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Fig. 1
Simplified Michaelson Interferometer |
The Problem:
To detect sound within a room (having an outside window) at a distance.
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. |
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Fig. 2
Angular, or Grazing Detection 45°
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There are several forms the LASER Mic can take; the several that
follow are, by no means all:
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. 2. The principle
being that sound vibrations will cause the window glass to move sufficient
to deflect the LASER beam across the receiving photo detector. |
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Fig, 3
Direct Reflection
(0° Angular, Bounce-back Detection) |
2)_ The second method might be
the same as above, with the difference being the LASER and detector are
co-located, i.e., incident and reflected beam boresighted. |
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Fig, 4
LASER Interferometer (Michaelson) |
Interferometry:
By using Interferometry, one stands the best chance for success. However,
the difficulty of use has increased somewhat. Also, this particular approach
has several deficienses--the most glaring (sorry) one is the very large
differences in the "leg" length. Ideally both legs of an interferometer
should be near 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 phase synchronized, the constructive and
destructive interference is degraded, or nonexistent, thus limiting the
device's sensitivity. |
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Fig. 5
LASER Interferometer (Modified Michaelson)
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3)_ A third approach might be
to use an Interferometer. This takes the form of fig. 5, except
one of the "arms" of the interferometer is the long path to and from the
window. This, of course, suffers "coherence degradation," as explained
above. |
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Fig. 6
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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
commom-mode path disterbances.
Note: There is nothing magic about the
LASER's Wave Length. The bound being between visible (e.g., < 670-nm),
and Infrared (e.g., >1500-nm). The ideal wavelength (WL) is what is termed
the "NIR," (Near-Infra-Red) band, e.g., ~ 790-nm.
There are only a finite number of WLs available
in this band, i.e., it's a Physics thing. Often the determining
factor is cost and ease of use. Speaking of which, alignment can be a 'Bitch,'
if you can't see the beam. So while experimenting, you might consider
using a visible
LASER!
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Dual Beam LASER Interferometer
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Some Suggested Optical Configurations |
LASER Beam Collimation, Expansion, and Focusing
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Circuit has Wide Dynamic Range
PIN Diode detector can be
substituted with less expensive devices. |
PIN Photo Detector Circuit
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Links: U
of W EE Dept., Project |
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Frequently-Asked-Questions
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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.
Could you explain to me if i'm wrong ? it would REALLY help me out.
How could I realized one? Does it worked well?
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.
I have tested this method, it Does Work.
While experimenting, aligning, and testing, you should
Seriously Consider
using a visible LASER!
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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.
You can fail utterly if you have to WAIT for parts
that either never get to you or when they finally do, are either the wrong
part or defective.
This is a lesson one hopes to learn sooner than later, especially
if the fortunes of your business is riding on same. |
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 (NIR) 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 NIR LASER.
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