Detecting Ultrasound

martin

Interesting.

The mixing process requires a non-linear device, which, for your purposes, I suspect the air is not. I've often considered, but never attempted, a similar mixing process using human ears (connected and intact, of course), since ears are quite non-linear. Ears won't work as mixers in your case since, at least for most adults, they are insensitive to ultrasonic frequencies (as well as to intelligent political analysis, it seems).

The suggested bat detector is far more promising.

Chuck

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Why worry about it... His yard, his pets, his life...

OK,

Take a microphone with a frequency response > 30Khz, and an amplifer. Monitor the amp's output with a scope.

Suspend a thin shaving razor blade between two pieces of dental floss and put your ear close to the blade. Watch the razor vibrate.

On a sunny day (10 Mar 2007 05:18:34 -0800) it happened snipped-for-privacy@burridge8333.fsbusiness.co.uk wrote in :

Google 'bat detector'.

That's pretty cool, MacGuyver. Does the razor blade do anything?

It cuts off your ear if you get too close ;)

clifto wrote:

The obvious "detection" would be oscilloscope observation of the amplified microphone signal. That's been mentioned.

Some commercial ultrasound detectors simply heterodyne the ultrasonic range down to audible frequencies...good if your hearing goes on up to the high end of human response. Expensive as portable devices but easily genned up on the average home workbench.

There are a couple of claims of outdoor advertising via sound through using high-power ultrasound generators in pairs, one modulated in amplitude the other unmodulated. The air acts as the non-linear "mixer" and the claim is that such beams of ultrasound can be focussed on particular locations. One such company is located in San Diego, California, if memory serves.

Be better if you could use it as an electric razor ...

JS

To get the air to be non-linear enough to mix the two frequencies, you would need very high sound pressure levels that could potentially be hazardous (or at least much more of a nuisance that whatever your neighbour is doing). I would suggest getting a wideband microphone (a very small electret might do, and would be less high-Q than a typical ultrasonic transducer), and then attach a preamplifier and an electronic mixer with an adjustable local oscillator (e.g. make a bridge from switches using a 4066 or FST3125 that alternately inverts or doesn't invert the signal). The output of the mixer can be fed to an audio amplifier and headphones. You can then test this receiver if you buy or borrow one of those cheap "pest annoyer" things. Once you know that your receiver works, you could make a parabolic reflector (e.g. spin cast a dish on an old record player from plaster of paris in a bin lid), so that you can search for sources of ultrasound.

Chris

Replace the dental floss with Litz wire and attach ends to 110 VAC.

Wait. Maybe not. Considering how many stations you can pick up with a razor-blade radio, that would probably interfere with vital communications all throughout the spectrum. What's the resonant frequency of a whisker?

You need your detector to be low Q since you don't know the offending frequency. Some of these fancy sound cards do 196kHz sampling. A piezo microphone with amp into the sound card might do the trick.

Also, as other have suggest, the bat detector.

Do you really want to hear it, or have a "field strength indicator"/locater?

Why not just a mike capable of "hearing" the ultrasonic freqs in question--feeding an opamp and meter?

Pointing the mike around should lead you into the correct direction and following the meter reading should lead you to the source(s).

Possibly can substitute a light or even a led in series with a pot and use a visual indication of strength ...

JS

Come to think of it, wouldn't take much more to square up the sine wave out of the opam and drop a cmos decade-divider onto that output of the opamp and feed an ear phone with the dividers output--30,000 becomes

JS

only if theres no other sound present in the mic signal.

and it will give no indication of signal amplitude.

I *think* what the OP is worried about is that his friends' pets are being scared off by his freinds' neighbor.

While I hate to discourage anyone from building something electronic, I do have to ask: what will one do if they learn that a bird-scarer *is* in use? Best of my knowledge, they aren't illegal.

If a bird-scarer works on dogs, then I find the details quite interesting. May have to work up a mobile version. Loose dogs allowed to roam a neighborhood are a serious safety issue for cyclists. (and I wonder if that's why the neighbor in question is trying to scare them off?)

They don't make the blue blades anymore.

Do they make any other kind of thin shaving razor blades?

Yes, but the blueing was what made a detector.