by using two mics you can just get a bearing with a two-sides
ambiguity. To resolve the ambiguity you need a third mic.
to get a real location of a sound source you need to find where two
bearings, calculated from two well spaced sets of mics, intersect.
you can use two spaced pairs to calculate two bearings and their
intersection, but you still have to face with two-sides ambiguity
that can be solved by a fifth mic.
o o o o
pair 1 pair 2
o
fifth
or, maybe, the design can be reduced to four mics only ? I mean, two
pairs, orthogonal each other ?
o
pair 1 o o pair 2
o
can you expand your formulas for these two configurations and then
evaluate which one is best ?
Gianni
At 23.13 26/01/2006, you wrote:
>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 = arcsin( V dt/D) here V=speed 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=333 m/sec)
>
>D = (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
>
>
>
>"Microphones are not ears,
>Loudspeakers are not birds,
>A listening room is not nature."
>Klas Strandberg
>Yahoo! Groups Links
>
>
>
>
>
>
>
>
>
>
>
>--
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--------------------------------------------------------------
Gianni Pavan
Email
Centro Interdisciplinare di Bioacustica e Ricerche Ambientali
Universita' degli Studi di Pavia
Via Taramelli 24, 27100 PAVIA, ITALIA
Tel +39-0382-987874
Fax +39-02-700-32921
Web http://www.unipv.it/cibra
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