Temporary Threshold Shifts from Prolonged Exposure to Noise

Abstract

Eight monaural chinchillas were trained to respond to tones, and their auditory thresholds were measured behaviorally. The animals were then exposed to octaveband noise centered at either 0.5 or 4.0 k Hz for periods of two to 21 days. Octaveband levels (OBL) between 65 and 105 dB SPL were used. The growth of temporary threshold shift (TTS) was measured during brief interruptions in the exposure. TTS increased for the first 24 to 48 hours of an exposure and then reached an asymptote and remained constant for as long as the exposure continued. At asymptote the relation between the TTS and the level of the octave band centered at 0.5 k Hz was TTS ₄ ∞ = 1.6 (OBL-65), where TTS ₄ ∞ was measured with a test tone of 0.715 k Hz at four minutes after an interruption of the noise. Decay of TTS after termination of the exposure was slow and approximately exponential with a time constant of about 29 hours. Empirically, the time for TTS to decay to near-zero values ranged from three to six days. The course of decay of TTS was independent of the duration of the exposure once asymptote had been reached.