Bats hunt for insects at night, using a high frequency system called echolocation. Bat calls are usually pitched at too high a frequency for humans to hear naturally - but they can be heard or recorded using a bat detector. Individual bat species emit calls with specific characteristics related to their their size, flight behaviour, environment and prey types. This means that with the aid of bat detectors we can identify many species by listening to their calls or recording them for sound analysis on a computer.
There is a wide range of different models of bat detector available. They utilise one or more of the following four main systems of enabling bat calls to be heard and/or recorded.
|How it works||Advantages||Disadvantages||Price range|
|Heterodyne||Tuneable detector. The bat calls are converted to distinctive sounds within the frequency range humans can hear.||A range of species can be identified in the field. Visual clues can be used as ID is made while the bat is present. They are simple to use.||Not suitable for spectrogram analysis. ID can't be verified at a later date. Only detects within a limited frequency range at a time so some species could be missed.||Cheapest of the detectors.|
|Frequency division (FD)||Broadband detector. Detected sound is outputted in real time. Frequencies divided by ten for recording. ||Allows continuous detection so bats are less likely to be missed.||Calls are recorded in less detail making sound analysis more challenging.||Cheapest of the broadband detectors. Overall mid-high price range.|
|Zero Crossing (ZC)||Broadband detector. Records sounds as data points which plot how frequency changes over time.||Creates very small files so can be left in the field for a long time before the SD or CF card fills up.||Spectrograms less detailed than from full spectrum and time expansion detectors.|
Mid-high price range.
|Time Expansion (TE)||Broadband detector. Captures a short sample of sound and plays it back at a slower speed for recording.||Sounds are recorded in very high detail making the call structure very clear and enabling precise measurements of call parameters. ||While the detector is playing back the slowed-down sounds it is not detecting so bats can be missed.||Mid-high price range.|
|Full spectrum/Direct Sampling||Broadband detector. Records sounds in real time at their original frequency.||"Best of both worlds" between FD/TE. Sounds recorded in high detail with continuous detection in real time.||Very high sampling rate required which creates large files.||Higher price range but cheaper models are also available.|