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Sensor determines sound direction through light

Posted: 21 Jun 2013 ?? ?Print Version ?Bookmark and Share

Keywords:optical position sensor? microphones?

Scientists from Scandinavian research organisation SINTEF have created an optical position sensor that assists microphones in identifying which direction a sound originates, pick up specific voices and filter out other sources of noise. The sensor is no more than a millimetre in diameter.

The reason for giving a position sensor such an important role is that a microphone is completely dependent on a membrane, which picks up the pressure waves produced by the sound. "In principle, a microphone acts like a drum. You have a membrane that vibrates when it is impacted by a sound-which is just a series of pressure waves. And then you have a reference surface in the background. The distance between these two surfaces registers the sound. We do this by measuring light waves from a microscopically small laser, so we can say that the sensor in microphones actually sees the sound," explains ICT researcher Matthieu Lacolle.

The sensor can measure incredibly small movements, and thus also extremely quiet sounds. If the membrane is light enough, and oscillates freely in the air, the microphone also becomes directionally sensitive. "That also tells us where the sound is coming from," says Lacolle, adding that the membrane is only 100nm thick, almost 1000 times thinner than a human hair.

Coloured by light

The technology that makes the microphone so sensitive is based on a combination of two optical phenomena; interference and diffraction, both of which are due to the wave character of light. What the SINTEF scientists did was to exploit optical diffraction and interference to measure membrane movements of less than the diameter of an atom by using the optimal sensor.

"We have created very special grooved microstructures on the reference surface, which lies directly underneath the microphone membrane. When the laser illuminates these microstructures, we can read off the direction in which the light is reflected by means of photodetectors, which transform the light into electrical signals," Lacolle details.

The microphone thus consists of several elements: an ultrathin membrane, tiny grooved microstructures, a miniaturised laser and a number of photodetectors. Everything is integrated into a tiny circuit that is mass-produced on a silicon wafer on which all the structures are etched.

Potential applications for the sensor include geophones for seismic shooting, photoacoustic gas sensors, accelerometers, vibration sensors, gyroscopes, pressure sensors, high-temperature versions of the above-mentioned sensors, sensors for highly irradiated sites (nuclear power stations, x-ray equipment) or with electromagnetic radiation (sensors in motors or magnetic resonance equipment).

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