Bats produce ultrasonic calls, which are too high pitched to be audible to the human ear, in order to navigate their environment and hunt prey. The echolocation calls produced by bats bounce off objects, and the reflected sounds are detected by the bats' highly sensitive hearing. This allows them to build up a picture of their surroundings.

Bat detectors convert bats' ultrasonic calls into audible sounds and so these devices allow the detection of bats flying in an area. The sounds converted in the bat detector can be listened to via the detector's speaker or headphones, and can be a fascinating range of ticks, clicks, warbles and trills.

Bats rely on sound to find food, navigate in the dark and communicate with other bats. Using a bat detector means you can find out about their activities and behaviour in the field without disturbing or endangering these amazing nocturnal animals.

There are over 1000 species of bat worldwide, with 17 species found in Britain alone. Each bat species echolocates (calls) within a specific ultrasonic frequency range, and the 'peak' frequency, combined with the distinctive type of sound each species makes (eg. trills or clicks) allows identification of species using bat detectors. There are guides which include audio CD recordings of typical bat calls and these can be useful aids to identification for beginners.

The sounds you hear have been electronically converted by the detector. Ultrasonic sounds are inaudible to the human ear, but they are picked up by the detector and output as audible sound, either through the detector's speaker or through headphones.

There are three main types of detector:

HETERODYNE
• The least expensive detectors
• Very good for beginners
• The best for immediate identification of bats in the field

You select the frequency range to listen to rather like tuning in to a radio.
How it works: A heterodyne detector mixes the bat sound with an ultrasonic signal made in the detector, then filters this sound to produce the difference between the two ultrasounds. The sound produced is audible to the human ear.
Recording: You can record on to minidisc, mp3, or other digital recorder, provided the detector has an audio output socket (most do), but the recordings cannot be used with sound analysis software.

FREQUENCY DIVISION
• Generally mid-price
• Detects all frequencies so you won't miss any bats
• Ideal research detector
• Sounds can be recorded and analysed as well as listened to in real time in the field

You hear the whole ultrasonic range without the need for tuning into a specific frequency.
How it works: The detector divides the incoming frequency (usually by 10) to give an audible output. For example 50kHz (inaudible) us output at 5kHz (audible).
Recording:You can record the sounds and use the recordings with sound analysis software.

TIME EXPANSION
• The more expensive detectors
• Broadband recording - all frequencies are detected
• Produces higher quality recordings/sonograms

These digitally record the sound, then play it back at a slower speed so you can hear details of the sound not normally audible.
How it works: A time expansion detector digitally records the ultrasonic call and then plays it back at a slower rate (usually 10,20 or 32 times).
Recording:You can record the time-expanded sounds and use sound analysis software with the recordings. Because a time expansion detector records digitally and at a slower speed, the recording is higher quality than with a frequency division detector so you can make a more detailed analysis of the sound.

Some detectors feature more than one type of sound conversion. For example, some frequency division detectors also have a heterodyne mode so you can use it in frequency division mode to find the sounds you are interest in, then switch to heterodyne mode to find out the frequency and identify the bat. Some of the more expensive detectors feature all three modes of sound conversion for maximum flexibility.

Yes, you can record the bat sounds directly from most detectors to audio recording devices such as minidisc, MP3 or other digital recorder. CDs of the different types of call made by each species are available to further help you identify the bats. With some detectors you can also analyse the sounds using PC software. Some detectors now have inbuilt recording facilities allowing you to record onto a digital memory card without additional equipment.