We are usually unaware of most of the sounds bats make when they are hunting because they are ultrasonic, that is, too high-pitched for people to hear. The pitch or frequency of sound is measured in cycles per second, or Hertz. Very high-pitched sounds, like those of the bats’ echolocation, are measured in kiloHertz (kHz). One kHz is one thousand cycles per second.

Normally human ears can pick up sounds in the frequency range 20 Hz to 20 kHz, though it is usually less in adults, but bat detectors allow you to eavesdrop on bats. Detectors contain a microphone which is sensitive to high frequencies, an electronic circuit which converts these to low signals and a speaker system which enables us to hear them. ‘Listening’ to bats adds a whole exciting new dimension to bat watching. You can sometimes hear dozens around you in the darkness, yet be unable to see a single one! With experience and practice, listening through a detector also helps us to identify the bats as their calls vary, though some are still very difficult to separate.

There are different types of bat detector. The most sophisticated are time expansion detectors which store the ultrasonic signal and replay it at a slower speed. The recording can then be played through a computer and analysed, helping to identify some of the more difficult species.