Dependence of distortion-product otoacoustic emissions on primary levels in normal and impaired ears. I. Effects of decreasing L2 below L1

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Abstract
The 2f 1−f 2 distortion‐product otoacoustic emission (DPOAE) is evoked by two primary tones of frequencies f 1<f 2, and levels L 1 and L 2. Previous reports indicate that decreasing L 2 below L 1=L 2 can; (1) increase DPOAE amplitude in normal ears, and (2) increase the degree to which DPOAE amplitudes are reduced by cochlear trauma. Although both of these factors could be advantageous for clinical applications of DPOAEs, neither has been explored in detail. In the present study, 2f 1−f 2 DPOAE‐amplitude frequency functions were collected from normal and impaired ears of rabbits and humans, with L 1=L 2, and with L 2<L 1, at each of three values of L 1. In rabbits, controlled tonal or noise overexposures were used to produce permanent reductions of DPOAE amplitudes. Comparison of pre‐ and postexposure DPOAE‐amplitude frequency functions demonstrated that the frequency‐specific reductions of DPOAEs were enhanced by decreasing L 2 below L 1. In humans, DPOAE‐amplitude frequency functions obtained with the various L 1 and L 2 combinations were collected from 16 normal ears to provide preliminary normative data for each stimulus‐level condition. The L 1−L 2 that produced the maximum DPOAE amplitude in normal ears was systematically dependent on L 1. Thus at most frequencies, decreasing L 2 below L 1=L 2 substantially reduced mean DPOAE amplitude when L 1≥75 dB SPL, but increased mean DPOAE amplitudes at L 1=65 dB SPL. However, the increase of mean DPOAE amplitude obtained by decreasing L 2 below L 1=65 dB SPL was small, being less than 3.5 dB at most frequencies. More importantly, at L 1=65 dB SPL,L 2 could be decreased considerably below L 1=L 2 without reducing mean DPOAE amplitude relative to that at L 1=L 2. Inspection of DPOAE‐amplitude frequency functions obtained from subjects with mild or moderate sensorineural hearing losses indicated that, in frequency regions of hearing impairment, decreasing L 2 below L 1 can enhance the degree of reduction of DPOAEs below the corresponding normative amplitudes, without reducing the normative amplitudes. It is concluded that decreasing L 2 below L 1=L 2 has the potential to enhance the performance of DPOAEs in clinical applications.

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