Hazard from an intense midrange impulse.

It has been hypothesized that the ear would become increasingly susceptible to impulses (gunfire) as the spectral peak of the impulse approached the frequency region where the ear was tuned best (about 4 kHz for the cat ear) [G. R. Price, J. Acoust. Soc. Am. Suppl. 1 62, S95 (1977)]. This prediction was counter to the predictions of the world's damage-risk criteria for impulse noise. It has been supported by experiments using exposures to 100-Hz and 800- to 1000-Hz impulses; but no test had been run at the point of predicted maximum susceptibility. In the present experiment, three groups of cats were exposed to 50 impulses produced by a primer explosion (spectral peak at 4 kHz) at peak levels of 135, 140, or 145 dB. Auditory thresholds were electrophysiologically measured from the vertex to 2-, 4-, 8-, and 16-kHz tone pips and losses were determined 30 min after exposure and more than 2 months post-exposure. Losses were greatest at 4 kHz, began to develop at 134-dB peak pressure, and the immediate losses grew at a rate of about 7 dB for every dB increase in peak pressure. About half of the loss measured immediately became permanent. The energy required to begin producing a permanent threshold shift was only about 0.07 J/m2, far lower than that required with continuous noises at lower sound pressures. The data were interpreted as supporting the original hypothesis of greater susceptibility in the midrange.