Interaction of spectrally‐mismatched continuous and impulse‐noise exposures in the chinchilla

Combinations of “safe,” spectrally overlapping continuous and impulse‐noise exposures have been shown to have a potentiating interaction [R.P. Hamernik et al., J. Acoust. Soc. Am. 55, 117 (1974)]. The present study was designed to determine whether matching of the impulse and continuous‐noise spectra is a necessary condition for the interaction effect. Ten monaural chinchillas were exposed to a combination of 60 min of 500–1000‐Hz noise at a level of 100 dB SPL and 50 impulses (CF ≈ 6 kHz; 40 μsec A duration) presented at a peak pressure of 158 dB SPL and a rate of 1/min. Thresholds were determined before and after exposure using either averaged‐evoked‐response AER) or shock‐avoidance conditioning audiometry. A control group of five monaural chinchillas was exposed to the continuous noise alone. The control animals developed no permanent threshold shifts and two of the five animals had slight cochlear damage. The aforementioned study (Hamernik et al., 1974) showed that the 158 dB, impulse‐noise exposure also produced no PTS and three of eight animals had slight cochlear damage. However, of the animals exposed to the combined noises, five of ten animals had a significant increase in TTS_max over the controls and these same five animals developed a significant PTS (>15 dB) after 30 days of recovery. Of the five cochleas which have been histologically examined, three show moderate to heavy damage. Our tentative conclusion is that the spectrally mismatched combination has an interaction with increased traumatic power, but the effect is not as great as when the spectral distribution of the impulse and the background noise is more closely matched.