Interaction of continuous and impulse noise: Audiometric and histological effects

Continuous and impulse noises were combined to model more realistic noise environments. Monaural chinchillas were exposed to one of the following conditions: (a) 50 impulses with 40-μsec A duration at 158-dB-SPL peak pressure, 1/min; (b) 95-dB-SPL continuous noise at 2-4 kHz for one hour; (c) superimposed combination of the continuous and impulse noise; (d) 50 impulses with a 40-μsec A duration at 175-dB-SPL peak pressure, 1/min; (e) interrupted combination of the continuous and impulse noise with the continuous noise off for 2 sec during the impulse delivery. Quiet thresholds were measured before and after exposure using the auditory evoked response and histology was obtained using the surface preparation technique. The audiometric and histological findings agree in showing that the superimposed combination of two noise exposures, shown to be safe for the chinchilla, produces traumatic effects that more than exceed the additive effects of either component. The existing damage risk criteria do not provide guidelines for such noise combinations.