We have only relatively recently been able to detect bat calls using electronic bat detectors. These detectors are able to transform the sound of the bat call from ultrasound to frequencies that people can hear. There are currently three basic types:
FREQUENCY DIVISION BAT DETECTORS
Frequency Division bat detectors do exactly that, they divide the true (or broadband) frequency that the bat calls at by a factor (usually 10). This means that if a bat is calling at (say) 55kHz the detector divides this frequency by 10 to output 5.5kHz to a built-in loudspeaker.
Advantages: Usually the lowest cost detector. Sound is output in real time (i.e. whilst the bat is calling).
Disadvantages: Some structure of the bat call is lost, though sufficient frequency information is preserved for sonogram analysis.
Another Frequency Division/ Zero Crossing analysis system that does not restore the amplitude information is the ANABAT system. ANABAT comes with its own sound analysis program ANALOOK W, which produces sonograms using Zero Crossing analysis.
TIME EXPANSION BAT DETECTORS
Time Expansion bat detectors record the broadband bat calls over a few seconds and then play it back over the loudspeaker slowed down by (say) a factor of 10. This means that a bat call of duration 5 seconds at 55kHz is played back over 50 seconds at 5.5kHz.
Advantages: Truer reproduction of the bat call characteristics.
Disadvantages: Replay time subtracts from time available to detect bat calls.