If I bounce the beam from a laser pointer off something that's moving, say, a window pane in a room with an ongoing conversation, the return beam will be frequency modulated. Any good ideas about how to recover this signal? This topic will get stranger.........

 

It's a common surveillance technique, but I couldn't say how it's done, presumably by comparing the received reflection with the transmitted beam?.

First off though, you don't want a visible beam :lol:

 

Visible is cheaper. Since I'm not spying on anyone, cheaper comes first.
However, it gets worse. I need to get the sounds from inside a strip of steel, which has an uneven surface and is going by at better than 50m/s.
The roar of the surface will swamp the signal in one beam, maybe if I use 3 or 4 and some sort of correlation technique? I think the DSP boys will be needed for this one.

 

I suspect you've got a big problem :lol:

 

better to draw a picture, your description lost some clarity during translation...

 

Try looking up a michelson inferometer on Google. I think this is the sort of system that the laser microphones use. Rather than measuring the doppler shift they measure the interfearance between a reflected beam and a non-reflected beam. The sensor is a simple photodiode.

measuring something with a laser that is going 50m/s is going to be tough. Can you mount a sensor to the metal itself?

Brent

 

Check this out!
http://www.williamson-labs.com/laser-mic.htm

Ante :roll: