Re: acoustic imager

On Wed, 16 Apr 2025 22:01:28 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:

On 2025-04-16 10:41, john larkin wrote:

On Wed, 16 Apr 2025 09:01:00 +0100, Cursitor Doom <cd@notformail.com>
wrote:
 

On Tue, 15 Apr 2025 15:04:15 -0700, john larkin <jl@glen--canyon.com>
wrote:
>

>
https://www.google.com/aclk?sa=L&ai=DChsSEwjTjaDVg9uMAxW3Hq0GHVmKOlYYACICCAEQARoCcHY&co=1&cce=2&sig=AOD64_3aGs74magNuXwdRGFo7oP8zK-LMQ&ctype=5&q=&adurl=

>
>
For 42,000 dollars? There's a product there you could develop, John.

 
Seems like it needs maybe a dozen electret mikes, one mux'd ADC, an
FPGA, and some code.

>
In the last few decades, there's been a lot of work done on imaging with
sparse arrays.
>
A full NxN rectangular antenna array has an enormous amount of
duplicated information from an imaging point of view. To make a good
image, you need spatial frequency information corresponding to all
values of dx and dy, with some regular spacing, i.e. in an NxN array,
>
dx and dy go from -N/2 to +N/2-1 (or equivalently, from 0 to N-1) in
integer steps.
>
In principle you only need one estimate per spacing, but in a dense
array, every pair of adjacent pixels gives an estimate of the dx = +-1
components, i.e. essentially the same information as every other
adjacent pair.  The redundancy is less at wider spacing, of course.
>
If one is willing to trade off SNR and computational expense, you can
get the resolution of a full array with far less than N**2 antennas--I
forget what the the number is, but it's a lot more like N log N than
N**2.  A pal of mine in grad school, Yoram Bresler, did his thesis on
that problem, which is where I first heard of it.
>
So a sparse array of microphones can in principle do quite a bit better
than one might suppose.

And it looks like the Fluke acoustic imaging is primitive, like those
hybrid visual+thermal gadgets.

>
Cheers
>
Phil Hobbs

I'd expect that a bunch of wideband antennas and ADCs listening to the
world would have the same effect, see everything. Radar without the
transmitter. No doubt that is being done.