So sensitive is the human ear that sound pressures light enough to vibrate the eardrum by mere tenths of an angstrom generate waves that are amplified in the cochlea and then passed on to the brain as electrical signals. Although details are murky at the cellular level, the basic physics of hearing is straightforward: Vibrational motion of the stapes bone in the middle ear creates a pressure difference across the basilar membrane, an elastic ribbon that partitions the fluid-filled cochlea lengthwise into two chambers. The pressure difference creates a slow traveling wave along the membrane, whose elasticity acts as a restoring force.
The membrane’s width increases along the cochlear spiral, from base to apex. The change in width is accompanied by a decrease in its stiffness, which makes the cochlea a frequency analyzer: Regions nearer its base resonate with higher-frequency signals; regions closer to its apex resonate with lower-frequency ones....
