On 16/11/17 12:29, Phil Allison wrote:
John-Del wrote:

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There have always been listening devices and there always will be.

I remember reading the clever way the Russians were snooping on the Americans with no bugs within half a mile of the room: they targeted a laser on a window pane of the room they were spying on. The window pane acted like a microphone diaphragm. They then demodulated the return signal which had the room audio modulated on it. Probably not hifi but I'll bet it was usable.


** By all accounts, bouncing a IR laser off a widow is not practical. The beam has to be precisely aligned *square on* to the window and the resulting sound quality is abysmal.

I don't understand that. Why "square on"? Wouldn't the beam have a
tendency to pass right through the glass, with little reflection?
Wouldn't it be best to have it meet the glass above the critical angle,
so it would experience maximum reflection? Of course, the detector would
then have to be well away from the laser, and aligning them would be
very difficult.

And in these days of double and triple glazing, it would be even less
viable.

OTOH this simple Soviet invention worked well and had no such issues:

https://en.wikipedia.org/wiki/The_Th...stening_device)

Thanks for that link. A very interesting read.

--

Jeff