THE COCHLEA, SURFACE ACOUSTIC WAVES, AND RESONANCE
Abstract
Within the distinctive spiral structure of the cochlea lies a remarkable geometric array of three rows of outer hair cells. Arranged precisely in a two-dimensional crystal-like lattice, the three rows resemble the interdigital fingers of a surface acoustic wave (SAW) resonator, and this chapter outlines how this geometry provides a good basis for explaining the mechanics of the cochlea. Like all SAW devices, the outer hair cells have both sensory and motor aspects, and operating together they are able to form a sharply tuned biological spectrum analyzer. In response to sound pressure, waves are generated between the rows and, via feedback, give rise to standing waves, which form the mechanical equivalent of an active "cochlear amplifier"that boosts hearing sensitivity and frequency selectivity. Because the spacing between the rows varies systematically from one end of the cochlea to the other, the result is a graded bank of sharply tuned filters, in many ways like a set of piano strings that resonate to incoming sound. Properties of this resonance-based system are explored and they are shown to match those of the actual cochlea. Unlike usual cochlear models based on a traveling wave, the SAW resonator system is completely specified - there are no free parameters.