  This paper discusses the role of 2-D/3-D cochlear fluid hydrodynamics in the
generation of the large nonlinear dynamical range of the basilar membrane (BM)
and pressure response, in the decoupling between cochlear gain and tuning, and
in the dynamic stabilization of the high-gain BM response in the peak region.
The large and closely correlated dependence on stimulus level of the BM
velocity and fluid pressure gain (Dong and Olson, 2013), is consistent with a
physiologically-oriented schematization of the outer hair cell (OHC) mechanism
if two hydrodynamic effects are accounted for: amplification of the
differential pressure associated with a focusing phenomenon, and viscous
damping at the BM-fluid interface. The predictions of the analytical 2-D WKB
approach are compared to solutions of a 3-D finite element model, showing that
these hydrodynamic phenomena yield stable high-gain response in the peak region
and a smooth transition among models with different effectiveness of the active
mechanism, mimicking the cochlear nonlinear response over a wide stimulus level
range. This study explains how an effectively anti-damping nonlinear OHC force
may yield large BM and pressure dynamical ranges along with an almost
level-independent admittance.
