Detectors 052
Detectors 052 : Particle Detectors: (4.2) Bat Detector: (1) 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 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 falls. Several species of bat use a composite FM and CF call starting with a rapid falling FM call which slows to become a CF call at the end, giving a "hockey stick" shape to the graph. This makes the call sound different on a bat detector: This gives a much wetter sound that the pure FM call. Pipistrelles generally use the hockey stick call for general echolocation, but use only the FM part at times. The end frequencies for the Common Pipistrelle and the Soprano Pipistrelle are around 45 kHz and 55 kHz respectively, but these frequencies can vary widely. There are three types of "real time" audio bat detector in common use: the heterodyne, frequency division, and time expansion. Some bat detectors combine two or all three types
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