Growth of distortion product otoacoustic emissions: dependence on frequency separation and frequency

The present study was conducted to determine the dependence of distortion product otoacoustic emissions (DPOAEs), specifically the 2f1-f2 distortion product (DP), on the frequency separation of the primary signals used to elicit the DPOAEs. DPOAEs were recorded in response to the experimental signals from nine human participants with normal auditory function in the test ear as determined by otoscopy, pure tone audiometry, tympanometry, ipsilateral acoustic-reflex thresholds and DPOAEs. The signals used to elicit 2f1-f2 DP were tone pairs (the primary signals, f1 and f2). Both primary signals were fixed at a level of 65 dB SPL. The higher frequency primary signal, f2, was fixed at one of four frequencies (1000, 2000, 3000 and 4000 Hz). The lower frequency primary signal, f1, was varied across ten different frequency separations, for each of the four f2 frequencies. For each experimental signal, a 2f1-f2 DP was measured in level (dB SPL). For each f2 frequency, a DP growth function was obtained. The shapes and slopes of the DP growth functions were studied to determine if the rate of growth of 2f1-f2 DPOAEs depend upon the frequency region of the primary tones. The results of the present study as well as those of Harris, Lonsbury-Martin, Stagner,Coats and Martin (1989) showed that the rate of growth of the 2f1-f2 DP depends on frequency separation and frequency. The onset of the DP growth functions was dependent on frequency, and the slopes of the DP growth functions increased systematically as a function of frequency. The increase in slope with frequency was the expected finding if these DPOAE phenomena are related to the interaction of the traveling wave envelopes created by the excitation of the two primary signals in the cochlea.