Hearing protection practice in the Canadian forces.
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In the Army, as in civil industry, there are many noise sources where the noise can not be reduced to safe levels except by use of hearing protectors, such as ear plugs, ear muffs or noise-excluding helmets. This paper shows how the effectiveness of hearing protectors can be determined for both continuous and impulse (gunfire) noise. It is shown that the results are in most cases in agreement with results for the real-ear attenuation at threshold method; the reasons for the exceptions are discussed. Hearing protection is often unpopular with users; reasons for this include discomfort, difficulty in speech communication, and incompatibility with other headgear. The degree of protection in practical usage is often much less than that measured in the laboratory. Possible solutions to these problems are described.
Speech perception was tested in high level noise under controlled laboratory conditions in noise-exposed workmen and normal subjects, with and without a hearing protector. The group was further divided by age and English fluency, the latter group being included because of the high proportion of non-fluent English speakers in the Canadian workforce. In normal-hearing subjects the highest discrimination scores were found without background noise, they were lower with white noise as a masker, and even lower with crowd noise as a masker; wearing of a protector had no effect on intelligibility. The results for non-fluent English speakers were parallel with these results, but the scores were lower in all test conditions. In the presence of a high frequency hearing loss speech discrimination was lower than in the normals in quiet and in noise. The addition of a hearing protector dropped their discrimination score even further. In a flat hearing loss, wearing of a protector also worsened the speech discrimination score. The results are discussed.
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Landing signal officers (LSOs), exposed to high noise levels during carrier operations, do not routinely wear hearing protectors. Noise exposure data obtained during carrier qualifications on the USS Lexington and the USS Forrestal suggested a clear risk for hearing damage. Moreover, a comparison of the hearing of LSO and non-LSO pilots matched for age and number of flight hours, indicated a trend for LSOs to have poorer hearing. Questionnaire data from 225 LSOs indicate they need full access to aircraft auditory cues, which is not attainable with current off-the-shelf hearing protectors. It is recommended that a hearing protector be developed for the LSO which would permit passage of critical auditory cues and, at the same time, provide hearing protection. There should be a redesign or elimination of the UHF handset. If eliminated, a boom-type noise-cancelling microphone in conjunction with earphones integral to the hearing protector should be considered. Additional hearing studies of LSO and non-LSO pilots should be undertaken.
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Austria's new legislation on the protection of employees and workers provides that the determination and assessment of noise-induced hearing defects caused at the working places may now also be carried out by physicians who are not otologists. Since language-audiometric data are not available to such physicians, the assessments are based only on sound-audiometric hearing results. Therefore, it is understandable that such physicians would like to be able to draw conclusions as to the extent of the noise-induced hearing defect from the sound-audiogram alone. The difficulties encountered in the attempt to determine language-hearing on the basis of sound-hearing are well-known. Among all defects the noise-induced hearing defect is still best suited for such attempts because of its stereo-typical curve in the sound-audiogram. 500 cases with a pure noise-induced hearing defect are subjected to a multiple regression analysis. The result of the investigation confirms the practical observations made. The sound-audiometric hearing loss at 1.5 kHz is closely related to the discrimination capacity for our language and thus gives some indication as to the degree of the noise-induced hearing defect.
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The possibility of further employment of workers in noise areas must be judged by the specific criteria of "Principle No. 20" of the Professional Associations. However, in many cases of already existing hearing damage it is possible to avoid a change of working place for the time being and to rate "No objection on health reasons under certain conditions". In cases of noise-induced, already existing hearing loss, a change of working place is generally not necessary if the worker uses ear protectors. There should also be no general objection in cases of pure disturbance of sound conduction. In combined hearing losses and pure labyrinthine deafness not created by noise, it must be supposed as a general rule that the worker is not fit for further working in noise areas. However, in special cases carefully analysed as to anamnesis and development, even a worker with such types of hearing loss might under special precautions be retained in the noise area.
In case of an impulse noise, the peak level in the auditory meatus is increased by about 5-10 dB. At the same time, the spectrum of the noise is shifted towards the higher frequencies. This is essential for assessing the damage risk criteria of the impulse noise and the evaluation of the efficacy of the noise protectors. The resonance of the meatal cavity should be taken into consideration to explain the loss of hearing in the range of high tones caused by high-energy impulse noise.
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