What is a bat detector?
A bat detector is a device which picks up the ultrasonic echolocation and social calls of bats and converts them into the human audible range. They work on three different principles:
Heterodyne :- This type of bat detector is similar in operation to and old fashioned radio with a tuning control knob, where you turn the knob to your required radio station, you can only listen to one station at a time. This is because the radio receiver has a very narrow receiving bandwidth which only accepts one station.
The bat detector operates in a similar fashion but instead of tuning over a range of radio frequencies, it tunes over a range of ultrasonic frequencies. Hence you may miss some bats calls if you are not at the correct tuned frequency. The detector sound output is usually a series of clicks, ticks and tocks and for some species a lovely warbling sound.
For the Techno- Anaraks :- The heterodyne technique is to beat the incoming bat frequency picked up by an ultrasonic microphone, with a local tuneable oscillator. This is done using a mixer. The output of the mixer contains the original ultrasonic frequency, the local oscillator frequency, and the sum and differences frequencies of both oscillator and ultrasonic. A low pass filter only allows the wanted sum and differences frequencies, usually restricted to ± 5kHz, which is then amplified and fed to a loudspeaker. This type of sound is completely artificial and does not bear much relationship to the actual bat call other that call repetition rate.
A Super Heterodyne receiver does the mixer trick twice and utilises an Intermediate Frequency ( IF) amplifier, before the final ± 5kHz output drive.
Frequency Division :- Out of fashion nowadays. For these detectors the ultrasonic bat call is divided down in frequency to cover human hearing range. On some detectors the amplitude variations of the ultrasonic waveform are transferred to the divided down frequency. These are wide band detectors capable of looking at the whole of the ultrasonic frequency range at once, and as such do not require a tuning control.
For the Techno-Anoraks :- Ultrasonic bat call is fed to a zero crossing detector. The output of this is a digital signal which is then fed to a binary counter. Favourite binary counter output used ÷32, but ÷16, and ÷ 64 can also be used. For amplitude transfer the ultrasonic bat call is demodulated, and this signal is used to re-modulate the digital counter output. Hence the amplified variation can be transferred to the divided down output.
Time Expansion :- This is the new cool kid on the block. It is wide band in operation and receives the whole of the ultrasonic range that is used by bats. No tuning is required. It records all bat calls at all frequencies then plays back at a much lower rate. This means that the ultrasonic frequency range is converted to an audible human hearing range. When using the detector on location the slowed down signal, referred to as a "time expansion" signal can be recorded onto a tape recorder. The tape recorder output can then be fed to a sound card in a PC for signal analysis, giving a very good tool for bat species identification. Of course, the PC must have suitable software. Using this method an objective assessment is available for determining bat species not subjective as in the past using heterodyne detectors. The sound of a time expanded signal is similar to the sound of whale calls, (whereas the whale call is speeded up and not slower down.) All the nuances of the bat calls can be heard.
For the Techno-Anaraks :- Signals are slowed down by ten for PC work and thirty two for human ears. Analogue to Digital sample rate typically 400 kHz to conform to the Nyquist sampling criteria, since signals up to about 200kHz required to be recorded. Eight bit resolution is generally considered as adequate. Length of ultrasonic recording up to 20 seconds, but typically only 1 second needed. An average bat calls last for 10 milli seconds usually an FM chirp but can also be a CF repeated tone.•