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Firing from Enclosures with 90mm Recoilless Rifle. Assessment of Acoustic Hazard.
HUMAN ENGINEERING LAB ABERDEEN PROVING GROUND MD
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In order to assess the acoustic hazard of exposure to the firing of a 90mm recoilless rifle from within an enclosure, eight cats, each with a plug in one ear, were exposed to one round fired from within a moderate sized reinforced concrete room. A similar exposure was made in the open with an additional group of animals. Hearing sensitivity was measured electrocochleographically 2 months after the exposure. The peak sound pressures were about 186 dB in both outdoor and indoor exposures however the B-duration was about 24 msec outdoors and 285 msec indoors. The mean threshold sensitivity of the group of protected ears exposed indoors suffered no significant loss, although one individual did. The group of unprotected ears exposed indoors did suffer significant permanent losses. Only one ear of the outdoor group provided interpretable data, and it showed no loss when exposed without an ear plug. Given that the cat is more sensitive to loss than the human, it was concluded that exposure to one round within the enclosure, with ear protection, would probably be safe for the human. Without ear protection, the exposure would probably be hazardous. The data are additionally interpreted as showing that the present criterion for estimating hazard, MIL-STD-1474AMI, overestimates the risk from pulses with most of their energy in the low-frequency region of the spectrum. Author
APPROVED FOR PUBLIC RELEASE