The Laser Listener - Espionage Technology for $3
Hundreds of yards away, in the dark of night, behind elaborate security and a pane of glass, a conversation ensues which will determine your future. (Just humor). Suppose you wanted to hear what was going on. What you need is a laser window-bounce sound reconstitution device. You could spend thousands of dollars on one, or you could head to radio shack with a couple $1 bills. This is how it works: In a microphone, sound waves (differences in the pressure of air) cause a diaphragm to vibrate back and forth, altering its resistance to an electric current. When an electric current passes through said diaphragm, the output forms a complex signal which can then be reinterpeted by speakers or a recording device as sound. The device shown here works in a very similar way. Sound bounces off of a window, causing miniscule vibrations (acting like the microphone's diaphragm). Click for Crude Diagram When a laser is trained on the window, its reflection will vibrate with the sound. The laser's reflection, if converted to an electrical signal can now be recorded as sound. I will outline how I made my own (cost me $3, hope yours is the same or less) - the technically savy among you will spot the countless variations possible on this basic idea.
~1 Pair of old headphones
~A laptop or other device capable of recording sound from a 1/8" jack
~1 package of Cadmium Sulfide Photocells from Radioshack
~Laser Pointer (red or green is fine, infrared would be extremely difficult to detect by the surveiled yet would also be difficult to use, at first)
~Tripod or two
~Room with window and a conversation
1) Cut the wire free from the old pair of headphones.
2) Solder a cadmium sulfide cell onto the headphones to form a circuit where the cell acts as a resisor.
3) Aim the (stabilized) laser at (the center of a large) window, and position the reflected dot onto the (stabilized) photo cell. Using a (stabilized) lens, focus the laser's beam onto the cell. If during the day, shade the photo cell from sunlight by placing it at the back end of a dark tube, so that only the laser will reach it.
4) Plug the headphone wire into your recording
device, make sure there's some action inside the room, and begin
5) Post editing: grab your favorite sound editing software, and play around with it, until through "denoising", "dehissing", and boosting the volume up, the conversation becomes clear. Alternatively, a circuit can be constructed to do this analog, before the signal reaches the recorder (which may be helpful for real-time listening).