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Microphone listens with light

Sep 2013
Listening to a conference call can leave you on the edge of your seat, craning to hear what the person on the other end of the call is trying to say. But a new sensor developed in Norway could change all that, giving microphones hyperacute hearing and a sense of direction.

With this technology, 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,” said Matthieu Lacolle, an information and communication technology researcher at SINTEF.

Microphones are completely dependent on a membrane, which picks up the pressure waves produced by the sound. Working much like a drum, the membrane vibrates when it is impacted by a sound, Lacolle said. “And then you have a reference surface in the background. The distance between these two surfaces registers the sound. We do this by measuring lightwaves from a microscopically small laser, so we can say that the sensor in the microphones actually sees the sound.”

The optical position sensor developed at SINTEF’s MiNaLab is no more than a millimeter in diameter and only 100 nm thick, and can measure minute movements and extremely quiet sounds. If the membrane is made light enough to oscillate freely in the air, the microphone can become directionally sensitive.

“That also tells us where the sound is coming from,” Lacolle said.

The microscope’s sensitivity is based on a combination of interference and diffraction. Exploiting these phenomena enabled the researchers to measure movement of less than the diameter of an atom using the sensor.

“We have created very special grooved microstructures on the reference surface, which lies directly underneath the microphone membrane,” Lacolle said. “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.”

Norsonic, a Norwegian company that supplies a variety of noise-measurement equipment, intends to use the new microphone to measure both sound pressure and acoustic power.

“The microphone is the very heart of the equipment that we supply,” said Norsonic senior scientist Ole Herman Bjor. “What is unique about this technology is that it can give us an extremely sensitive microphone that is capable of registering sound waves far beyond the range that microphones in this price class can do today.

“Our version is also much smaller, which is an advantage in itself, because the physical size of the microphone actually affects the sound field that it is measuring.”

Additional applications for the sensor include accelerometers, gyroscopes, vibration and pressure sensors, geophones for seismic shooting, photoacoustic gas sensors, and sensors for highly irradiated sites or areas with electromagnetic radiation.

1. The additive process whereby the amplitudes of two or more overlapping waves are systematically attenuated and reinforced. 2. The process whereby a given wave is split into two or more waves by, for example, reflection and refraction of beamsplitters, and then possibly brought back together to form a single wave.
Euro NewsEuropeFiltersimaginginterferenceMatthieu Lacollemicrophonemicrophone sensitivityMiNaLabNewsNorsonicNorwayOle Herman Bjoroptical diffractionopticsSensors & DetectorsSINTEFlasers

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