Time-domain demonstration of distributed distortion-product otoacoustic emission components

Distortion-product otoacoustic emissions (DPOAEs) were measured in rabbits as time waveforms by employing a phase-rotation technique to cancel all components in the final average, except the 2f(1)-f(2) DPOAE. Subsequent filtering allowed the DPOAE waveform to be clearly visualized in the time domain. In most conditions, f(2) was turned off for 6 ms, which produced a gap so that the DPOAE was no longer generated. These procedures allowed the DPOAE onset as well as the decay during the gap to be observed in the time domain. DPOAEs were collected with L-1 = L-2 = 65-dB sound pressure level primary-tone levels for f(2)/f(1) ratios from 1.25 to 1.01 in 0.02 steps. Findings included the appearance of complex onsets and decays for the DPOAE time waveforms as the f(2)/f(1) ratio was decreased and the DPOAE level was reduced. These complexities were unaffected by interference tones (ITs) near the DPOAE frequency place (f(dp)), but could be removed by ITs presented above f(2), which also increased DPOAE levels. Similar outcomes were observed when DPOAEs were measured at a sharp notch in the DPOAE level as a function of the f(2) primary tone frequency, i.e., DP-gram. Both findings were consistent with the hypothesis that the DPOAE-ratio function, and some notches in the DP-gram, are caused by interactions of distributed DPOAE components with unique phases.