Latent Effects of Low Noise Levels on the Temporary Threshold Shift (TTS)

Reasonable extrapolation of existing experimental data suggests that, while there may be no measurable change in the temporary threshold shift (TTS) following exposure to low noise levels, such exposure may have a sensitizing effect upon the hearing system. The present study measures the TTS following high-level noise exposure under two conditions: (1) a control condition where the high-level noise is preceded by a period of silence; (2) an experimental condition where the high-level noise is preceded by noise levels which alone are not associated with a measurable TTS. Four listeners were exposed to a series of noise conditions for 18 min per condition. The control condition consisted of 15 min of silence followed by a 3 min exposure to a noise level of 118 db SPL. The experimental condition consisted of a 15-min exposure to sensitizing noise levels ranging from 18 to 88 db followed by a 3-min exposure to 118 db. Threshold measurements were obtained at 4000 and 6000 cps for 5 min preceding the sensitizing exposure, and continuously for 10 min after cessation of the 18-min exposure duration. In general, when the sensitizing noise preceded the high-level noise, a significantly greater TTS resulted than when silence preceded the high-level noise. That is, a “sleeper effect” was generated by the sensitizing, low noise levels. This held for most of the listeners, and for most of the sensitizing levels employed. It is concluded, then, that (1) noise exposure levels which in themselves produce no TTS may have a sensitizing, or residual effect upon the hearing system and (2) that zero db of hearing loss following noise exposure does not necessarily constitute complete recovery of the auditory system.