condenser microphone array

[pezman]

Just wanted to extract this one item from the acoustic camera thread.

I think that you could make a 32x32 array by arranging 32 parallel
strips on a background and then suspending 32 parallel strips that are
perpendicular to these "background" strips. If you record all
signals, I believe that you can isolate the signal arriving at any one
element using a suitable linear combination of the resulting signals.
You should also (in principle) be able to beam-steer the sensor and
post-process signals after the fact in much the same way as a
plenoptic camera. In fact, you have the enormous advantage that you
can preserve temporal information at pretty high resolution, so you
can do all sorts of post-processing to manipulate spatial, phase and
directional characteristics of the array (again, this is all "after
the fact" processing of the signals).

Copper tape on masonite is probably a decent way to build the
background. You would need something very low-mass for the
"suspended" strips, so that they can move with relative ease. And you
would probably need to calibrate each microphone in order to linearize
it for post-processing.

At any rate, not tough to build such a critter. However, it might be
challenging to collect signals at adequate resolution -- and there
might be something fundamental that I have overlooked. Also, storing
64 signals at high resolution is no trivial task. You could make a
smaller array, of course. For example, a 4x4 should be pretty
feasible.

[pezman]

Did some back-of-the-envelope math, assuming that the diaphragm was
1"x1" and that the plates were 1mm apart, and came up with a
capacitance of something like 5.4E-12 farads per mic. I think that
would require a rather high bias voltage to yield a workable signal.

Gain can be improved somewhat with thinner spacing. A business card
is approximately 0.25 mm thick, so that would give you 4 X the
capacitance per unit area. Doubling the width of the strips would
quadruple the capacitance of a single element (and quadruple the
overall area of the array).

Also, after doing some drawings, it became apparent that the size of
the sensor influences its directional response at any given frequency
-- so each element does have an effective, frequency-dependent comb
filter-ish "aperture". The net effect of all apertures at all
frequencies is a little obscure ...

[Jack Zylkin]

You can get your 1000 electret condenser mics here at $290 for the lot. Seems worth it... They are through hole though unfortunately.

http://www.surplussales.com/microphones-audio/MicroAudio-4.html

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[Dave]

I'd put the diaphragm much closer than 1mm. The one I linked to in the other thread placed theirs 0.005" away (0.127mm). Also, common aluminum foil is typically about 0.0005" (half a thousandth of an inch, "five tenths") -- there are thinner materials out there, but everybody's got aluminum foil.

I'm not sure how well it would work to do the array in the way that you're talking about, if I understand it right. I'm not sure how well the individual suspended strips would work as mics either, it seems like it might be tough getting them to move being so open to the air on all sides like that.

It would be fun to do a more sophisticated phased array some time.

-Dave

[Sean McBeth]

But aluminum foil us conductive, so it won't work as a dialetric.

You could spray paint the base layer.

--

[Dave]

Maybe I missed something, usually you use an air gap as the dielectric. Maybe we're talking about different things.

[Sean McBeth]

Nope, it's me missing something. I thought Mike was talking about making a capacitor array. Herpaderp, don't mind me :)