Re: Laser-Accurate microphone

On 22 Sep 2009 11:41:58 -0400, kludge@xxxxxxxxx (Scott Dorsey) wrote:

Don Pearce <spam@xxxxxxxx> wrote:
On 22 Sep 2009 07:27:06 -0400, kludge@xxxxxxxxx (Scott Dorsey) wrote:

Again a mechanical system using a moving mirror, like Bell patented many
years earlier. These days I am sure that the trenchcoat guys can get
much longer range with lasers although getting your position absolutely
parallel to the glass becomes that much harder the farther away you are.

A laser would be no good for this - the coherence length is far too
great, and all you would receive is a jumble of interference fringes.
Any signal would be utterly buried. The original worked by simple
amplitude modulation by slight alteration of the direction of the beam
under reflection.

As far as I know, devices today exist to do that also, but they use an IR
laser. Because the laser is very narrow, you can be a great distance
away from the source before there is an issue with spreading. They
still measure the position, not the interference fringes.

In the original you didn't actually need to be perpendicular to the
glass. The light source and receiver didn't need to be in the same
place. As long as you could view a reflection, it worked.

Right, but ultimately you want to have a single device at a single position
rather than two widely separated devices. Less conspicuous.


Well, the light source (even IR) will always be conspicuous, and I
rather like the idea of being nowhere near it while I am listening.

There is an obvious counter of course; something like a mobile phone
vibrator glued to the window would do nicely.

d
.

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