Overview / Physics / Fluids / Stria
Drawing : S. Blatrix

Propagation of sound waves into the cochlea:
resonance and/or travelling waves?

The ossicular chain ensures the transmission of acoustic pressure from the air to the cochlear fluids (see animation). For more than a century, the physical mechanisms explaining the propagation of signal and the tonotopic pattern of the cochlea have been explained by two main theories, namely: resonance or travelling waves.

In 1863, Helmholtz (ref. b1) described the cochlea as being formed of basic elements, each one resonating at a different frequency, like the strings of a harp.

Travelling waves
In the late 1930s, Békésy (ref. b2) developed the travelling wave theory, for which he was rewarded the Nobel prize in 1961.

As shown in this schematic drawing, as the frequency varies from high to low, the site of maximum displacement of the basilar membrane shifts toward the apex. This corresponds to a passive tonotopy .

Current concept
Because of the discovery of the outer hair cell (OHC) active mechanism which explains the exquisite properties of sensitivity and tuning of the cochlea, the travelling wave versus resonance theory is not a hot debate any longer. Most acousticians and physiologists now think that the physical model of damping resonance, based upon OHC properties, is close to reality; the travelling wave tends now to be considered an epiphenomenon.

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