The cochlea as a graded bank of independent, simultaneously excited resonators: Calculated properties of an apparent 'travelling wave'

There have been two main theories of how the cochlea works: resonance and travelling wave. The first says the cochlea comprises a bank of tiny resonating elements, like piano strings, which respond directly to sound pressure (the excitation is in parallel to the elements). The second considers that differential pressure across the basilar membrane causes a hydrodynamically coupled wave to propagate, like a ripple on a pond, from base to apex (i.e., the excitation is in series). Yet a bank of graded, independent resonating elements, if simultaneously excited, will give rise to an apparent travelling wave, as each element builds up and decays, governed by its Q. Here we model a bank of resonators ranging from 1 to 10 kHz and possessing Q values from 12 to 25, in line with reported values and in accord with a recent surface acoustic wave (SAW) model of the cochlea. When simultaneously excited, the bank shows an apparent travelling wave moving from base to apex with a speed of several metres per second, a value similar to experiment. We conclude that the 'travelling wave' can be interpreted as arising from resonant activity.