Search

Distortion Product Otoacoustic Emissions (DPOAE) offer great potential for hearing diagnosis, but are complicated by the interaction of components generated by different mechanisms. Separation of these components from DPOAE measurements may allow conclusions to be drawn about the functionality of these separate mechanisms of a cochlear. However, the signal processing methods for performing this separation are imperfect. Existing methods are based on time windowing of DPOAE generated from frequency sweep stimuli. This paper presents a method in which the entire spectra of both distortion (D) and reflection (R) components are simultaneously estimated. This approach has several advantages. Firstly, the method removes the need for a compromise between frequency precision and signal to noise ratio. Secondly, the method can be made to include models of the stimulus signals, so that the stimulus does not strongly interfere with the estimation process. Thirdly, the method can be arranged to make efficient use of data that has been corrupted by measurement artefacts. Fourthly, the method can be easily adapted to track DPOAEs that are changing in response to chemical or acoustic treatments. The basic modelling assumptions made are that the sum of R and D measurements can be represented as the sum of convolutions with the stimulus signal, the frequency representation of the D component is more smooth than the R component, and that a reasonable estimate of the noise level in the signal is available. These assumptions are combined into a linear convex problem. In this paper we compare the proposed approach with three other methods. While it is not superior to the earlier methods at every frequency, it does offer some improvement, particularly with regards reducing the contamination of D by R.

Two hydrodynamic effects are introduced in the standard transmission-line formalism, the focusing of the pressure and fluid velocity fields near the basilar membrane and the viscous damping at the fluid-basilar membrane interface, which significantly affect the cochlear response in the short-wave region. In this region, in which the wavelength is shorter than the cochlear duct height, only a layer of fluid of order of the wavelength is effectively involved in the traveling wave. This has been interpreted [8] as a reduced fluid contribution to the system inertia in the peak region, which is a viewpoint common to the 3-D FEM solutions. In this paper we propose an alternative approach, from a slightly different physical viewpoint. Invoking the fluid flux conservation along the traveling wave propagation direction, we can derive a rigorous propagation equation for the pressure integrated along the vertical axis. Consequently, the relation between the average pressure and the local pressure [4] at the fluid-BM interface can be written. The local pressure is amplified by a factor dependent on the local wavenumber with respect to the average pressure, a phenomenon we refer to as "fluid focusing", which plays a relevant role in the BM total amplification gain. This interpretation of the hydrodynamic boost to the pressure provides a physical justification to the strategy [10] of fitting the BM admittance with a polynomial containing both a conjugated pole and a zero. In the short-wave region, the sharp gradients of the velocity field yield a second important effect, a damping force on the BM motion, proportional to the local wavenumber, which stabilizes active models and shifts the peak of the response towards the base, with respect to the resonant place. This way, the peaked BM response is not that of a proper resonance, corresponding to a sharp maximum of the admittance, but rather a focusing-driven growth toward the resonant place, which is "aborted" before reaching it by the sharply increasing viscous losses. The large values of the wavenumber that ensure strong focusing are ultimately fueled, against viscosity, by the nonlinear OHC mechanism, hence the otherwise puzzling observation of a wide nonlinear gain dynamics with almost level-independent admittance.