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Chris Jones

wrote:

Hi chaps,

I suspect a neighbour of a friend of mine is using an ultrasonic bird-
scarer to frighten off his pets. The man concerned won´t admit to it,
but there are times when his dog and two cats just seem to get
suddenly very distressed and hypermanic for no apparent reason. I`d
like to at least eliminate this possibility before considering any
others. So the question is, what´s the simplest way to detect
ultrasound? My web research leads me to believe the area of interest
is between 20 and 30khz. Most common bird scarers warble between these
two limits which are of course above the range of human hearing. I´ve
acquired an ultrasonic transducer that transmits on 41khz. If I couple
this up to a wien-bridge oscillator trimmed to the same frequency, I
figure I ought to be able to hear a warble if indeed this guy is using
a birdscarer, because the difference between 41khz and 20khz-30khz
will be audible to me. Is this feasible to "air mix" the two
frequencies in this simple way and hear a result, or is something more
complicated required?
Thanks!

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