Artificial cochlea tells tones apart

Researchers from the University of Michigan have constructed a hydromechanical device that is the size of and emulates the basic function of the cochlea structure of the mammalian ear. The cochlea converts sound waves to nerve impulses.

The human cochlea senses sounds that vibrate from 20 to 20,000 cycles per second and can distinguish tones that differ by less than half a percent, covers a loudness range of 120 decibels, can separate audio signals into 3,500 channels of frequency information, and is less than one square centimeter in size.

The artificial cochlea makes it easier to study the precise mechanics of the ear. It also paves the way for economical microphones that work as well as ears, and could eventually be used as cochlear prostheses, according to the researchers.

The researchers constructed the device using micromachining methods similar to those used to make computer chips, which means it is possible to produce the cochlear devices in bulk.

Like the biological ear, the device uses a fluid-filled channel to determine the frequency of a sound. An incoming sound causes a wave in the fluid. Because the thickness of the membrane covering the channel varies, a sound of a particular frequency displaces the membrane most at a specific spot. Adding sensors along the membrane will make it possible to convert the mechanical changes to electrical signals.

The researchers' device will be ready for practical use as a microphone that performs real-time frequency analysis in two to five years, and as a cochlear prosthesis in five to ten years, according to the researchers. A cochlear prosthesis also requires electronics that read the membrane signals and active feedback elements that emulate the other complex processes that occur in ears to increase sound sensitivity, discrimination and range.

The work appeared in the January 21, 2005 issue of the Proceedings of the National Academy Of Sciences.