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Turning sound into light

September 18, 2003

Actors who perform in musicals often sweat in torrents when they have to zap around the stage on roller-skates or sing a ballad under a burning spotlight, dressed in bearskin. To allow the audience to hear them clearly, the artists wear cleverly hidden microphones underneath their make-up and costume. But as soon they break into a sweat, the microphone can short-circuit within seconds. This is because conventional microphones convert sound waves directly into electrical energy, by vibrating a diaphragm. Dynamic microphones employ a principle similar to a bicycle dynamo, in which the diaphragm displaces a coil inside a magnetic field. Condenser microphones convert sound by means of charge transfer and pass the signal on to an amplifier. When sound is transmitted over optical fibers, however, moisture has no effect at all on the microphone. Researchers at the Fraunhofer Institute for Applied Optics and Precision Engineering IOF in Jena have been working on just such an optical microphone since 2002 on behalf of Sennheiser electronic GmbH.

"With this optical microphone, a beam of light from an LED is transmitted through an optical fiber and a set of lenses", explains Dr. Andreas Br'¤uer, who heads the IOF Microoptics Department. "This array of optical components splits the beam and focuses it onto a reflective membrane. The reflected light passes through the lenses to a fiber-optic receiver." At each change in the optical signal, the membrane vibrates and its movement is converted into electrical current by a remote photodetector. "The IOF has been especially helpful in working with us to improve the signal-to-noise ratio", says Dr. Wolfgang Niehoff, Research Director at Sennheiser electronic. "In other words, how can we make the process of transforming sound into modulated light even more efficient?"

Because the optical microphone can be made without metal parts, it is totally immune to electromagnetic and magnetic interference. This allows the technology to be used in other applications such as scanners that use magnetic resonance imaging for medical examinations. Nervous patients can maintain contact with the attending physician via the microphone without affecting the results of the examination. "The optical microphone can also be used to attenuate the noise level inside the scanner," adds Niehoff. "Computer-generated anti-noise cancels out the noise recorded with the microphone. This makes the examination more bearable. We're already supplying to a manufacturer of such systems."

Fraunhofer-Gesellschaft