for a complete beginner's intro to the fascinating world of neuroscience.
This animation shows a simulation of "travelling wave motion" in the basilar membrane in response to a sound composed of two frequencies (1000 and 2500 Hz). The sound waveform is shown in the top panel, the basilar membrane response is shown below. Since the frequency components of the input are separated by more than an octave, they are well separated by the mechanical filtering of the cochlea, producing clearly separated "travelling waves" for each frequency component.
Since the amplitude, and hence the mechanical energy, of airborne sounds is tiny, the cochlea mechanically amplifies the incoming vibrations. The motors which supply this mechanical amplification are the outer hair cells. Like inner hair cells, they use stretch receptors associated with the stereocilia at their tips to sense vibrations and convert them to electrical currents. But only in outer hair cells are these currents used to control length changes which parallel, and reinforce, the incoming mechanical vibration.
The job of the ear is to turn sound waves in the air into electrical nerve impulses that travel to the brain. The ear thus occupies a key "interface" position between the physical and the psychological aspects of sound. The ear is an interesting organ on many levels. Because sound waves usually contain only minuscule amounts of physical energy, the ear has to be phenomenally sensitive, and it is a marvel of "biological engineering".