The mechanical waveform of the basilar membrane. II. From data to models -and back

Mechanical responses in the basal turn of the guinea-pig cochlea are measur ed with low-level broad-band noise as the acoustical stimulus []for details see de Beer and Nuttall, J. Acoust. Sec. Am. 101, 3583-3592 (1997)]. Resul ts are interpreted within the framework of a classical three-dimensional mo del of the cochlea that belongs to a very wide class of nonlinear models. T he use of linear-systems analysis for this class of nonlinear models has be en justified earlier [de Beer, Audit. Neurosci. 3, 377-388 (1997)]. The dat a are subjected to inverse analysis with the aim to recover the "effective basilar-membrane impedance." This is a parameter function that, when insert ed into the model, produces a model response, the ''resynthesized'' respons e, that is similar to the measured response. With present-day solution meth ods, resynthesis leads back to an almost perfect replica of the original re sponse in the spatial domain. It is demonstrated in this paper that this al so applies to the response in the frequency domain and in the time domain. This paper further reports details with regard to geometrical properties of the model employed. Two three-dimensional models are studied; one has its dimensions close to that of the real cochlea, the other is a stylized model which has homogeneous geometry over its length. In spite of the geometric differences the recovered impedance functions are very similar. An impedanc e function computed for one model can be used in resynthesis of the respons e in the other one, and this leads to global amplitude deviations between o riginal and resynthesized response functions not exceeding 8 dB. Discrepanc ies are much larger (particularly in the phase) when a two-dimensional mode l is compared with a three-dimensional model. It is concluded that a styliz ed three-dimensional model with homogeneous geometric parameters will give sufficient information in further work on unraveling; cochlear function via inverse analysis. In all cases of a sensitive cochlea stimulated by a sign al with a stimulus level of 50 dB SPL per octave or less, the resulting bas ilar-membrane impedance is found to be locally active, that is, the impedan ce function shows a region where the basilar membrane is able to amplify ac oustic power or to reduce dissipation of power by the organ of Corti. Final ly, the influence of deliberate errors added to the data is discussed in or der to judge the accuracy of the results. (C) 2000 Acoustical Society of Am erica. [S0001-4966(00)02703-X].