  High auditory sensitivity, sharp frequency selectivity, and otoacoustic
emissions are signatures of active amplification of the cochlea. The human ear
can also detect very large amplitude sound without being damaged as long as the
exposed time is not too long. The outer hair cells are believed as the best
candidate for the active force generator of the mammalian cochlea. In this
paper, we propose a new model for the basilar membrane oscillation which
successfully describes both the active and the protective mechanisms by
employing an energy depot concept and a critical velocity of the basilar
membrane. One of the main results is that thermal noise in the absence of
external stimulation can be amplified leading to the spontaneous basilar
membrane oscillation. The compressive response of the basilar membrane at the
characteristic frequency and the dynamic response to the stimulation are
consistent with the experimental results as expected. Our model also shows the
nonlinear distortion of the response of the basilar membrane.
