GENERATION OF DISTORTION PRODUCTS IN A MODEL OF COCHLEAR MECHANICS

Considerable evidence implicates outer hair cells (OHC) as being responsible for the generation of distortion products (DP) in cochlear mechanics. Forces exerted on the basilar membrane by OHCs must saturate at high levels and, thereby, become a major source of nonlinearity. Realistic growth of distortion products requires that the saturation of OHC forces (with increasing level) be gradual. The apparent band-pass filtering of the distortion product otoacoustic emission (DPOAE) observed in ear canal sound pressure can be explained as the combination of two effects. (1) The DPOAE becomes smaller as the primary frequencies are separated because the basilar membrane vibration becomes smaller at the place where the two tones interact. (2) The DPOAE becomes smaller as the primary frequencies are brought closer together because reflected energy from the DP place partially cancels the retrograde signal. This explanation does not require the presence of any “second filter ” in the mechanics of the cochlear partition. With a nonlinear, active model of cochlear mechanics, we have explored whether a simulated hearing loss at the DP frequency can influence the apparent band-pass filtering of DPs, which are generated primarily at the f2 place. Results of this simulation are compared with DP measurements from a subject with low-frequency hearing loss.