Real-ear attenuation of earmuffs in normal-hearing and hearing-impaired individuals

Many of the 9 million workers exposed to average noise levels of 85 dB (A) and above are required to wear hearing protection devices, and many of these workers have already developed noise-induced hearing impairments. There is some evidence in the literature that hearing-impaired users may not receive as much attenuation from hearing protectors as normal-hearing users. This study assessed real-ear attenuation at threshold for ten normal-hearing and ten hearing-impaired subjects using a set of David Clark 10A earmuffs. Testing procedures followed the specifications of ANSI S12.6-1984. The results showed that the hearing-impaired subjects received slightly more attenuation than the normal-hearing subjects at all frequencies, but these differences were not statistically significant. These results provide additional support to the finding that hearing protection devices are capable of providing as much attenuation to hearing-impaired users as they do to normal-hearing individuals.     

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.     

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.     

Traffic noise annoyance and speech intelligibility in persons with normal and persons with impaired hearing

Annoyance ratings in speech intelligibility tests at 45 dB(A) and 55 dB(A) traffic noise were investigated in a laboratory study. Subjects were chosen according to their hearing acuity to be representative of 70-year-old men and women, and of noise-induced hearing losses typical for a great number of industrial workers. These groups were compared with normal hearing subjects of the same sex and, when possible, the same age. The subjects rated their annoyance on an open 100 mm scale. Significant correlations were found between annoyance expressed in millimetres and speech intelligibility in percent when all subjects were taken as one sample. Speech intelligibility was also calculated from physical measurements of speech and noise by using the articulation index method. Observed and calculated speech intelligibility scores are compared and discussed. Also treated is the estimation of annoyance by traffic noise at moderate noise levels via speech intelligibility scores.     

Effects of hearing protector devices on speech intelligibility

The purpose of this study was to determine the influence of hearing protection devices (HPDs) on the understanding of speech in young adults with normal hearing, both in a silent situation and in the presence of ambient noise. The experimental research was carried out with the following variables: five different conditions of HPD use (without protectors, with two earplugs and with two earmuffs); a type of noise (pink noise); 4 test levels (60, 70, 80 and 90 dB[A]); 6 signal/noise ratios (without noise, +5, +10, zero, −5 and −10 dB); 5 repetitions for each case, totalling 600 tests with 10 monosyllables in each one. The variable measure was the percentage of correctly heard words (monosyllabic) in the test. The results revealed that, at the lowest levels (60 and 70 dB), the protectors reduced the intelligibility of speech (compared to the tests without protectors) while, in the presence of ambient noise levels of 80 and 90 dB and unfavourable signal/noise ratios (0, −5 and −10 dB), the HPDs improved the intelligibility. A comparison of the effectiveness of earplugs versus earmuffs showed that the former offer greater efficiency in respect to the recognition of speech, providing a 30% improvement over situations in which no protection is used. As might be expected, this study confirmed that the protectors' influence on speech intelligibility is related directly to the spectral curve of the protector's attenuation.     

Industrial noise and its effects on human hearing

The effects of industrial noise on hearing loss of workers in Cyprus was studied over the period from 1996 to 1999. Measurements of the noise exposure doses of more than 200 workers from 90 Cypriot industries, ranging from timber to food and beverage were evaluated. Audiometric examinations of the studied workers showed that 27.8% suffered some hearing damage while 7.7% suffered serious hearing loss.     

Earmuff comfort

In many industrial and military situations it is not practical or economical to reduce ambient noise to levels that present neither a hazard to hearing nor annoyance. In these situations, personal hearing protection devices are capable of reducing the noise by 20–30 dB. Although the use of a hearing protector is recommended as a temporary solution until action is taken to control the noise, in practice, it ends up as a permanent solution in most cases. Therefore, hearing protectors must be both efficient in terms of noise attenuation and comfortable to wear. Comfort in this case is related to the agreement of the user to wear the hearing protector consistently and correctly at all times. The purpose of this paper is, firstly, to provide some background on the publications related to earmuff comfort, most of which are based on measurement of the total headband force and subjective evaluation using questionnaires. Most of the published results show a weak correlation between total headband force and subjective evaluation. Secondly, this paper presents a new method to measure the contact pressure distribution between the earmuff cushions and the circumaural flesh of the human head and estimate a comfort index. The comfort parameters were investigated and equations developed to calculate comfort indices and overall quality indices. The most important calculated comfort index is measured from the contact pressure distribution and correlated with a subjective evaluation. Measurement results for the pressure distribution of 10 earmuffs show good correlation with the subjective evaluation.     

SELF-EVALUATION SCORES OF HEARING DIFFICULTIES AND QUALITY OF LIFE COMPONENTS AMONG RETIRED WORKERS WITH NOISE-RELATED HEARING LOSS

This study is intended to clarify the relationships between hearing loss caused both by noise exposure and aging and self-rated scores of hearing disabilities and to elucidate the structure of the quality of life (QOL) determinants and their moderating conditions among retired workers with noise-related hearing loss. The questionnaire consisted of three parts: background questions, the hearing disabilities and handicap scale (HDHS), and questions regarding QOL covering five areas, self-rated health, personal health practice, social support network, life satisfaction, and life events. Two hundred ten retired workers aged 56-65 years old (60·6±1·6) with noise-related hearing loss responded to our questionnaire. All were previously engaged in noise exposed work such as shipbuilding, steel and woodwork. According to the hearing disability score (DIS score), subjects were divided into three groups and comparisons were made of the hearing handicap score (HD score) among those groups. Although groups with a higher DIS score showed a higher HD score, a large individual difference in HD score was observed in each of the three groups with the same DIS score level. The results of multiple regression analysis including two variables (life satisfaction and HD score) as the dependent variables and seven variables as the independent variables showed that the strongest explanatory variable for life satisfaction was social support network, followed by handicaps caused by hearing disabilities, self-rated health and personal health practice. It was demonstrated that hearing disabilities and handicap measured by the Japanese version of the HDHS were directly associated with the deterioration in QOL. Measures of the social support network, life satisfaction, and hearing disabilities and handicaps may assist in the detection of workers who can be targeted for a variety of interventions, such as audiological rehabilitation or the creation of a barrier-free community that is supportive of the hearing impaired.     

Survey of noise in coal preparation plants

In response to the continuing problem of noise induced hearing loss (NIHL) among mine workers, the National Institute for Occupational Safety and Health (NIOSH) has conducted numerous noise surveys in coal preparation plants. The research, consisting of worker dose monitoring, task observations, and equipment noise profiling, was completed in eight separate preparation plants. Worker dose monitoring was conducted for three shifts in most cases. Workers experiencing higher than allowable doses were task-observed for one full shift to correlate dose to noise source(s). Finally, noise levels on all floors, and in lunch rooms and control rooms, were characterized. Results indicate that only workers who routinely spend a significant portion of their shift in the plants (away from the control rooms) are susceptible to overexposure from noise. Certain pieces of equipment (screens, centrifuges, sieve bends) are the loudest primary noise sources responsible for the worker noise exposures.     

Noise exposure and hearing conservation for farmers of rural Japanese communities

Agricultural mechanization in Japan has progressed dramatically since 1955 with the introduction of tractors, harvesters, and processing machines. These technological developments have resulted in an increase in exposure to sources of noise that are not only annoying, but damaging to hearing. The present study was undertaken to determine, whether Japanese farmers are at risk for noise-induced hearing loss in comparison with office workers, and by evaluating the present conditions regarding occupational noise levels among agricultural workers.The results suggest that farmers, especially male farmers, have a high prevalence of hearing loss in the higher frequency ranges. Daily noise exposure levels in L_Aeq ranged from 81.5 to 99.1 dBA for tea harvesting and processing, and from 83.2 to 97.6 for sugar cane harvesting. Taking into account their rather long working hours and excessive noise from farm machinery, it is concluded that farmers are at risk for noise-induced hearing loss. These findings clearly indicate a strong need for implementation of hearing conservation programs among agricultural workers exposed to machinery noise.     

An acoustic head simulator for hearing protector evaluation. II: Measurements in steady-state and impulse noise environments

The attenuation characteristics of hearing protection devices (HPDs) were measured using a modular acoustic head simulator. The effect in changes in the head configuration was assessed in a steady-state diffuse sound field. The use of artificial circumaural skin had a relatively small influence on the insertion loss of earmuffs (max. 6-7 dB at low frequencies). This contrasts to the very large effects found for the artificial intraaural skin on the insertion loss of earplugs (in excess of 40 dB at low frequencies for some devices). Results were also compared with real-ear attenuation at threshold (REAT) data (ANSI S3.19-1974). In general, there is good agreement between the two methods, especially for earmuffs. Design improvements are proposed for earplugs. The result of an exploratory study aimed at measuring the complex (amplitude and phase) insertion loss of HPDs using an impulse noise source are also reported.     

Industrial noise levels and annoyance in Egypt

Annoyance and increase of accident risk of workers from industrial noise levels in Egypt were studied. 683 workers from 15 Egyptian sites of industry, ranging from food to metal industry were evaluated. The goals of this study are to carry out measurements to evaluate industrial noise levels, are these levels exceeded the permissible levels set by Egyptian noise standard and policy to protect public health of workers?, to examine worker’s attitudes towards industrial noise, to know the relationship between industrial noise levels and degree of annoyance. Results showed that equivalent continuous noise levels ranged from 70 to 100 dB (A). Annoyance of respondents showed that 47.1% were highly annoyed, 5.8% their hearing were harmed. There was a strong relationship between industrial noise levels and percentage of highly annoyed respondents. By increasing industrial noise level possibility of workers to make accident was also increased. Respondents suggest less maximum daily exposure duration than those set by Egyptian law.     

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.     

汽车车内噪声主动控制系统仿真

摘要：降低汽车空腔的振动,是抑制汽车车内噪声的有效途径之一。以激振器、作动器和控制器等为主要部件,搭建了简化的汽车车内噪声主动控制系统,该系统通过将汽车空腔模型简化为板件,以减弱板件振动为目标,实现了汽车车内噪声主动控制。采用简谐正弦及余弦信号作为激振器发出的激励,用于模拟板件的初始振动,控制器通过采用模糊控制算法直接控制压电陶瓷作动器的振动,压电陶瓷作动器的振动用于抑制板件的振动,完成了汽车车内噪声主动控制系统仿真。仿真结果表明,研究采用的汽车车内噪声主动控制系统,使汽车空腔振动降低23%,为解决由汽车发动机和动力总成的振动所引发的汽车车内噪声问题提供了一个有效途径。     

Development of a new standard laboratory protocol for estimation of the field attenuation of hearing protection devices: sample size necessary to provide acceptable reproducibility

The mandate of ASA Working Group S12/WG11 has been to develop "laboratory and/or field procedure(s) that yield useful estimates of field performance" of hearing protection devices (HPDs). A real-ear attenuation at threshold procedure was selected, devised, tested for one earmuff and three earplugs via an interlaboratory study involving five laboratories and 147 subjects, and incorporated into a new standard that was approved in 1997 [Royster et al., "Development of a new standard laboratory protocol for estimating the field attenuation of hearing protection devices. Part I. Research of Working Group 11, Accredited Standards Committee S 12, Noise," J. Acoust. Soc. Am. 99, 1506-1526; ANSI, S12.6-1997, "American National Standard method for measuring real-ear attenuation of hearing protectors" (American National Standards Institute, New York, 1997)]. The subject-fit methodology of ANSI S12.6-1997 relies upon listeners who are audiometrically proficient, but inexperienced in the use of HPDs. Whenever a new method is adopted, it is important to know the effects of variability on the power of the measurements. In evaluation of protector noise reduction determined by experimenter-fit, informed-user-fit, and subject-fit methods, interlaboratory reproducibility was found to be best for the subject-fit method. Formulas were derived for determining the minimum detectable difference between attenuation measurements and for determining the number of subjects necessary to achieve a selected level of precision. For a precision of 6 dB, the study found that the minimum number of subjects was 4 for the Bilsom UF-1 earmuff, 10 for the E.A.R Classic earplug, 31 for the Willson EP100 earplug, and 22 for the PlasMed V-51R earplug.     

Risk factors for hearing loss at different frequencies in a population of 47,388 noise-exposed workers.

Weighted regression analysis was applied to determine the dependence of the hearing thresholds of 47,388 noise-exposed workers on age, sex, noise immission level, ear disease, head injury, tinnitus, hearing protector usage, and audiometric frequency in the range from 0.5 to 6 kHz. It could be shown that the hearing thresholds at any frequency are dominated by the age of the worker and that women, after equivalent exposure conditions, hear better than men. The relative effects of sex, noise immission level, ear diseases, tinnitus, and hearing protector usage are related to the audiometric frequency. Users of hearing protectors at the last audiometric investigation hear worse than nonusers. Hearing protector usage is strongly related with the hearing threshold in the low-frequency range. The noise immission level does not noticeably affect the hearing threshold below 3 kHz. The most important frequency of the noise immission level is as expected 4 kHz. For 4 kHz, it was shown that the variables age, noise immission level, tinnitus, head injuries, and ear diseases act in a good approximation additively on the pure-tone hearing threshold.     

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.     

Design of a pulse generator and shock tube for measuring hearing protector attenuation of high-amplitude impulsive noise

The effectiveness of hearing protectors against high amplitude impulse noise levels remains the subject of research with objective testing techniques using acoustic test fixtures offering the only realistic method of providing rapid performance data for protector design and qualification. The work presented in this paper examines a prototype test method based on a shock tube and acoustic test fixture for the evaluation of protectors against high-level impulsive noise where established real ear attenuation at threshold methods would be impractical to apply. The results show that the system is capable of producing controlled repeatable high amplitude pressure pulses of variable duration for testing hearing protection devices in a grazing wave type test. A series of pilot tests illustrate how the system can have a sufficient self-insertion loss to reject flanking noise and allow the measurement of protector attenuations of up to 45 dB with little corruption from flanking noise.     

Simplified hearing protector ratings—an international comparison

A computer was programmed to model the distributions of dB(A) levels reaching the ears of an imaginary workforce wearing hearing protectors selected on the basis of either octave band attenuation values or various simplified ratings in use in Australia, Germany, Poland, Spain or the U.S.A. Both multi-valued and single-valued versions of dB(A) reduction and sound level conversion ratings were considered. Ratings were compared in terms of precision and protection rate and the comparisons were replicated for different samples of noise spectra (N = 400) and hearing protectors (N = 70) to establish the generality of the conclusions. Different countries adopt different approaches to the measurement of octave band attenuation values and the consequences of these differences were investigated. All rating systems have built-in correction factors to account for hearing protector performance variability and the merits of these were determined in the light of their ultimate effects on the distribution of dB(A) levels reaching wearers' ears. It was concluded that the optimum rating is one that enables the dB(A) level reaching wearers to be estimated by subtracting a single rating value from the dB(C) level of the noise environment, the rating value to be determined for a pink noise spectrum from mean minus one standard deviation octave band attenuation values with further protection rate adjustments being achieved by the use of a constant correction factor.