Studies on the Aural Reflex. II. Reduction of Temporary Threshold Shift from Intermittent Noise by Reflex Activity; Implications for Damage-Risk Criteria

Abstract

The temporary threshold shift (TTS) produced by intermittent noise was compared with that produced by steady noise. It was found that an on-fraction of 0.50 (30-sec bursts of noise alternating with 30 sec of quiet) resulted in a reduction of 50% in the TTS produced by 1200–2400- and 2400–4800-cps octave bands of noise. However, in the case of 300–600- and 600–1200-cps noise, the same on-fraction reduced the TTS to about one-third the value observed after continuous stimulation. The difference is attributed to the action of the middle ear muscles, which attenuates low-frequency sounds more than high-frequency sounds. Burst duration of 600–1200-cps noise was varied, holding the on-fraction constant at 0.50; results indicate that the same fractional reduction in TTS is produced by burst durations from 7.5 sec to 1 min. The significance of these results in regard to damage-risk criteria is discussed. In particular, the inadequacy of the equal-energy rule in assessing the potential danger of intermittent noise exposures is emphasized.