Device used for listening to high frequency sounds produced by bats. The most widely available bat detectors work on a principle known as heterodyning. This method uses a chozen frequency and compares it with an incoming sound. It produces sounds that are the sum and difference of two frequencies in which the sum signal is ignored.

Thus if the bat detector is set to 50kHz and an incoming bat call is at 49kHz then the difference is 1kHz which we can hear. By adjusting the tuning frequency of the bat detector we can "listen" to different portions of the bat call.

The sounds produced by heterodyne bat detectors may vary depending upon the main characteristic of the call being used by the bat.

A short burst of constant frequency sounds like "smack", a longer burst of constant frequency like a "warble", Rhinolophus
(Rhinolophus hipposideros detector set to 105kHz).
a steep frequency sweep like a sharp "click" or a "tick"Myotis (Myotis daubentonii detector set to 45kHz) and a shallow sweep like a "tock".
Nyctalus noctula detector set to 25kHz.

The output of such a bat detector is not an accurate reproduction of the original bat call.

Other more sophisticated bat detectors include those that work by recording the whole of the bat sounds and replaying them at a slower rate. This results in the frequency also to be much lower. This can then be heard by us with the advantage that the entire structure of the call can be heard and studied (see also sonogram) in its original form, but these "time expansion" detectors are more expensive.
N. noctula time expanded (10x) (echolocation calls)

Recordings used coutesy of:
- The Bat Conservation Trust, London.

Alternative forms for bat detector : heterodyning, time expansion.