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. The sound you can hear is from a noctule bat through a heterodyne detector. The 'chip-chop' sound is characteristic of noctules.
The heterodyne detector gives good sensitivity and the output is a good representation of the sound the bat actually makes. It is also useful to be able to tune for one particular bat species – but don’t forget while you're listening to pipistrelles you may miss the horseshoe bats!
If you are recording using a heterodyne detector don't forget to note the frequency the detector dial is set to!