laser digital microphone

[riggler]

So I was up all night with a crazy idea:

Take an existing condenser mic capsule but make the diaphragm optically reflective, like a mirror.
Now mount a small laser at 45 degrees to the vertical plane of the diaphragm.
The laser beam is "read" by a conventional optical pickup, like out of a CD player at the other 45 degrees to the diaphragm.

So at rest, the beam is *almost* aligned nicely to the optical pickup. Pickup gets the highest strength when capsule fully deflected inwards. Weakest when deflected outwards.

So as the capsule deflects in and out, the pickup sees varying strength, and we have in essence a "bias" power at rest that we'll need to subtract. So now we take the optical pickup output and subtract the "bias".

So if any that would even work, instead of sending a beam, could you send pulses at like 96kHz and simply measure the strength of each pulse to ascertain a pure digital signal? Obviously you'd need a clock, and there will be some deviation in time arrival of the pulse due to the path length difference from in-out positions of the diaphragm.

OK well I'm going to bed now to think of more crazy stuff, lol!!

 

[PRR]

I think you are 40 years too late.

There's an old rumor that the Russians (or Americans?) bugged the American (or Russian?) embassy by bouncing a laser off the outside of a conference room. (An alternate version has one side mounting a lightweight mirror in the building before turning the building over to the other side...)

Yes, you can sense vibration this way. The devil is in the details.

> a conventional optical pickup, like out of a CD player

CD pickups don't read linearly. They read the edges of the strong light/dark pattern and reconstruct the bits.

CD pits are scaled to the size of light-waves. (Why BluRay has more/smaller pits than older red-light formats.) The motion of a microphone diaphragm extends to excursions smaller than a ray of light. Yeah, the large 120dB SPL excursions may be almost big enough to see, but we also want the 15dB SPL of background sound (to be sure of getting all significant sound).

Some work with diffraction gratings could paradoxically bend the sub-lightwave fuzz and resolve smaller motion. I don't think we can construct a grating fine enough.

> 96kHz and simply measure the strength

How are you going to digitize "strength"? Basically you extract an analog voltage and run that to an ADC. Then prematurely chopping at 44K or 96K is just making the job harder. Let your photocell preamp follow the full wave in real-time. Leave the sampling to the ADC; that's part of its job.

> some deviation in time arrival

Yes, and if you could clock VERY fine slices of time, this doppler shift would be the way to extract the signal. Light travels one foot in one nanosecond. 0.01 inch resolution wants a picosecond clock. Large diaphragm motion is probably less than that; noise level is say a million times smaller. Atto seconds? 1,000,000,000,000,000 Hz? Conversely, teraHertz (that's more than GHz).

There's some side-tricks. The edge-clamped diaphragm (or window!) does not move parallel, it bows. So an off-center beam will be deflected in angle. The angle is very small, but if you can let it run-off a few blocks away from the embassy it may wobble some detectable distance. A simple tapered-density optical filter and large photocell will recover the deflection information. Also all the air turbulence.

I have real doubt that embassy-lasering ever caught any good secrets. Of course if just one unusual word comes through, you can casually drop that word the next time you meet with the other side, and send them into a panic looking for bugs, leaks, and finally funding futile laser-tricks.

 

[zebra50]

It's going back a bit but I recall discussing this before...

http://www.groupdiy.com/index.php?topic=1397

 

[riggler]

I knew it was too good to be true! 

 

[zapnspark]

For those of you who might be interested in the "Great Seal" bugging scandal, here is an article that I found:
http://tinyurl.com/2s8h8
It's about a passive, radio wave activated microphone.
An invention of Leon Theremin
It's not an entirely accurate article but most of the device details are OK.

Cheers.

ZAP

 

[xmvlk]

http://tinyurl.com/2s8h8

I do not think that the mic is simple resonant circuit,
because direct immersion of diaphragm detuned resonant circuit to
air degrades so much its Q factor.
May be it is some kind of pumped circuit tuned by mic.
Pumping would be done by kind of pulse modulation.
When the frequency of pulse modulation is near resonance
of the internal circuit, detection circuit (diode) is not working into short.