Distortion product otoacoustic emissions (DPOAEs) are composed of distortion and reflection components. Much is known about the influence of the stimulus frequency ratio (f2/f1) on the overall/composite DPOAE level. However, the influence of f2/f1 on individual DPOAE components is not as well examined. The goals of this pilot study were to systematically evaluate the effects of f2/f1 on DPOAE components in clinically normal-hearing young adult ears. To extend the limited reports in the literature, this examination was carried out over an extended frequency range using two stimulus-level combinations.
DPOAEs were recorded from seven normal-hearing, young adult ears for f2 frequencies between 0.75 and 16 kHz over a range of f2/f1 using two stimulus-level combinations. The distortion (DPOAED) and reflection (DPOAER) components were separated using an inverse fast Fourier transform algorithm. Optimal ratios for the composite DPOAE and DPOAE components were determined from smoothed versions of level versus ratio functions in each case.
The optimal ratio for the composite DPOAE level increased with stimulus level and decreased as a function of frequency above 1 kHz. The optimal ratios for the DPOAE components followed a similar trend, decreasing with increasing frequency. The optimal ratio for DPOAED was generally higher than that for DPOAER. The overall level for DPOAED was greater than that of DPOAER, both decreasing with increasing frequency. DPOAER, but not DPOAED, became unrecordable above the noise floor at the higher frequencies.
DPOAE components behave similarly but not identically as a function of f2/f1. The ear canal DPOAE is generally dominated by DPOAED. The behavior of DPOAED as a function of f2/f1 is entirely consistent with known properties of cochlear mechanics. The behavior of DPOAER is more variable across ears, perhaps reflective of the increased number of parameters that influence its final form. Attempting to use an f2/f1 that would allow a greater bias of the ear canal DPOAE toward one component or the other does not appear to be practical.