An Immersed Boundary Model of the Cochlea with Parametric Forcing

The cochlea or inner ear has a remarkable ability to amplify sound signals. This is understood to derive at least in part from some active process that magnifies vibrations of the basilar membrane (BM) and the cochlear partition in which it is embedded, to the extent that it overcomes the effect of viscous damping from the surrounding cochlear fluid. Many authors have associated this amplification ability to some type of mechanical resonance within the cochlea, however there is still no consensus regarding the precise cause of amplification. Our work is inspired by experiments showing that the outer hair cells within the cochlear partition change their lengths when stimulated, which can in turn cause periodic distortions of the BM and other structures in the cochlea. This paper investigates a novel fluid-mechanical resonance mechanism that derives from hydrodynamic interactions between an oscillating BM and the surrounding cochlear fluid. We present a model of the cochlea based on the immersed boundary method, in which a small-amplitude periodic internal forcing due to outer hair cells can induce parametric resonance. A Floquet stability analysis of the linearized equations demonstrates the existence of resonant (unstable) solutions within the range of physical parameters corresponding to the human auditory system. Numerical simulations of the immersed boundary equations support the analytical results and clearly demonstrate the existence of resonant solution modes. These results are then used to illustrate the influence of parametric resonance on wave propagation along the BM and explicit comparisons are drawn with results from another two-dimensional cochlea model.