Here another animation showing the mechanical response of the basilar membrane, but this time the incoming sound is not the sum of two sine waves, but a single "ideal impulse", or click.
The top trace shows the click stimulus. Think of the click as travelling through air, but also impinging on the cochlea at time zero. The bottom trace shows the basilar membrane, with distance from the basal end on the x-axis.
Clicks are broad band sounds, and you might therefore expect clicks to excite all parts of the basilar membrane, but they do not excite all parts at once. Each point on the basilar membrane has its own "impulse response", and each will resonate at its own characteristic frequency. Also note that the response of the high frequency part of the basilar membrane starts earlier and dies away quicker than that of the low frequency part. This allows the basilar membrane to carry out a multiresolutionanalysis, using long integration time windows to analyse the low frequency content of sounds, and shorter windows at the higher frequencies.