Abstract
A box shape with constant area is often used to represent the complex geometry in the cochlea, although variation of the fluid chambers areas is known to be more complicated. This variation is accounted for here by an "effective area," given by the harmonic mean of upper and lower chamber area from previous measurements. The square root of this effective area varies linearly along the cochleae in the investigated mammalian species. This suggests the use of a linearly tapered box model in which the fluid chamber width and height are equal, but decrease linearly along its length. The basilar membrane (BM) width is assumed to increase linearly along the model. An analytic form of the far-field fluid pressure difference due to BM motion is derived for this tapered model. The distributions of the passive BM response are calculated using both the tapered and uniform models and compared with human and mouse measurements. The discrepancy between the models is frequency-dependent and becomes small at low frequencies. The tapered model developed here shows a reasonable fit to experimental measurements, when the cochleae are cadaver or driven at high sound pressure level, and provides a convenient way to incorporate cochlear geometrical variations.
Highlights
The cochlea is a sensory organ of the hearing system responsible for converting sound-induced motion into electrochemical impulses for perception
As an improvement and extension to the previous work, a modification of the widely used uniform box model is developed here with a linear variation of fluid chamber cross-sectional areas and the basilar membrane (BM) width, which both play an important role in the BM passive response
The BM velocity calculated using the linearly tapered model is greater than that using the uniform models and the difference in BM velocity peak is frequency dependent, e.g., the difference is about 20 dB at 5000 Hz but about 5 dB at 500 Hz
Summary
The cochlea is a sensory organ of the hearing system responsible for converting sound-induced motion into electrochemical impulses for perception. The assumption that the box model is rectangular may appear to be physically unrealistic; de Boer (1991) has shown that similar results for the three-dimensional, 3D, fluid coupling are obtained if the cross section is assumed circular This reflects the fact that the far-field, plane wave component of the fluid pressure depends mainly on the cross-sectional area of the chambers rather than their shape. Shera et al (2004) derive a similar expression, Eq (A1) in Shera et al (2004), Appendix A, for the far-field, long-wavelength, fluid coupling in a tapered cochlear model with a constant BM width Geometrical features, such as the cross-sectional area of the fluid chambers, are crucial for quantitative modeling of the cochlea. V, the coupled response of the cochlea is calculated using the linearly tapered and uniform models of the human and mouse cochleae and compared with experimental measurements
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