Abstract
Three monaural chinchillas were exposed to noise (300–600 Hz, 100-dB SPL) for an effective duration of 193 min. The experiment was replicated six times each at two-week intervals. Tests of quiet thresholds at 715 Hz were interspersed within each exposure. Threshold levels increased from 0- to 38-dB SPL with the logarithm of exposure duration. Postexposure audibility curves were uniform across low frequencies but increased to maximum at 2.0 kHz for two of the animals. All thresholds returned to normal in about 48 h. Two chinchillas were then exposed to this noise for seven days. Thresholds measured during brief quiet periods increased during the first day of exposure and remained about constant thereafter. After completion of the exposure, threshold shifts were found throughout the audible range and they recovered nearly to normal in about one week. However, small permanent threshold shifts are probably present below 1 kHz. [Work supported by the National Institutes of Health, Public Health Service, U. S. Department of Health, Education, and Welfare.]
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