TURNING SOUND INTO LIGHT

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TURNING SOUND INTO LIGHT
15 September 2003 - Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.

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 Bräuer, 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."

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About: Fraunhofer-Gesellschaft zur Förderung der angewandten Forschung e.V.
The Fraunhofer-Gesellschaft undertakes applied research of direct utility to private and public enterprise and of wide benefit to society. Its services are solicited by customers and contractual partners in industry, the service sector and public administration. The Fraunhofer-Gesellschaft maintains over 80 research units at more than 40 different locations throughout Germany. A staff of some 12,700, predominantly qualified scientists and engineers, works with an annual research budget of over one billion euros. Of this sum, more than € 900 million is generated through contract research. Two thirds of the Fraunhofer-Gesellschaft’s contract research revenue is derived from contracts with industry and from publicly financed research projects. The remaining one third is contributed by the German federal and Länder governments, as a means of enabling the institutes to pursue more fundamental research in areas that are likely to become relevant to industry and society in five or ten years’ time.

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