The laser microphones primarily found its use in the surveillance industry. Laser microphone is basically a device which detects sound vibrations in the distant object. It can be used to eavesdrop with minimal chance of exposure.


Laser Microphones History


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. In the ever-constant quest to eliminate distortion from microphones, a laser-based mic has been developed. We can say that it is still in the experimental stage. However, it deserves attention because it could revolutionize microphone design.


The Technology Behind Laser Microphones


The technology is described as a particulate flow detection microphone. A stream of air infused with particles passes through a chamber within the outer casing near the top, where sound waves entering from the back perturbs the particles. A laser beam passes through the stream, which modulates the intensity. Furthermore, a photosensor converts that modulated light into an electric signal analogous to the original sound wave. The process eliminates all nonlinearities in the conversion from acoustic to electric energy, hence making the sound distortion free. Most commonly it main marts are the Sound port, Airstream exit, Laser, Photosensor, a Sensing chamber, Airstream entrance, and Electrical conductors.


Converting Vibrations Into an Audio Signal


The human voice can generate sound waves in the range of 300 Hz to 3400 Hz [8]. These sound waves vibrate nearby objects, making it possible for an analog electronic device to convert these vibrations into an audio signal. One way to accomplish this conversion from movement to audio is to use a “laser microphone”. It reflects a laser off the vibrating object and uses a receiver to capture the laser’s reflection. The reflection of the laser deflects as vibrations shift the surface of the vibrating object. Therefore, if a receiver takes in the oscillating laser signal from a fixed location, the receiver will detect the laser deflections caused by the vibrations that were originally produced from an audio signal.

The receiver can then filter and amplify this signal, and output it as audio. Through this process, the laser microphone effectively reproduces the audio that induced the object’s vibrations. The laser microphone is able to reproduce audio detected from a vibrating surface with relatively high accuracy: less than 8% distortion. As an additional feature, the laser microphone is also able to transmit audio. It uses an amplitude-modulated laser signal, capture the laser signal, and output the audio. Thus by using a laser-based system that captures oscillations in the position of the laser, the laser microphone is able to accurately reproduce both the audio that induced an object’s vibrations and audio transmitted via laser communication.


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