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How to Make a Laser Microphone

Anything is more exciting if you do it with a laser. That's one reason to want to make a microphone with a laser. But there are also some technical advantages that make it a good choice. One of those advantages is that a light beam can transmit information across a distance. There are several different types of laser microphones. One type uses reflections off a window to sense and transmit sound from inside a room.

Things You'll Need

Laser diode
Laser mount and tripod
Detector mount
Optical bandpass filter
Photodetector
Audio amplifier
Speaker

Instructions

- 1
  Attach the laser diode to the mount, and attach the mount to the tripod. Aim the laser at the window of interest. Set up almost perpendicular to the surface of the window, at an angle of incidence of just a degree or two.
- 2
  Place the optical bandpass filter in front of the photodetector. Select the bandpass to match the laser wavelength. The filter will block wavelengths away from the laser wavelength, which means the noise will be reduced.
- 3
  Put the detector assembly on the mount. Adjust the detector position and angle so that the detector intercepts the laser reflection off the glass. The detector will end up pointing back at the window at an equal but opposite angle from the laser --- they'll be two points on a long, skinny isosceles triangle. The intensity of light on the detector will change as the window ever-so-slightly changes its shape in response to the sound waves striking against it.
- 4
  Connect the output of the photodetector to the amplifier. Connect the amplifier to the speaker.

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