MadSci Network: Physics

Re: Can parabolic microphones pick up sound through windows?

Date: Thu Jan 6 00:58:32 2005
Posted By: Vernon Nemitz, , NONE, NONE
Area of science: Physics
ID: 1104201776.Ph

The answer is a qualified "yes".  Although it is true that a window
transmits some sound, it also absorbs some sound.  So, the amount of sound
that gets through will be diminished.  This means that a larger-than-normal
parabolic dish will be needed by the eavesdropper, to compensate for the
diminished sound.

Perhaps you don't know this, but there exists a much less obvious way to
eavesdrop on the sound coming through a window, than using a parabolic
microphone.  This trick involves shining a low-power infrared laser beam
(invisible!) against the glass, and catching the reflected beam with an
appropriate SMALL sensor.  It happens that the vibrating glass (caused by
sound waves) distorts the reflected laser light in a way that is fairly
easily translated back into good-quality sound waves, for the benefit of
the eavesdropper.

With respect to protecting your in-house conversations from eavesdroppers,
several different things can be done, and each is somewhat effective.  If
you could implement all of them, you will probably obtain reasonable
security from remote eavesdroppers ("bugs" that are planted are an
altogether different problem, of course).

First, get double-paned or even triple-paned windows.  EACH layer of glass,
after all, will diminish the sound that passes through it.  Next, get FULL
curtains, both the relatively thin type near the glass, and the fairly
heavy-cloth type that can block the light.  Curtains will also absorb sound
on its way through the windows, of course.  Finally, you can put working
(not decorative!) storm shutters outside the house.  Their job is to
protect the glass during storms, but they also can help diminish escaping
sounds.  (And shutters, at least, can also block laser beams)

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