Two-tone Distortion Products in a Suppression Model of the Basilar Membrane

Abstract

When we hear two tones of different frequencies (f₁<f₂), the additional tones called combination tones (e.g. 2f₁-f₂, 3f₁-2f_1…) are audible in addition to these primary tones. These combination tones are considered to be generated by the nonlinear responses of the basilar membrane to the primary tones. Distortion product otoacoustic emissions (DPOAEs) are the phenomena reflecting these response. It is well known that the amplitude of DPOAEs and the intensity of combination tones are far larger at 2f₁-f₂ than at 2f₂-f₁. The present study was designed to explain the generation mechanism of DPOAEs and the difference in amplitude between 2f₁-f₂ and 2f₂-f₁ using a mathematical model of the cochlea based on the current theory in cochlear physiology.
When two primary tones are input to the cochlea, the movement of the basilar membrane is suppressed in the area between the displacement maxima for f₁ and f₂ so as to obtain better discrimination. Since the tuning curve of the basilar membrane movement is asymmetrical (sharper at the apex side), this suppression effect is larger on the lower frequency tone (f₁) than on the higher one (f₂). It was theoretically explained using a mathematical model how this difference in suppression effect makes the 2f₁-f₂ amplitude higher than of 2f₂-f₁.