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.
You need:
- 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
- Soldering Equipment
- 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
How To:
- Cut the wire free from the old pair of headphones.
- Solder a cadmium sulfide cell onto the headphones to form a circuit where the cell acts as a resisor.
- 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.
- Plug the headphone wire into your recording device, make sure there's some action inside the room, and begin recording.
- 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).
