# Laser microphone

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{{Short description|Acoustic surveillance device}}
A '''laser microphone''' is a [surveillance](/source/surveillance) device that uses a [laser](/source/laser) beam to detect [sound](/source/sound) [vibration](/source/vibration)s in a distant object. It can be used to eavesdrop with minimal chance of exposure.

The object is typically inside a room where a conversation is taking place and can be anything that can [vibrate](/source/vibration) (for example, a picture on a wall) in response to the pressure waves created by noises present in the room. The object preferably should have a smooth surface for the beam to be reflected accurately. The laser beam is directed into the room through a window, reflects off the object, and returns to a receiver that converts the beam to an audio signal. The beam may also be bounced off the window itself. The minute differences in the distance traveled by the light as it reflects from the vibrating object are detected [interferometrically](/source/interferometer). The interferometer converts the variations to intensity variations, and electronics are used to convert these variations to signals that can be converted back to sound.

== History ==
The technique of using a light beam to remotely record sound probably originated with [Léon Theremin](/source/L%C3%A9on_Theremin) in the Soviet Union at or before 1947, when he developed and used the ''Buran'' eavesdropping system.{{sfnp|Galeyev|1996}} This worked by using a low power infrared beam (not a laser) from a distance to detect the sound vibrations in the glass windows.{{sfnp|Galeyev|1996}}{{sfnp|Glinsky|2000|pp=260-261}} [Lavrentiy Beria](/source/Lavrentiy_Beria) had used this Buran system to spy on the U.S., British, and French embassies in Moscow.{{sfnp|Glinsky|2000|pp=260-261}}

On 25 August 2009, {{US patent|7580533}} was issued for a device that uses a laser beam and smoke or vapor to detect [sound](/source/sound) [vibration](/source/vibration)s in free air ("Particulate Flow Detection Microphone based on a laser-photocell pair with a moving stream of smoke or vapor in the laser beam's path"). Sound pressure waves cause disturbances in the smoke that in turn cause variations in the amount of laser light reaching the photo detector. A prototype of the device was demonstrated at the 127th Audio Engineering Society convention in New York City from 9 through 12 October 2009.{{sfnp|Galeyev|1996}}{{sfnp|O'Malley|2009}}

== See also ==
* [Photophone](/source/Photophone)
* [Passive radar](/source/Passive_radar)
* [Laser Doppler vibrometer](/source/Laser_Doppler_vibrometer)
* [The Thing (listening device)](/source/The_Thing_(listening_device))
* [List of laser articles](/source/List_of_laser_articles)
* [Laser turntable](/source/Laser_turntable)
* [Optical heterodyne detection](/source/Optical_heterodyne_detection)

== Notes ==
{{Reflist}}

== References ==
{{refbegin}}
* {{cite journal |last=Galeyev |first=Bulat M. |date=1996 |title=Special Section: Leon Theremin, Pioneer of Electronic Art |url=https://leonardo.info/isast/journal/journal96/LMJ6/galeyevintro.html |journal=Leonardo Music Journal |volume=6 |translator-first=Vladimir |translator-last=Chudnovsky |accessdate=2007-11-22}} linked from [https://web.archive.org/web/20030426143613/http://theremin.info/leon-theremin.shtml LMJ6]
* {{cite book |last=Glinsky |first=Albert |url=https://archive.org/details/thereminethermus00glin |title=Theremin: Ether Music and Espionage |date=2000 |publisher=University of Illinois Press |isbn=0-252-02582-2 |location=Urbana, Illinois |url-access=registration}}
* {{cite web |last=O'Malley |first=Owen |date=20 Oct 2009 |title=Schwartz Engineering & Design Laser-Optic Microphone Prototype |url=http://www.gearwire.com/schwartzengineeringanddesign-smokemic-aes2009.html |url-status=dead |archiveurl=https://web.archive.org/web/20121124062946/http://www.gearwire.com/schwartzengineeringanddesign-smokemic-aes2009.html |archivedate=2012-11-24 |accessdate=21 Oct 2009 |work=Gearwire}}
{{refend}}

== External links ==
* [https://web.archive.org/web/20121124062946/http://www.gearwire.com/schwartzengineeringanddesign-smokemic-aes2009.html ''Smoke & Laser Microphone''] (archived from [http://www.gearwire.com/schwartzengineeringanddesign-smokemic-aes2009.html here]), Gearwire 
* [https://www.youtube.com/watch?v=Gqe79RpbPxU New Mic Proof of Concept], David M. Schwartz
* [https://www.youtube.com/watch?v=V2K5LJHTouQ Smoke and Laser Mic Prototype Two Demo], David M. Schwartz

{{DEFAULTSORT:Laser Microphone}}
Category:Optical devices
Category:Microphones
Category:Surveillance
Category:Covert listening devices
Category:Inventions by Leon Theremin

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Adapted from the Wikipedia article [Laser microphone](https://en.wikipedia.org/wiki/Laser_microphone) by Wikipedia contributors ([contributor history](https://en.wikipedia.org/wiki/Laser_microphone?action=history)). Available under [Creative Commons Attribution-ShareAlike 4.0 International](https://creativecommons.org/licenses/by-sa/4.0/). Changes may have been made.