The cochlear's spiral could amplify low frequencies by 20 decibels.© SPL

Why is our cochlea, the key organ of hearing, curled into a spiral? It has been often thought to be a space-saving measure. But researchers in the United States have shown that the spiral could be vital for increasing our ear's sensitivity to sound, particularly at low frequencies.

Daphne Manoussaki of Vanderbilt University in Nashville, Tennessee, and her colleagues believe that the snail-shell curve of the cochlea focuses sound waves at the spiral's outer edge, making it easier for vibration-sensitive cells to detect them¹.

If the researchers are right, then the ear is more sophisticated than we thought. "It would show we need to take a step back from the cell biology and see how the cochlea works as an integrated system", says Karl Grosh of the University of Michigan in Ann Arbor, who studies the ear's structure.

The findings also suggest that artificial cochlear implants for the hard of hearing could be improved. Grosh, who is working on such microscopic devices, says that the work will encourage him to think about mimicking the coiled structure, which was thought previously to have no real function.

Whispering along walls

The cochlea is a fluid-filled, coiled tube, about one cubic centimetre in volume, that narrows towards one end. Sitting in the inner ear, it separates the different frequencies of a sound, picking up sound waves from 20 to 20,000 hertz.

Different frequencies peak at different positions along the tube: high frequencies near the spiral's broad mouth, and low frequencies further up the tube.

The tube also carries nerve cells that fire in response to vibrations in the watery cochlear fluid.

The separation of frequencies happens just as effectively in a straight tube, and so until now it hadn't been clear that the cochlea's coiling did anything other than keep it compact. Some designs for an artificial cochlea have represented it as a straight, tapering channel².

But Manoussaki and her colleagues have calculated the way that sound waves travel in a cochlear tube with a realistic coiled shape. They find that the wave energy is not evenly distributed throughout the tube, but becomes concentrated along the outer wall, the more so the further up the duct the wave travels.

The researchers say this is similar to the way sound in a cylindrical space such as St Paul's cathedral in London, UK, gets concentrated around the walls; known as the 'whispering gallery' effect, it allows a listener at one part of the wall to hear a murmuring speaker on the other side of the cathedral.

Turn up the volume

The concentration of energy in one part of the tube could help the membrane cells to detect sound, if they are clustered in that region. Thus the cochlea may be more sensitive further up the tube, where lower frequencies are detected.

The researchers estimate that this amplification means that sound at the inner tip of the spiral is boosted by 20 decibels relative to sound at the outer face: the difference between the volume of a normal conversation and that of a vacuum cleaner.

A boost of 20 decibels would be significant in an artificial cochlea, says Grosh: "we'd love that." He says that it would be relatively easy to make miniaturized channels in the shape of a coil.

Visit our spiralrespondstobass.html">newsblog to read and post comments about this story.  

• References

  1. Manoussaki D., et al. Phys. Rev. Lett, 96 . 088701 (2006).
  2. White R. D..& Grosh K. . Proc. Natl Acad. Sci. USA, 102 . 1296 - 1301 (2005).