Evidence for the distortion product frequency place as a source of distribution product otoacoustic emission (DPOAE) fine structure in humans. I. Fine structure and higher-order DPOAE as a function of the frequency ratio f2/f1

Critical experiments were performed in order to validate the two-source hyp othesis of distortion product otoacoustic emissions (DPOAE) generation. Mea surements of the spectral fine structure of DPOAE in response to stimulatio n with two sinusoids have been:performed with normal-hearing subjects. The dependence of fine-structure patterns on the frequency ratio f(2)/f(1) was investigated by changing f(1) or f(2) only (fixed f(2) or fixed f(1) paradi gm, respectively), and by changing both primaries at a fixed ratio and look ing at different order DPOAE. When f(2)/f(1) is varied in the fixed ratio p aradigm, the patterns of 2f(1)-f(2) fine structure vary considerably more i f plotted as a function of f(2) than as a function of f(DP) Different order distortion products located at the same characteristic place on the basila r membrane (BM) show similar patterns for both, the fixed-f(2) and Sop para digms, Fluctuations in DPOAE level up to 20 dB can be observed. In contrast , the results from a fixed-f(DP) paradigm do not show any fine structure bu t only an-overall dependence of DP level on the frequency ratio, with a max imum for 2f(1)-f(2) at f(2)/f(1) close to 1.2. Similar stimulus configurati ons used in the experiments have also been used for computer simulations of DPOAE in a nonlinear and active model of the cochlea. Experimental results and model simulations give strong evidence for a two-source model of DPOAE generation: The first source is the initial nonlinear interaction of the p rimaries close to the f(2) place. The second source is:caused by coherent r eflection from a re-emission site at the characteristic place of the distor tion product frequency. The spectral fine structure of DPOAE observed in th e ear canal reflects:the interaction of both these sources. (C) 1999 Acoust ical Society of America. [S0001-4966(99)02812-X].