The influence of distributed source regions in the formation of the nonlinear distortion component of cubic distortion-product otoacoustic emissions.

Distortion product otoacoustic emissions (DPOAEs) are evoked by two stimulus tones with frequency f and f of ratio f/f  in the range between approximately 1.05 and 1.4. This study theoretically and experimentally analyzes the cubic 2f-f DPOAE for different stimulus levels of one of the tones while the other is constant. Simulations for f/f of 1.2 and moderate stimulus levels (30-70 dB sound pressure level) indicate that cubic distortion products are generated along a relatively large length of the basilar membrane, the extent of which increases with stimulus level. However, apical from the place of maximum nonlinear force, the wavelets generated by these distributed sources mutually cancel. Therefore, although the spatial extent of the primary DPOAE sources broadens with increasing stimulus level (up to 1.5 oct), the basilar-membrane region contributing to the DPOAE signal is relatively narrow (0.6 oct) and level independent. The observed dependence of DPOAE amplitude on stimulus level can be well-approximated by a point source at the basilar-membrane place where the largest distortion product (maximum of the nonlinear force) is generated. Onset and offset of the DPOAE signal may contain amplitude overshoots (complexities), which are in most cases asymmetrical. Two-tone suppression was identified as the main cause of these onset and offset complexities. DPOAE measurements in two normal-hearing subjects support the level dependence of the steady-state DPOAE amplitude and the asymmetry in the onset and offset responses predicted by the theoretical analysis.