Susceptibility of the noise-toughened auditory system to noise-induced trauma.

The auditory system 'toughened' by an interrupted noise exposure has been shown in several reports, to be less affected by (or protected from) a subsequent high level noise exposure. A group of chinchillas (n = 12) was exposed to an interrupted noise at 95 dB SPL, 0.5 kHz octave band, 6 h/day for 10 days. Threshold shifts measured over the 10 day exposure showed that the animals responded by either (1) developing a large toughening effect (i.e., thresholds after day 10 of the exposure were considerably better than at the end of day 1) (n = 5) or (2) not showing any toughening, instead thresholds continued to get worse over the course of the exposure (n = 7). After a 5 day interval, during which thresholds of all the animals returned to normal, they, along with a control group (n = 10) not exposed to the interrupted noise, were exposed to an asymptotic threshold shift producing traumatic noise (127 dB peak SPL narrow band impact, 1 kHz center frequency, 24 h/day for 5 days). Auditory evoked potential audiometry and surface preparation histology showed that there were no statistically significant differences in the response of any of the above groups to the traumatic noise. The interrupted noise exposure, whether it produced a toughening or not, did not provide any protection from a subsequent high-level noise.