Determination of occupational noise exposure limits for very high intensity impulses when hearing protection is used

Abstract

The U.S. Army needs validation of safe limits for exposure to impulse noise produced by heavy weapons. Current impulse noise limits are based on data from small arms. Recent studies indicate these standards may be overly conservative. In order to define new limits, a systematic study of the effects of high-intensity impulse noise on human volunteers is underway. The number of impulses, the peak pressure level, and spectral distribution of energy are being varied systematically. Four groups of at least 60 volunteers will be given a series of exposures to one of four impulse types. The impulse spectrum is varied by changing the distance between the volunteer and an explosive detonation. The peak pressure level is varied in 3-dB steps by changing the weight of the explosive charge. The number of impulses is 6, 12, 25, 50, or 100. Volunteers wear hearing protection for all exposures. After each exposure, TTS is determined. Each volunteer starts with an exposure of six impulses at the lowest intensity. If the TTS is less than 15 dB, the subject receives six impulses at the next higher level the next day. This continues through all intensities. Then the number of impulses is increased using the maximum intensity permitted by nonauditory injury limits. The first group, using an earmuff as a protective device, has completed all exposures at the first impulse detonation distance of 5 m. The peak sound pressure levels varied from 172 to 192 dB with A durations of approximately 3 ms and B durations of approximately 20 m. No TTS in excess of 15 dB was observed for any condition. The percentages of volunteers unwilling to undergo the higher intensity exposures also will be presented.