Temporary Threshold Shifts Produced by High Intensity Freefield Impulse Noise inHumans Wearing Hearing Protection

摘要

Exposure to high intensity impulse noise produced by modern military weapons isknown to be hazardous to hearing. Hearing protection is required; however, there is no generally accepted theoretical way to predict whether protection will be adequate for the highest noise levels. This had led us to empirically determine the safe limits of exposure to impulse noise when hearing protection is used by exposing human volunteers under controlled conditions. Over the past 5 years, a series of studies has been conducted to determine the maximum safe exposure to high intensity freefield impulse noise. An exposure was considered to be safe if it produced only a small temporary threshold shift (TTS < 25 dB) in a small percentage of the volunteers exposed. Three different impulses were used with A-durations of 0.8, 1.4, and 2.9 ms. Both the level and number of impulses were varied to find the maximum tolerable exposure for combinations of these parameters. The peak sound pressure levels ranged up to 196 dB. The number of impulses was varied from 6 to 100. Approximately 60 volunteers were exposed to each type of impulse, allowing high confidence estimates of the exposures which would produce no significant TTS in 95 percent of the exposed population. The hearing protection used was an ear muff which had been modified to simulate a poor fitting protector. The results of these studies indicated that even with a relatively poor hearing protector, combinations of level and number of impulses which far exceed our currently accepted exposure limits could be tolerated by 95 percent of the volunteers.