The inner ear functions like ‘an underwater piano’ – highly tuned despite being immersed in water

Aug 2 2004

The inner ear functions like ‘an underwater piano’ – highly tuned despite being immersed in water – according to a new explanation of how we hear by two ANU researchers.

Their paper, in the respected Journal of the Acoustical Society of America, provides a new understanding of how our delicate sense of hearing might work, explaining why most human ears continuously emit soft, pure tones that can be picked up with a microphone placed in the outer ear.

The hypothesis was formulated by Mr Andrew Bell, a PhD student at the Research School of Biological Sciences and developed mathematically by Professor Neville Fletcher of the Research School of Physical Sciences and Engineering.

“Like a piano, the cochlea is highly tuned, and yet, paradoxically, it is filled with watery fluid”, Mr Bell said. “Our paper explains how the ear can overcome this apparent contradiction by using a special wave known as a ‘squirting wave’, which so far has only been recorded by scientists working in ultrasonics.”

Squirting waves occur when two closely-spaced flexible plates, surrounded by fluid, vibrate. Fluid jets are created as the plates alternately squeeze together and pull apart – much like clapping hands underwater – and the jets, interacting with the bending of the plates, give birth to squirting waves.

The paper demonstrates how the unique properties of squirting waves allow the cochlea to respond sympathetically to the lowest audible frequency, such as the deepest drone of a pipe organ; to the highest, such as the tinkle of a triangle.

“Resonance is what makes a wind chime work, and the cochlea seems to have thousands of such chimes,” says Mr Bell. “But because the cochlea is filled with liquid, it has to overcome what seems an insuperable obstacle – viscosity.

“Lowering a ringing wind chime into water will silence it. Several decades ago a key auditory scientist, Thomas Gold, well described the cochlea’s problem: like a piano with the sustain pedal on, it needs to vibrate in sympathy with incoming sound, but given water’s viscosity, how do we get such an ‘underwater piano’ to work?”

The cochlea does the trick by using special piston-like cells to create squirting waves between two thin membranes, according to Mr Bell. It makes the surfaces extra slippery by coating them with oil and then uses positive feedback to overcome the remaining viscosity. Working together, these techniques create a fully functioning ‘underwater piano’ that can respond to the slightest whisper.

“Our paper might help provide a clearer picture of what is going on with tinnitus, a mysterious ringing in the ear which millions of people around the world experience,” Mr Bell said. “At the most fundamental level, it could provide a basis for reinstating the long-neglected resonance theory of hearing which has, for more than a century, taken a back seat to the currently-accepted travelling wave theory.”

Hear a recording of tones emitted by the ear recorded by Andrew Bell (2.5MB)