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The Problem:

To detect sound within a room (having an outside window) at a distance.  The principle being that sound vibrations will cause the window glass to move sufficient to deflect the LASER beam across the receiving photo detector.

There are several methods that might accomplish this, with some fidelity; [1] however this page will describe the use of a LASER Microphone. 

The use of a LASER to transduce sound from a window pane/glass is commonly used in movies which leave the mistaken impression that any such task is easy and the LASER Microphone, itself, is highly portable and easily set up. My recent experience is just the opposite. 

There are several forms the LASER Mic. can take

1)  The one that is probably the most intuitive is a system that "grazes" a LASER beam at an angle to the plane of the glass (e.g., 45-deg.), with the photo detector at a complementary angle, and located at a near distance on the other side of the window, see fig. 1.


2)  The second method might be the same as above, with the difference being the LASER and detector are co-located; either with a slight angle between the incident beam and the reflected beam; or boresighted where the incident and reflected beam share the same superimposed.

    LASER Interferometer (Michaelson)

3) By using Interferometry, one stands the best chance for success (fidelity wise). However, the difficulty of use has increased somewhat. Also, this particular approach has several deficienses--the most glaring one is the very large differences in the "leg" length. Ideally both legs of an interferometer should be of equal length. This is due to temporal or longitudinal coherence: where the phase coherence of a LASER beam changes over time. If the two jointly arriving beams are not synchronized the constructive and destructive interference is degraded, or nonexistent, thus limiting the device's sensitivity.
4) The final approach is an interferometer similar to the one above, but having both legs of equal length--the so called "Dual Beam LASER Mic." 

The main principle is the differential measurement of glass movement (acoustic vibration) across a small section of the glass pane. 

This has the advantages of ~equal leg length for temporal coherence; common mode rejection of gross window movements, and some rejection of common-mode path disturbances. 

Dual Beam LASER Interferometer


 Suggested Optical Configuration

LASER Beam Collimation, Expansion, and Focusing


Circuit has Wide Dynamic Range 

PIN Diode detector can be substituted with less expensive devices.

PIN Photo Detector Circuit



 Q: I would like to make an optical microphone using your method , but there is a thing that i don't understand (my teacher too):  in the dual beam microphone, if both legs of the interferometer are on the window, they vibrate together, and there is no path difference?! (or just a little phase difference) and it would not be possible to get the frequency of the vibration.

A: There is a differential measurement between the beams. This tends to cancel the common mode sound waves, especially in the lower frequencies; caused by wind, elevator noise, heating and air conditioning, etc. Of course the wider the spacing of the beams, the lower the frequency response.

There are several other things that complement this situation:
1) the audio bandwidth for optimum intelligibility in a noisy environment is in the area of 400 Hz to 2800 Hz (military), and the telephone industry uses 300 Hz to 3600 Hz (all measured at -3 dBm).

2) The speed of sound in air is ~ 1100 ft/sec, in glass the speed of sound is in the neighborhood of  ~20,000 ft/sec.

Misc Notes:

Finding the components for this and other projects: 

In the majority of cases, in today's consumer market, it is significantly cheaper to purchase either new or used devices and cannibalize them for the parts. 

Also, and more importantly, this assures you of not being the victim of promises not kept by parts vendors. 

In the case of the LASER microphone, finding an inexpensive red LASER pointer, of which there are many, would be a good starting point. 

As for photo detectors, capable of detecting reflected LASER light, with voice frequencies impressed on it, TV remote control detectors (that reside in the TV set) might be a good source. 

As I mention on this page, if possible use a RED visible LASER; as oppose to using an invisible Near Infrared LASER.  Alignment is a bitch if you can't see the LASER light!!  Of course, once you have perfected the system, then you can substitute the invisible LASER. 





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© 2002 =Functional combinations in solid states= h.dr. V.Gavryushin, h.dr. A.Žukauskas