Laser Microphone:Design,Working,7 Advantages

by

What is a laser microphone?

A laser microphone is a modern monitoring device that incorporates the use of laser beams for sensing and analyzing vibrations of sound in an object placed at a distance. This instrument primarily serves the purpose of eavesdropping with a negligible chance of getting exposed.

laser listening device spectra laser microphone m
laser microphone demonstration. image source: https://www.detective-store.com/5721-thickbox_default/laser-listening-device-spectra-laser-microphone-m.jpg

When was the laser microphone developed?

The basic technique or idea of using a laser light beam for remotely recording sound originated with Léon Theremin of the Soviet Union in 1947. He developed the Buran eavesdropping system with this technique specifically.
A patent was issued particularly on flow detection microphone based on laser for an instrument that incorporates the use of a laser beam along with vapor or smoke for detecting vibrations of sound in the air medium and An updated version of the instrument was first shown (in the year 2009) at the 127th Audio Engineering Society convention organized at New York City.

How does a laser microphone work?

An object, which is placed inside the room in which a conversation is taking place, is selected. This object can be anything that has the ability to vibrate (for example, a laminated picture hanging on a wall) as a response to the pressure waves generated by sounds present in the room. The object chosen is preferred to have a smooth surface so that the laser beam can be reflected accurately.

Drum vibration mode21
Vibrations result in generating a minute difference in distance covered by the light beam. Image source: Oleg Alexandrov, Drum vibration mode21, marked as public domain, more details on Wikimedia Commons

The laser beam is typically sent into the room directed towards the object through a window. This beam then reflects back from the object to a receiver which then transforms the beam into an audio signal. At times, the laser beam directly bounces off the windowpane itself. When the light reflects off the surface of the vibrating object, the fine differences between the distance traveled by the light during each vibration is detected interferometrically. The interferometer is used for converting the distance variations into intensity variations, and then these intensity variations are converted into signals that can be transformed back to sound electronically.

laser microscope
Interferometry demonstration. Image source: User:Stannered – Image:Interferometer.png
Simple Michelson interferometer diagram
GFDL original, this version own work.View more
CC BY-SA 3.0

What are the components of a laser microphone?

Laser microphone has 3 main components:

Laser

Infrared laser beams are used for carrying out the technique. Carbon dioxide or CO2 lasers operates and emits infrared light. The atmosphere of the Earth is more transparent for IR radiations. Based on this fact, Carbon dioxide or CO2 lasers are usually used for this process. Apart from carbon dioxide lasers, Nd:YAG lasers that emit light in the IR range (near the wavelengths of 946, 1064, 1120, 1320, and 1440 nm) are also used. These lasers are pumped by using near-infrared laser diodes or flash tubes or continuous gas discharge lamps.

Receiver

The receiver plays an important role in this technique. The receiver picks up the beam that is reflected back. It is generally placed at an angle of 90 degrees to the original beam. This device then processes the laser light signal and transmits it to the demodulator for further processing.

Demodulator

The demodulator is the device that converts the intensity variation signal into sound electronically. This device is computer-controlled with the help of software for removing noises such as traffic, wind, etc. to deliver a purer sample.

Advantages of laser microphone:

Some advantages of laser microphone are:

  • Laser microphones are capable of eavesdropping on a conversation through the window glass without actually entering the room that is to be monitored.
  • It has an average range of 400m.
  • An invisible infrared laser beam is used.
  • Laser microphones have separate receiver and transmitter modules that make it possible to hear a conversation even when the perpendicular orientation is not attainable.
  • It is portable and has a very compact design.
  • Modern-day laser microphones are capable of providing double signal output (original and converted signal).
  • This instrument has a multi-band equalizer for digital sound processing.
  • The multi-band tonal signal range lies between 50Hz and 20kHz.
  • Laser microphones have innovatory beam modulation technology that marks them untraceable for other instruments.
  • Most laser microphones come with an integrated digital recorder that automatically stores all the information.
  • These instruments are equipped with an optical aiming system that enables the process of properly directing a laser at the target.
  • Laser microscopes allow micrometrical regulation in all three dimensions for definitive precision.
  • Most laser microphones have encrypted radio link that does not allow any external source to intercept the instrument’s internal communications.
  • It has double power feeding mode i.e. it can be both battery-powered or DC powered with an adapter.

To know more about CO2 lasers visit here

Also Read:

     

Leave a Comment