Re: acoustic imager

On Fri, 18 Apr 2025 10:34:25 -0400, Phil Hobbs
<pcdhSpamMeSenseless@electrooptical.net> wrote:
 

On 2025-04-17 03:45, John R Walliker wrote:

On 17/04/2025 03:12, john larkin wrote:

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.

 
It is.� Look up "passive bistatic radar"
For example:
https://sspd.eng.ed.ac.uk/sites/sspd.eng.ed.ac.uk/files/attachments/basicpage/20171219/Session%201.0.pdf
 
 
John
 

 
For a long time, too.
IIRC the first successful radar experiment used the reflection from a
BBC transmitter.
 
Cheers
 
Phil Hobbs

 
Radar and code-breaking really saved Britain's bacon in WW2. Plus a
bit of assistance from the old colonies. :->