Most bats supplement their eyesight by echolocation, a system a bit like radar or the sonar used by submarines. Although the sounds made by bats for echolocation are very loud you can't normally hear them, because the frequency (pitch) is too high. By reducing the pitch to match our hearing range we can listen to the bats. This helps in observing them, and identifying their species. This graph shows the range of frequencies used by bats:
All detectors pick up the sound with a sensor
called a microphone or transducer. The sound is converted to a changing voltage and the
signal is processed electronically to change its frequency.
There are three main ways in which this can be done:
New techniques are being developed to use digital signal processing techniques to overcome the limitations of these simple detectors, but the methods used are too mathematical to explain in simple terms.
Anything that makes an ultrasonic noise can be picked up with a bat detector. Keys jangling, nylon jackets rustling or gravel scrunching will all be picked up – so if you are looking for bats check your clothing is suitable. Other sources of ultrasound include dog training whistles, or gas or steam escaping. Many small animals make high pitched sounds – for example crickets, beetles and small rodents.