Distortion-product source unmixing: A test of the two-mechanism model for DPOAE generation

This paper tests key predictions of the "two-mechanism model" for the gener ation of distortion-product otoacoustic emissions (DPOAEs). The two-mechani sm model asserts that lower-sideband DPOAEs constitute a mixture of emissio ns arising not simply from two distinct cochlear locations las is now well established) but, more importantly, by two fundamentally different mechanis ms: nonlinear distortion induced by the traveling wave and linear coherent reflection off pre-existing micromechanical impedance perturbations. The mo del predicts that (1) DPOAEs evoked by frequency-scaled stimuli (e.g., at f ixed f(2)/f(1)) can be unmixed into putative distortion- and reflection-sou rce components with the frequency dependence of their phases consistent wit h the presumed mechanisms of generation; (2) The putative reflection-source component of the total DPOAE closely matches the reflection-source emissio n (e.g., low level stimulus-frequency emission) measured at the same freque ncy under similar conditions. These predictions were tested by unmixing DPO AEs into components using two completely different methods: (a) selective s uppression of the putative reflection source using a third tone near the di stortion-product frequency and (b) spectral smoothing (or, equivalently, ti me-domain windowing). Although the two methods unmix in very different ways , they yield similar DPOAE components. The properties of the two DPOAE comp onents are consistent with the predictions of the two-mechanism model. (C) 2001 Acoustical Society of America.