Hearing threshold shifts from repeated 6-h daily exposure to impact noise.

Exposure of chinchillas to broadband, high-level impact noise on an interrupted 6-h daily schedule over 20 days has shown that pure-tone thresholds measured immediately following each daily exposure improve as much as 30 dB despite the continuing noise exposure. The time constant of this recovery effect (toughening) and the magnitude of the effect are related to the audiometric test frequency and the exposure energy. The trauma, quantified by permanent threshold shifts and sensory cell losses, produced by the interrupted exposure paradigm is generally less than that produced by an equal-energy uninterrupted exposure. The wide variations in the temporal pattern of threshold shift across similarly exposed animals suggest that the toughening effect reflects the underlying susceptibility of that animal to noise trauma.