DEPENDENCE OF DISTORTION-PRODUCT OTOACOUSTIC EMISSIONS ON PRIMARY LEVELS IN NORMAL AND IMPAIRED EARS .1. EFFECTS OF DECREASING L(2) BELOW L(1)

The 2f1-f2 distortion-produoctto acoustiecm ission(D POAE) is evokedb y two primaryt oneso frequencis f1 < f2, and level L1 and L2. Previousr eportsi ndicatet hat decreasin L2 below L1 =L2 Can; (1) increase DPOAE amplitudein normalears, and (2) increase the degree to which 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, 2f1-f2 DPOAE-amplitude frequency functions were collected from normal and impaired ears of rabbits and humans, with L1 - = L2, and with L2 < L1, at each of three values of L1. In rabbits, controlled tonal or noise overexposures were used to produce permanent reductions of DPOAE amplitude. Comparison of pre- and postexposure DPOAE-amplitude frequency function demonstrated that the frequency-specific reductions of DPOAEs were enhanced by decreasing L2 below L1. In humans, DPOAE-amplitude frequency functions obtained with the various L1 and L2 combinations were collected from 16 normal ears to provide preliminary normative data for each stimulus-level condition. The L1 = L2 that produced the maximum DPOAE amplitude in normal ears was systematically dependent on L1. Thus at most frequencies, decreasing L2 below L1 = L2 substantially reduced mean DPOAE amplitude when L1 75 dB SPL, but increased mean DPOAE amplitudes at L1 =65 dB SPL. However, the increase of mean DPOAE amplitude obtained by decreasing L2 below L1 =65 dB SPL was small, beingless than 3.5 dB at most frequencies. More importantly, at L1 =65 dB SPL, L2 could be decreased considerably below L1 =L2 without reducing mean DPOAE amplitude relative to that at L1 = L2. Inspection of DPOAE-amplitud frequency functions obtained from subjects with mild or moderate sensorineural hearing losse indicated that, in frequency regions of hearing impairment, decreasing L2 below L1 can enhance the degree of reduction of DPOAEs below the corresponding normative amplitudes, without reducing the normative amplitudes. It is concluded that decreasing L2 below L1 = L2 has the potential to enhance the performance of DPOAEs in clinical application. © 1995, Acoustical Society of America. All rights reserved.