I've been kicking around this idea recently, and I think it just might
work. The idea is for a camera that takes pictures with sound instead of
light. If you have to ask "Why?" then you don't need one, and you may be on
the wrong website! ;) I just think it would be fascinating to play with.

Most light cameras use a lens, but the simplest way to focus the incoming
wavefronts onto your pixel array is with a pinhole. The pinhole can be
almost any material that is opaque to light at around 550 nm wavelength.
The sound equivalent might be some type of sound-deadening baffle, maybe a
slab of drywall, or cardboard, or other insulating board. For light, the
pinhole is often literally a hole made with a pin, but for sound the
wavelengths are many orders of magnitude larger, and so everything has to
get correspondingly bigger. For example if our acoustic camera were tuned
to pick up roughly 10 kHz audio (wavelength in air is about 1.3 inches), if
we had just 32x32 pixels at 1 wavelength pitch, the pixel array would be
nearly 4 feet on a side. For a focal length of 2 meters (79 inches) the
pinhole would be about 20 inches in diameter, for a 30 degree field of
view. Potentially you could capture these low-res images and mosaic a bunch
of them together for a much larger image.

That's all well and good, but 32x32 pixels means you need 1024 total
microphones! Even if they cost 50 cents a piece (less than the cheapest mic
on DigiKey) and could solder each one in 10 seconds, it would cost over
$1000 and take 3 hours of soldering just for the mics! Plus you might need
an amp for each one, some number of ADCs, some way to pull all that data
together, etc. So here's what I'm thinking: you make the imaging array out
of large PCB panels that you etch yourself, and you build the mics in
yourself as part of it. I'm leaning towards homebrew condenser microphones
using aluminum foil as the diaphragm. You'd need to develop a good way to
make an array of these mics yourself but it seems doable to me. Here's a
page showing one guy's own homebrew condenser microphone, clearly not
streamlined to be made hundreds at a time, but still. I think with some
experimentation you could get a system down for making 100 mics on a panel
in just a few minutes.

http://kt4qw.com/condenser.pdf

I think with the right mic design, you could multiplex them all together
using just two inexpensive mux ICs, a single amp, and a single ADC. It
would probably take several seconds to capture each image since you'd only
be pulling in one pixel at a time, but the processing would be really
simple. You could even do an FFT and extract out different frequencies to
be displayed in different colors. Different materials would show up in
different colors. Practical? No way! It would sure make some cool pictures
though.