The generation of distortion products in a nonlinear transmission line model of the cochlea

A method is presented to study the generation of intermodulation distortion products (IDPs) in a nonlinear model of the human cochlea in detail. The model used is a simple one-dimensional (long wave) transmission line model, using a simple mass-stiffness-damping combination for the local cochlear partition mechanics. Nonlinearity is introduced in the damping term, causing the generation of IDPs. First it is shown that these IDPs have properties in common with experimental data, measured both psychophysically and in otoacoustic emissions. Although there are clear (quantitative) differences with the experimental data, some of the (qualitative) behavior is similar. The method presented here for studying the generation process in more detail assumes that the total excitation at any section of the model and at a certain (IDP) frequency can be regarded as the sum of contributions from all model sections where distortion generation takes place. The resulting patterns of generated distortion and of contributions to emission and excitation at the characteristic position (DP-place) of the 2f(1)-f(2) component explain some of the behavior of this IDP component. It is shown that the generation at the DP-place is influenced by reflection of energy at the stapes boundary, whereas in the case of the emission the phase differences of contributions from the main generation region play a crucial role