Asymptotic threshold shift in chinchillas exposed to impulse noise.

Five monaural chinchillas were exposed to a repetitive, reverberant, impulse noise for ten days. The impulse-noise source was a mechanized hammer hitting a steel plate at a rate of 1/s. The average, peak over pressure within the holding cage was 113 dB SPL. Auditory thresholds were determined before and after exposure at 0.5, 1, 1.4 2, 2.8, 4, 8, and 16 kHz utilizing shock-avoidance conditioning. During exposure thresholds were monitored on a daily basis at 0.5 and 8 kHz. Within one hour from the start of the exposure, threshold shift at 8 kHz had reached an asymptotic level, while threshold shift at 0.5 kHz leveled off after 24 h of exposure. Asymptotic threshold shift (ATS) levels of both frequencies varied between 30 and 50 dB across animals. Median threshold shift for both frequencies recovered to within 10 dB of normal after 40 days of recovery. Maximum permanent threshold shift (PTS) was at 2 and 2.8 KHZ with median PTS of 17 and 13 dB, respectively. After final thresholds were obtained (50-70 days), the cochleas were examined histologically using the surface preparation technique. A comparison is made between impulse-noise-induced ATS and ATS produced by continuous-noise sources.