Bat Detectors: Heterodyne
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.
In the figures below the blue line represents a pipistrelle "hockeystick" call. There is a descending sweep from about 55kHz to 45kHz. The red line shows the shape of the sound from the detector. It's 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.
Figure 1: the Detector is tuned to 43kHz so the resulting sound sweeps from (55 - 43 =) 12kHz down to (45 - 43 =) 2kHz.
Figure 2: As you can see below, incorrect tuning can give some very strange and misleading results. Note the dip to very low frequency
Figure 3: shows the result of tuning to a higher frequency. The sound you hear is a sweep UP to a constant frequency.
Here you can play the sound of a pipistrelle recorded from a heterodyne detector.
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!