A vibrating pane can convert conversation sound into modulated laser light

Evidence

A late twentieth century device for spying on conversations employs much the same principles as Lissajous’s mirror on a tuning fork. For about $45,000, you can own one yourself. Of course, it is illegal to use it except if everyone inside knows you are recording their conversations. The role of the mirror is played by a window pane in the room where the conversation is taking place. Instead of a tuning fork causing the mirror to vibrate, the window vibrates ever so slightly in response to the sound waves from nearby conversations. An infrared (invisible) laser beam is directed at the window from the outside, perhaps from an adjacent building. Detection is by projection of the reflected beam onto a sensitive photodiode, which modulates electric current according to the amount of infrared light it is receiving. These modulations are picked up and transformed electronically into audible sound. The device was reputedly a favorite trick played on the U.S. Embassy in Moscow during the Cold War. It can be defeated by taping a small vibrator to the window, overwhelming the signature of a conversation with noise. — Eric J. Heller, Why You Hear What You Hear- An…, 320

Categorization

• CANDY: Cold War spy trick plus cheap defeat by taping a vibrator to the window — fun dinner-party material.
• SHARE: Send to anyone who works in surveillance, photonics, or acoustic eavesdropping tech.
• XREF: Mirrors Lissajous’s tuning-fork mirror principle introduced earlier in this chapter; also resonates with military/espionage sound-weapons material in the reader’s silent-noise book cluster.