Techniques for detecting bat calls

There are many ways in which the bat call sound can be made into a sound we can hear. This page describes the most widely used techniques.

Approaches

All bat detectors use the same design for picking up the call. A microphone (or sensor) picks up the sound from the bat call, and converts it into an electrical signal. This signal is very small, so it needs to be pre-amplified. Our designs use different microphone types and different pre-amplifier circuits, but they do both HAVE a microphone and pre-amplifier.

The approaches to detection differ in the way the signal is then processed to make sounds we can hear.

Three main techniques

Most bat detectors use one or more of the following techniques to render the bat call as an audible signal. For example the BatBox Duet offers both heterodyne and frequency division conversions. The aim is to provide an audible signal that gives the listener as much information as possible about the bat call - and also to provide a signal suitable for recording and later analysis. Because the heterodyne detector shifts the sound by an amount that can't later be restored, recordings from Heterodyne detectors arent suitable for this purpose.

Time expansion

Time expansion detectors work by recording the bat call and playing it back more slowly.

If you record a pipistrelle call at 48kHz and play it back ten times slower you get a frequency of 48/10 = 4.8 kHz. However if the original call lasted 0.1 sec the output will last ten times longer - 1 second. The pitch is changed by a fixed amount - its divided by 10. The amplitude (loudness) of the original call is preserved. Because its played back more slowly its easier to hear the changes in the call. However Time Expansion detectors work in a two-stage process of recording, then playback; so any bat calls that occur during a playback phase will not be detected.

Heterodyning

Another way of changing the call frequency is to SUBTRACT a frequency. So for example if a pipistrelle calls at 48kHz and we subtract 46kHz the detector will produce an output at 48-46 = 2kHz. The way they work preserves the loudness of the call so a quiet call creates a quiet sound.

Heterodyne detectors are very popular because they make it easier to identify the type of bat that is making the sound. However like a radio they need to be tuned in for the kind of bat you are hearing. If you are tuned to 46kHz a horseshoe bat call at 120kHz will be rendered at 120-46 = 74kHz - and you will not hear it.

Frequency division

A frequency division (or FD) bat detector takes the sound made by the bat and divides the frequency by a constant factor - often 16, then amplifying this and outputting it through a speaker. So a noctule bat call at 24kHz will be played at 24/16 = 1.5kHz, while a Horseshoe bat call at 120kHz will be played at 120/16 = 7.5kHz, Frequency division detectors work by making the bat call into a digital signal for division, and this process removes the loudness information from the signal. The big advantage of the FD detector is that ANY bat call loud enough to be detected will result in an audible output.

Our Simple Digital Bat Detector (SDBD) is a frequency division detector.

In all figures below the blue line represents a pipistrelle call as a chirp starting at 55kHz and dropping to 45kHz with a duration of 6msec. The red line represents the output from each type of of detector.

The sound files are synthesized so they provide an accurate indication of the result of processing the SAME bat call in each of the three different ways desribed above.

The heterodyne process keeps the range of the call the same e.g. 55kHz - 45 kHz is mapped to 12kHz-2kHz as shown above.

Time expansion and frequency division techniques reduce the freqency range by the expansion or division factor but this does mean the whole bat call spectrum (200kHz - 20kHz) gives an audio output.

Real-time recording

Modern recording equipment will allow very high frequency signals to be recorded if a suitable microphone is used. These microphones have built-in circuits which record the signal at very high rates - much faster than would be needed for normal audio recording. They produce a digitised signal that can be recorded on a laptop or tablet, and you can get a visual display of the bat calls.

However they have no mechanism for producing an audible output. The recordings can not be bettered for analysing and identifying bats from their calls, but they aren't ideally suited to the rigors of wandering around woods in the dark. That is where self-contained detectors with an audible output show their worth.

Frequency division with amplitude restored

One development of the simple frequency division detector is to measure the amplitude (loudness) of the signal from the microphone - and reinstate this at the output so that the sound produced does mimic the pitch and loudness changes in the original call. This helps give you a picture of the distance and direction of the bat you are detecting.

Our Frequency Division Amplitude Restore (FDAR) detector does this.