On Mar 10, 9:18 am, 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!

If you want to compress the range of 0-30 kHz to something like 0-12
kHz you can do that with a switched capacitor delay chip like the
Panasonic MN3007.

It will work like the bat detector, except it won't need to clip and
threshold the audio. You will need to use a slow ramped VCO
(continually ramping the sampling frequency down) in order to do this.
There are some projects at:
http://www.geofex.com/
You may be able to adapt a flanger, for instance, to suit your
purpose.

The control voltage to the sampling VCO will be a sawtooth wave.
Unfortunately, you will hear the sawtooth period as an artifact in the
output. Maybe you can filter it out.

I guess an all-digital solution is better ;-)

Frank Raffaeli