Helmholtz ’ s piano strings : reverberation of ripples on the tectorial membrane

In 1857 Helmholtz proposed that the ear contained an array of sympathetic resonators, like piano strings, which served to give the ear its fine frequency discrimination. Since the discovery that most healthy human ears emit faint, pure tones (spontaneous otoacoustic emissions), it has been possible to view these narrowband signals as the continuous ringing of the resonant elements. But what are the elements? It is noteworthy that motile outer hair cells lie in a precise crystal-like array with their sensitive stereocilia in contact with the tectorial membrane, a gelatinous structure with an observed surface tension. This paper therefore speculates that ripples (surface tension waves) on the lower surface of the tectorial membrane propagate to and fro between neighbouring cells. This mechanism defines a surface acoustic wave (SAW) resonator, and relies on the outer hair cells directly sensing intracochlear fluid pressure through their cell bodies; in this way the theory revisits the resonance theory of hearing. The SAW resonator acts as a regenerative receiver of acoustic energy, a topology which was invoked in 1948 by Gold, who later drew the analogy to an 'underwater piano' to describe the cochlea's problem of how it could vibrate with high Q while immersed in fluid. The proposal also gives a physical description of the cochlear amplifier postulated by Davis in 1983. An active array of resonating cavities driven by outer hair cells can explain spontaneous emissions, the shape of the basilar membrane tuning curve, and evoked emissions, among others, and could relate strongly to music. At levels above which the cochlear amplifier saturates, ripples on the tectorial membrane can still be identified, this time due to vibration of the tectorial membrane against the sharp vestibular lip. This second putative mechanism provides time delays between initiation of the ripple by acoustic pressure variations and its detection by the inner hair cells, and so represents an alternative way of interpreting the traveling wave. Thus, by invoking two ways of generating ripples on the tectorial membrane, a comprehensive account of cochlear mechanics can be constructed, unifying a resonance theory (at low levels) with a traveling wave picture (at high levels).