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Hypersensitive mic detects sound origin

21 Jun 2013

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Scientists at the SINTEF MiNaLab in Norway have developed hypersensitive microphones that can sense which direction a faint sound is coming from. This breakthrough is ideal for applications involving geophones for seismic shooting, photoacoustic gas sensors, accelerometers, vibration sensors, gyroscopes and pressure sensors.

"With technology of this sort, a microphone will be able to see where the sound comes from, pick up the voice of the person speaking, and filter out other sources of noise in the room," explains ICT researcher Matthieu Lacolle, who emphasises that acoustics scientists at SINTEF have also contributed to this innovative solution.

The microphone is packed full of microelectronics. What makes it really special is an optical position sensor that is no more than a millimetre in diameter.

Hypersensitive microphone

In MiNaLab’s clean room, production of the hypersensitive microphone takes place in a highly controlled environment.

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.

The sensor can measure incredibly small movements, and thus also extremely quiet sounds. If we make the membrane light enough and let it oscillate 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.

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. The team exploited 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."

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, using special equipment within a clean room.

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