Long‐term, asymptotic threshold shift in chinchillas exposed to repetitive, reverberant, impulse noise

Five monaural chinchillas were exposed to a repetitive, reverberant, impulse noise for ten days. The impulse‐noise source was a mechanized hammer pounding on a steel plate at a rate of 1 impulse/sec. 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 by means of a behavioral technique utilizing shock‐avoidance conditioning. During exposure, thresholds at 0.5 and 8 kHz were monitored on a daily basis. Within eight hours from the start of the exposure, threshold shifts at both frequencies had reached asymptotic levels of from 30 to 40 dB. Upon removal from the noise, the threshold shift at 8 kHz recovered more rapidly than at 0.5 kHz; however, at 40 days, both had returned to within 10 dB of the pre‐exposure thresholds. A maximum PTS of 20–30 dB was found at either 2 or 2.8 kHz. Cochleagrams were obtained for each animal 60 days following the exposure. The histological examination provided support for the audiological findings.