Sensitivity of distortion-product otoacoustic emissions in humans to tonal over-exposure: Time course of recovery and effects of lowering L₂

An important concern of industrial hearing-conservation programs is detecting the onset of noise-induced hearing loss. If it can he shown that otoacoustic emissions are sufficiently sensitive to reliably detect auditory fatigue and the permanent hearing loss that eventually develops, they could become an important part of the hearing-conservation test battery. The present study in humans was designed to examine the influence of overall primary-tone level and the effects of lowering the f₂ primary on the sensitivity of distortion-product otoacoustic emissions (DPOAEs) to acoustic overstimulation. One ear from each of 14 subjects with normal hearing was exposed to a 105-dB SPL pure tone at 2.8 kHz for 3 min using a protocol consisting of distinct pre-exposure, exposure, and post-exposure periods. As a quantitative index of the functional status of the outer hair cells, 2f₁-f₂ DPOAEs were monitored systematically over time using four stimulus-test conditions consisting of either one of two levels of equilevel primary tones, or one of two levels of offset primaries, with L₂ set 25 dB lower than L₁. The overall finding was that the DPOAE protocol incorporating both the lowest level of stimulation and an f₂-primary tone that was 25 dB below the level of the f₁ stimulus [i.e., L₁ (55 dB SPL) - L₂ (30 dB SPL) = 25 dB]was most sensitive to the exposure effects. The results establish that DPOAEs elicited with unequal, in contrast to equal-level primaries, have comparable signal-to-noise ratios, but are considerably more sensitive to reductions in emission levels induced by exposure to short-lasting, moderately intense tones. The recovery of DPOAE amplitudes over the first 15 min post-exposure appeared to be roughly linear in log time and, in many cases, could be closely approximated by fitting a logarithmic curve to the post-exposure data. From these functions, the initial amount of loss (γ-intercept) and the slope of recovery were identified as potential measures of vulnerability to acoustic exposure in that these variables appeared to be related to the susceptibility of some of the subjects, who also participated in a subsequent experiment on the behavioral effects of the exposure stimulus. Finally, compared to behaviorally measured temporary threshold shift (TTS), the time course of the recovery for DPOAEs was very similar, suggesting that, with the appropriate parameters, DPOAEs can be as sensitive to TTS as routine pure-tone audiometry.