The echolocation of bats has grown to a field of science since its detection and much is still being discovered. For the beginner, a bat detector is a way to better find bats, like some smaller species hunting very low or species emerging very late in almost complete darkness. As experience grows one learns to distinguish the sound patterns and associate them with certain species – but this subject fills entire books on its own…
There are three different methods of detection:
- Heterodyning detectors – a frequency can be adjusted and is combined with an internal oszillator, thus producing an audible result
- Divider – a wide range of frequencies is scanned by dividing all frequencies to audible sound
- time expansion – Ultrasonics are recorded digitally and replayed more slowly
Detectors may be built relatively easily for the first two systems. Dividers have the advantage that bats will be detected over a wide range of frequencies even if their call frequency is not known. The result, however, is a coarse clicking sound and distinguishing species is impossible. Heterodyning detectors monitor only a small chosen range of frequencies but retain the feel of the call, enabling the observer to distinguish species by frequency and sound characteristics with some accuracy (at least where we live, with only 5 species frequent of around a dozen which may occur in the vicinity). In addition to the call, the flight patterns, size, surrounding habitat, and behaviour of the bat should be taken into account when trying to put a name to the species.
Based on some designs from Betrik Sikken I created my own circuit board and made a kit, which has later been replaced by the „bat-scanner“ with additional functions (display, scan-mode):
NE612 mixing receiver
- good sensitivity (20..60 kHz)
- relatively linear frequency response and low noise
I also recommend Tony Messinas Homepage. Tony presents a kit for a very simple but effective dividing detector.