Temporary and Permanent Threshold Shifts Produced by Nine-Day Exposures to Noise

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Abstract
Four groups of monaural chinchillas, trained in behavioral audiometry, were exposed for nine days in a diffuse sound field to an octave-band noise centered at 4.0 k Hz. Each group (N = 4) was exposed to a different level of noise-80, 86, 92, or 98 dB SPL. Threshold shifts measured about four minutes after exposure (TS4) increased during the first 24 hours of exposure and then reached an asymptote that was maintained for the remainder of the exposure. TS4 at asymptote (TS4∞), in the frequency region of maximum shift, can be described by the equation TS4∞ = 1.7 (SPL-47.0). Whereas the threshold shifts of the group exposed at 80 dB were purely temporary and decayed from an asymptote to zero in three to six days, the threshold shifts of the group exposed at 86, 92, or 98 dB had both temporary and permanent components and decayed from an asymptote to a final value in about 15 days. The magnitude of the permanent threshold shifts and their spread along the frequency dimension are related directly to the level of the noise.

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