All bat detectors pick up the ultrasonic sound produced by bats and change it to a sound that we can hear, helping us to observe and identify them.
Frequency division detectors work by amplifying the signal from the bat, then frequency dividing it with a digital IC. The amplitude (loudness) of the signal is usually lost in this process, and the result is a crackly sound. A frequency division detector does not need tuning, so it will allow you to listen for all types of bat at the same time. The output from frequency division detectors can be recorded for later spectral analysis.
The BatBox "Baton" and "Duet" are examples of this type of detector.
Heterodyne detectorsThe 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 detector has a tuning knob that allows the internal frequency to be changed. Its just like tuning in a radio.
Listening to the sound it produces and noting the tuning frequency can help in identifying the species of bat.
Magenta and batbox 3 detectors are examples of this type of detector.
Time expansion detectorsThe time expansion system is based on recording the signal from the bat electronically and playing it back more slowly than it was recorded. The result of stretching out the signal means that the duration of the bat call is also stretched out. This provides a very true likeness of the pitch and loudness variation in the signal from the bat, and gives you more time to hear the changes; but it does give a rather false impression of the duration of the bat call.
Wildlife Acoustics and Petterson are examples of manufacturers of this type of detector