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A Microphone made of smoke

A smoke plume

What good would a microphone made of smoke be? Potentially incredibly sensitive it turns out. Microphones turn sound - waves of movement of the air - into electrical signals. Most microphones have a moving part inside them, a diaphragm whose movement is used to detect the moving air and so pick up the audio signal. Rather than use a diaphragm, father-and-son team David and Daniel Schwartz have invented a microphone that uses a single plume of smoke instead.

Why? The physical mass of the diaphragm in a normal microphone limits how responsive it can be. Soft sounds may not be strong enough to push the diaphragm so would not be detected. David Schwartz noticed that smoke rising from an oil lamp moved in response to his voice, and wondered if that movement could be detected using a laser, to create an ultra-lightweight way of sensing sound. Son Daniel wondered if he could actually make one. A few months later and the Schwartzes had made a prototype of their new microphone.

The device shines a laser pointer through a thin plume of smoke rising through a tube. Any sound disturbs the smoke and makes it vibrate, and that changes the amount of laser light that can pass through. A sensitive light detector on the other side detects the light and turns it into an electrical signal. Out of the smoke you have a microphone!

To find out more and see a video of the smoke microphone, visit the IEEE Spectrum site