Low-frequency modulation of the 2f(1)-f(2) distortion product otoacoustic emissions in the human ear

Low-frequency masking is a recent clinical procedure for the differential d iagnosis of sensory hearing loss. Currently this requires the recording of the phase-dependent masked subjective threshold, which is time consuming an d not always accurate. As an objective method, the recording of modulated d istortion product otoacoustic emissions (DPOAEs) can be performed continuou sly, and with better frequency specificity. Results of measurements of the low-frequency modulated two-tone DPOAE 2f(1)-f(2) in the human ear, and its dependence on various acoustic parameters, are presented here for the firs t time. Similar to the masked hearing threshold, the pattern of the phase-d ependent modulated DPOAEs displayed two minima, at the phases of maximal ra refaction and condensation, respectively, with a latency of about 4 ms (sup pressor frequency 32.8 Hz). The smaller dip, at maximal condensation, appea red only for a high suppressor level, and for a low level of the primary to ne f(2). The modulating effect measured for the primary frequencies f(1) = 2.5 kHz and f(2) = 3 kHz, decreased for 4 and 4.8 kHz, and vanished for 5 a nd 6 kHz. The results are discussed using a cubic distortion model based on the Boltzmann function for mechano-electrical transduction of the hair cel ls. The saturation behavior of the increase of the DPOAE level at different phases is compared with the growth rates of the DPOAE level in normal hear ing and in sensory hearing loss. (C) 1999 Elsevier Science B.V. All rights reserved.