(see my guide on flying bats and bat detectors)
bats use sonar . The bat makes a loud noise, at a pitch above our hearing range 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.
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 it to a sound that we can hear. This can be done in different ways:
Heterodyning. This involves mixing the signal with another signal and amplifying the difference signal.
Frequency division. Digital circuits divide the frequency of the signal from the bat, usually by 16 or 32, so that it lies in the range humans can hear.
Time expansion. The signal is literally stretched out in time, so that the frequency is reduced, and a 10ms call lasts 100ms or even 1 second.
DSP Digital Signal Processing: once the signal has been sampled there are many possibilities in processing it to produce audible signals relating to the bat call.
For anyone new to the hobby of observing bats a simple heterodyne detector is recommended.
Further information can be found on the Durham Bat Group website (ebay doesnt allow a link)
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19 November 2011
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