Rich:
I live in a city so I hear lots of hand guns too. But I'm always pretty
sure where
the shots are coming from: the 24 hour store on the corner.
Here's back-of-the envelop stuff to see if we're close enough to what
you could do to make more calculation
worth it. The basic geometry is sketched in
(http://math.uc.edu/~pelikan/Dora/Instructions.html)
A formula for finding a bearing theta to a distant object based on time
of arrival of sounds at two microphones is
theta =3D arcsin( V dt/D) here V=3Dspeed of sound, D is distance between=
mics and dt is difference in time of arrival.
To locate something to within 100 feet at a mile means getting the angle
right to a couple degrees.
This boils down to (assuming middling values of theta and V=3D333 m/sec)
D =3D (333/0.02) X
where X is how accurately the time of arrival difference can be
measured. With values between 0.001 sec (generally easy to do) and
1/(22050) (very tricky to do with 44.1KHz sampling) this gives D in the
range 1 -17 meters. How well you can "align" the tracks pretty much
determines X --- gunshots should be pretty easy, but ambient noise (like
wind) makes things harder.
With 3 microphones the baselines for the pairs are all oriented
differently and usually one pair has an unfortunate value of theta. So
probably you'd be looking at trying to put mics at the corners of an
equilateral triangle with side length on the order of 30 feet or so.
Don't start hauling cable! But if this is close to doable, let me know
and I'll get out a calculator.
Thanks,
Steve P
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