Cochlear generation of intermodulation distortion revealed by DPOAE frequency functions in normal and impaired ears

Distortion product otoacoustic emission (DPOAE) frequency:functions were measured in normal-hearing and hearing-impaired ears. A fixed-f(2)/swept-f(1) paradigm was used with f(2) fixed at half-octave intervals from 1 to 8 kHz. L-1 was always 10 dB greater than L-2, and L-2 was varied from 65 to 10 dB SPL in 5-dB steps. The responses were quantified by the frequency and amplitude of the Peak response. Peak responses were closer to f(2) in higher frequency regions and for lower intensity stimulation. Results from hearing-impaired subjects suggest that audiometric thresholds at the distortion product frequency, f(dp), in addition to hearing status at f(2), can affect DPOAE results. Results are discussed in terms of several manifestations of a second resonance model, as well as a dual source model for the generation of DPOAEs as measured in the ear canal of humans. It appears that a dual source model accounts for the data better than second filter models. (C) 1999 Acoustical Society of America. [S0001-4966(99)03911-9].