Heterodyne detector

The heterodyne bat detector is a very popular and established system. By mixing the signal from the bat with an internal signal from the detector the signal frequency is reduced by a fixed amount. So for example the frequency of a Pipistrelle call at about 42kHz mixed with a 40kHz signal would give an output at 2kHz. However a Noctule call at 25kHz would not be audible. To hear Noctules we would use a local signal at about 20kHz.

How heterodyne bat detection works:

Here are two examples of bat call signals.  The blue signal represents a small part of a noctule's call at a frequency of about 24kHz. The purple signal represents a small part of a pipistrelle's call at a frequency of about 44kHz. Neither of these is audible - only signals that fall below about 12kHz (depending on your age and gender) can be heard.
Here you can see the result of mixing the pipistrelle's call (purple) with a single frequency at 36kHz (black).  You can see that there is an output (blue) signal in the audio range, between about 2 and 12 on the scale.  The width of this signal is the same as the pipistrelles call - i.e. the frequency range of the output you hear is the same as what the pipistrelle produces. There is also a signal at about 80kHz.  This is filtered out in the detector.
Its important to always tune your detector to the lowest frequency that will let you hear the bat calls.  As you turn the tuning dial up the frequency you hear should go down. Here you can see that the detector is tuned to 50kHz. yet the pip's call is still audible.  However the "chirp" is now reversed because high frequencies in the bat call are rendered as low frequencies in the audio output due to this mis-tuning.

The heterodyne detector gives good sensitivity and the output is a good representation of the sound the bat actually makes, in terms of frequency range, amplitude and duration. It is also useful to be able to tune for one particular bat species – but don’t forget while you're listening to Pipistrelle you may miss the horseshoe bats! If you are recording bat calls using a heterodyne detector don't forget to note the frequency the detector dial is set to!

 

 

Technical stuff:

Mixing (multiplying) two single-frequency waves (sinewaves)  at frequencies (a * 2 * pi)  and (b * 2 * pi) gives
cos(a) * cos(b) = 0.5 [ cos( a + b) + cos( a - b )]  (product of two cosines - fundamental maths formula.)

A simpler way of expressing this is that mixing signals with frequencies f1 and f2 gives an  output with signals at (f1+f2) and (f1 - f2). In a heterodyne bat detector its the (f1-f2) signal that we use to provide the output.  The amplitude (size) of the output signal is controlled by the strength of the received signal.  This is why heterodyne detectors don't need any way of reinserting the signal amplitude - because its never lost.