The growth of and recovery from TTS in human subjects exposed to impact noise.

It is now recognized that temporary threshold shift (TTS) grows to an asymptotic level (ATS) after a prolonged exposure to steady-state and intermittent noise. Few studies have been conducted to verify this process of acquisition for impact noise. However, results obtained on animals have demonstrated that an asymptote is reached after 1-2 h of exposure. These results have been confirmed in the present study on four human subjects using a 470-ms (B duration) impact noise at a rate of 1 pps. The above pattern of acquisition has been retested on two subjects exposed to the same impact for periods of 15-150 min. An ATS of 15-20 dB was measured (Lp = 102-104 dBA) after 30-60 min of exposure. Recovery curves were determined for each exposure duration. They appeared to follow a logarithmic function of post-exposure time. It took from 30-50 min for one subject and from 4-5 h for the other to completely recover. The time for total recovery did not seem to vary as a function of exposure time. Implications are drawn for damage risk criteria and for the study of the effects of impact noise on hearing.