Military studies of temporary threshold shift on protected and unprotected ears following impulse noise exposure.
Dancer A; Cabanis P; Gorzerino P; Grateau P; Vaillant T
Proceedings: 1992 hearing conservation conference, April 1-4, 1992. Cincinnati, OH: U.S. Department of Health and Human Services, Public Health Service, Centers for Disease Control, National Institute for Occupational Safety and Health, task order 91-37982, 1992 Apr; :97-102
Studies of temporary threshold shift in protected and unprotected ears exposed to impulse noise were conducted in French Army personnel. Subjects were soldiers who had hearing levels (HLs) better than 30 decibels (dB) at any frequency. Bekesy audiograms were made on each subject just prior to exposure, and 5 minutes, 1 hour (hr), and 4hr afterwards. Delayed temporary threshold shifts (TTS) were sought. Three groups of subjects wearing no hearing protection were exposed in the field for over 2 days to impulse noises produced by a rifle. Group 1 was subjected to ten shots at 10 second (s) intervals, Group 2 to a burst of ten shots at 10s intervals, and Group 3 to 100 shots at 10s intervals at a distance of the rifle for which the peak sound pressure level was 152dB. After the first exposure, significant TTS were evident in 21 ears of 28 subjects, and on 18 ears after the second exposure. Delayed TTS were observed in 29% of the TTS cases after the first exposure, and in 56% after the second exposure. Average TTS was largest 1hr after the exposure. Spacing of impulses was of some significance in that in Group 1, nine subjects developed tinnitus and six had medium or severe hyperemia around the malleus, while among Group 2 subjects, hyperemia was seen only in one subject. Damage risk criteria indicated that exposure to 100 rounds was less hazardous than exposure to ten rounds. TTS occurred more frequently and was larger following exposure to short duration impulses. The E-A-R and Gunfender earplugs tested on subjects exposed to rifle, antitank weapon, and Howitzer noise showed that TTS larger than 20dB were seen only in few subjects. The authors conclude that earplugs are sufficient protection against weapon noise, and that a nonlinear earplug seems to be the best answer to this problem.
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