A bat detector is a device used to detect the presence of bats by converting their echolocation ultrasound signals, as they are emitted by the bats, to audible frequencies, usually about 120 Hz to 15 kHz. There are other types of detectors which record bat calls so that they can be analysed afterward, but these are more commonly referred to by their particular function. Bats emit calls from about 12 kHz to 160 kHz, but the upper frequencies in this range are rapidly absorbed in air. Many bat detectors are limited to around 15 kHz to 125 kHz at best. Bat detectors are available commercially and also can be self-built. Using bat detectors Bat detectors are used to detect the presence of bats and also help form conclusions about their species. Some bat calls are distinct and easy to recognise such as the horseshoe bats; other calls are less distinct between similar species. While bats can vary their calls as they fly and hunt, the ear can be trained to recognise species according to the frequency ranges and repetition rates of the echolocation calls. Bats also emit social calls (non-echolocation calls) at ultrasound frequencies. A major limitation of acoustic bat detectors is their range, which is limited by the absorption of ultrasound in air. At mid range frequencies around 50 kHz, the maximum range is only about 25 to 30 metres in average atmospheric conditions when bats fly. This decreases with increasing frequency. Some bat calls have components around 20 kHz or even lower and sometimes these can be detected at 2 or 3 times the usual range. However, only the lower frequency components will be detected at a distance. The usable range of bat detectors decreases with humidity and in misty conditions the maximum range can be very low. It is important to recognise three types of bat echolocation call: frequency modulation (FM), constant frequency (CF) (sometimes called amplitude modulation), and composite calls with both FM and CF components. The following illustrates a bat making an FM type call followed by a bat which uses a CF type call: The FM call is heard as rapid dry clicks and the CF call as peeps. These vary in frequency due to the Doppler effect as the bat flies past. A heterodyne bat detector exaggerates the Doppler effect. As the bat making the CF calls flies toward the detector, the pitch