Spontaneous otoacoustic emissions (SOAE) are low level sound pressures appearing in the ear canal without any external excitation. The most salient feature of SOAE is its frequency spectrum being composed of almost discrete spectral lines. Different theories have been proposed to explain the origin of SOAE. It it is widely accepted that the active outer hair cells (OHC) play an important role. In most models the active properties are described by some amplification or negative damping influencing the vibrations of the basilar membrane (BM). In contrast, we use a model that is "close to physiology", although this cannot mean that all the acoustic, mechanic, electric processes are truly covered in the model. Actually the model is a pure circuit model basically representing only one-dimensional vibrations. The active part of the model (the cochlear amplifier) approximates the spatial interaction of the passive and active structures in the cochlea using physiological elements as OHC, active hair bundles, and "Corti resonators". The model reproduces tuning curves and input/output characteristics of the cochlea correctly. Measured input/output characteristics supply valuable data to design the characteristics of OHC. However, it turned out that the effect of OHC alone is not compatible with correct SOAE amplitudes: the SOAE levels generated using the OHC as decisive nonlinear element are too high, showing levels of up to 70 dB SPL. To generate SOAE of natural levels below 10 dB SPL further nonlinear elements (the active hair bundles) are necessary which saturate at 60 dB lower amplitudes. Furthermore the model gives insight into the details of standing and travelling waves on the BM associated with SOAE.
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