The object is typically inside a room where a conversation is taking place, and can be anything that can vibrate (for example, a picture on a wall) in response to the pressure waves created by noises present in the room. The object preferably has a smooth surface. 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. 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.
However, countermeasures exist in the form of specialized light sensors that can detect the light from the beam. Rippled glass can be used as a defense, as it provides a poor surface for a laser microphone.
The technique of using a light beam to remotely record sound probably originated with Léon Theremin in the Soviet Union at or before 1947, when he developed and used the Buran eavesdropping system. This worked by using a low power infrared beam (not a laser) from a distance to detect the sound vibrations in the glass windows. Lavrentiy Beria, head of the KGB, had used this Buran device to spy on the U.S., British, and French embassies in Moscow.
It has been reported that the National Security Agency makes use of laser microphones.
On 25 August 2009, U.S. patent 7,580,533 was issued for a device that uses a laser beam and smoke or vapor to detect sound vibrations 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.
Examples of use
|This section needs additional citations for verification. (April 2012)|
The director of GlobalSecurity.org, John Pike, suggested laser microphones may have been used as part of the sophisticated surveillance to determine whether Osama Bin Laden was hiding in the suspected compound in Abbottabad, Pakistan, in conjunction with spy drones, spy satellites, and other technologies. Although the CIA believed that Bin Laden was hiding in the compound, they had not been able to obtain visual confirmation that he was there. There was visual intelligence gathered to determine how many people had been seen within the compound, and then laser microphones were used to demonstrate that there was someone else inside the building that had not been seen. This was done by bouncing lasers off of the glass panes in the building's windows, which would act like diaphragms to pick up voices inside. The CIA operatives could not tell what was being said, but they could distinguish individual voices, from each person's unique pitch and tone. As a result of this data, they determined that there was another person within the compound that they had not seen with visual confirmation, which led to the conclusion that Bin Laden was probably inside the compound.
- Glinsky 261
- "The rapid decline of transparency and privacy in America". Future of Freedom Foundation. 2008-06-07. Retrieved 2008-10-31.
- "Schwartz Engineering & Design Laser-Optic Microphone Prototype". Gearwire. 20-Oct-2009. Retrieved 21-Oct-2009.
- "CIA Used Satellites To Prep For Bin Laden Raid". National Public Radio. 2011-05-04. Retrieved 10 June 2012. "Mr. PIKE: ... shine a laser beam on those windows, you can detect those vibrations"
- Galeyev, Bulat M.; Translated by Vladimir Chudnovsky (1996). "Special Section: Leon Theremin, Pioneer of Electronic Art". Leonardo Music Journal (LMJ) 6. Retrieved 2007-11-22. linked from LMJ6
- Glinsky, Albert (2000). Theremin: Ether Music and Espionage. Urbana, Illinois: University of Illinois Press. ISBN 0-252-02582-2.