doubling the recoverytimes,for the two sound pressure levels 95 dB and 105 dB,occurs at a rate of near-ly 3 dB.The comparison of the recovery curve at 3 kHz with that calculated usingWard's general equation for recovery was made.Finally,to study the values ofTTS produced by exposure to certain noise at different test frequencies,distribu-tion curves for two recovery times were plotted representing TTS values,for anexposure