Assessing the hazards of impulse noise superimposed on background noise

Abstract

Impulsive noise, either in the military or industrial setting, rarely occurs without some substantial background noise. This paper reviews a series of animal experiments which show synergistic interactions between relatively “safe” continuous and impulsive noise. In all the experiments, chinchillas were either exposed to (1) 60 min of background noise (either 2–4 kHz or 0.5–1 kHz) at levels ranging from 95 to 89 dB; (2) 50 spark discharge‐generated impulses with peak sound pressure levels ranging from 158 to 137 dB and A durations of 32 to 64 μs; (3) various combinations of (1) and (2). The animals' hearing was measured immediately after the exposure, and at regular intervals over a 30‐day period. After 30 days, the cochleas were analyzed for losses of sensory cells. The results show that certain combinations of individually “safe” impulse and continuous noise can (1) produce a synergistic interaction; (2) that these interactions behave systematically as either the level of the impulse or the continuous noise is reduced; and (3) that the interaction effect is enhanced as the frequency spectra of the two noises begin to overlap. The results of the experiments will be discussed in terms of the mechanisms of noise‐induced hearing, implications for measurement of noise environments loss, and the implications for public health standards. [Work supported by NIOSH.]