Bat detector
A bat detector is a device used to detect the presence of bats by converting their echolocation ultrasound signals, as they are emitted by the bats, to audible frequencies, usually about 120 Hz to 15 kHz. There are [|other types of detectors] which record bat calls so that they can be analysed afterward, but these are more commonly referred to by their particular function.
Bats emit calls from about 12 kHz to 160 kHz, but the upper frequencies in this range are rapidly absorbed in air. Many bat detectors are limited to around 15 kHz to 125 kHz at best. Bat detectors are available commercially and also can be self-built.
Using bat detectors
Bat detectors are used to detect the presence of bats and also help form conclusions about their species. Some bat calls are distinct and easy to recognise such as the horseshoe bats; other calls are less distinct between similar species. While bats can vary their calls as they fly and hunt, the ear can be trained to recognise species according to the frequency ranges and repetition rates of the echolocation calls. Bats also emit social calls at ultrasound frequencies.A major limitation of acoustic bat detectors is their range, which is limited by the absorption of ultrasound in air. At mid range frequencies around 50 kHz, the maximum range is only about 25 to 30 metres in average atmospheric conditions when bats fly. This decreases with increasing frequency. Some bat calls have components around 20 kHz or even lower and sometimes these can be detected at 2 or 3 times the usual range. However, only the lower frequency components will be detected at a distance. The usable range of bat detectors decreases with humidity and in misty conditions the maximum range can be very low.
It is important to recognise three types of bat echolocation call: frequency modulation, constant frequency , and composite calls with both FM and CF components. The following illustrates a bat making an FM type call followed by a bat which uses a CF type call:
The FM call is heard as rapid dry clicks and the CF call as peeps. These vary in frequency due to the Doppler effect as the bat flies past. A heterodyne bat detector exaggerates the Doppler effect. As the bat making the CF calls flies toward the detector, the pitch falls.
Several species of bat use a composite FM and CF call starting with a rapid falling FM call which slows to become a CF call at the end, giving a "hockey stick" shape to the graph. This makes the call sound different on a bat detector:
This gives a much wetter sound that the pure FM call. Pipistrelles generally use the hockey stick call for general echolocation, but use only the FM part at times. The end frequencies for the Common Pipistrelle and the Soprano Pipistrelle are around 45 kHz and 55 kHz respectively, but these frequencies can vary widely.
There are three types of "real time" audio bat detector in common use: the heterodyne, frequency division, and time expansion. Some bat detectors combine two or all three types.
Bat detector types
Heterodyne
detectors are the most commonly used, and most self-build detectors are of this type. A heterodyne function is often also built into the other types of detector. A heterodyne bat detector simply shifts all the ultrasound frequencies downward by a fixed amount so we can hear them.A "heterodyne" is a beat frequency such as can be heard when two close musical notes are played together. A heterodyne bat detector combines the bat call with a constant internal frequency so that sum and difference frequencies are generated. For instance, a bat call at 45 kHz and an internal frequency of 43 kHz produces output frequencies of 2 kHz and 88 kHz. The 88 kHz frequency is inaudible and is filtered out and the 2 kHz frequency is fed to a loudspeaker or headphones. The internal frequency is displayed on a dial or on a display.
A better quality version of a heterodyne, or direct conversion, bat detector is the super-heterodyne detector. In this case the bat signal is mixed with a high frequency oscillator, typically around 450–600 kHz. The difference frequency is then amplified and filtered in an 'intermediate frequency' or i.f. amplifier before being converted back to audible frequencies again. This design, which is based on standard radio design, gives improved frequency discrimination and avoids problems with interference from the local oscillator.
In more recent DSP-based detectors, the heterodyne conversion can be done entirely digitally.
It is also possible to use a 'comb spectrum' generator as the local oscillator so that the detector is effectively tuned to many frequencies, 10 kHz apart, simultaneously.
Some early bat detectors used ex-Navy low frequency radio sets, simply replacing the aerial with a microphone and pre-amplifier. It is also possible to modify a portable Long Wave radio to be a bat detector by adjusting the tuning frequencies and replacing the ferrite rod aerial with a microphone and pre-amplifier.
How it is used
The operator guesses the likely species to be present and tunes the frequency accordingly. Many users will start listening around 45 kHz. If a bat is seen or a bat-like call is heard, the frequency is tuned up and down until the clearest sound is heard.Species like Pipistrelles which end their call with a "hockey stick" CF component can be recognised according to the lowest frequency which gives the clearest "plop" sound. Horseshoe bats give a peeping sound at a frequency depending on their species. FM calls all tend to sound like clicks, but the start and end frequencies and the call repetition pattern can give clues as to the species.
Pros and cons
The advantages of a heterodyne bat detector is that it works in real time, exaggerates the frequency changes of a bat call, is easy to use, and is the least expensive. It is easy to recognise a doppler shift in CF calls of flying bats due to their speed of flight. Stereo listening and recording is possible with models such as the CSE stereo heterodyne detector, and this can help to track bats when visibility is poor.The disadvantages of a heterodyne bat detector are that it can only convert a narrow band of frequencies, typically 5 kHz, and has to be continually retuned, and can easily miss species out of its current tuned range.
Frequency division
Frequency division bat detectors synthesise a sound which is a fraction of the bat call frequencies, typically 1/10. This is done by converting the call into a square wave, otherwise called a zero crossing signal. This square wave is then divided using an electronic counter by 10 to provide another square wave. Square waves sound harsh and contain harmonics which can cause problems in analysis so these are filtered out where possible. Some recent all-digital detectors can synthesise a sine wave instead of a square wave. One example of a detector which synthesises a sine-wave FD output is the Griffin.Some FD detectors output this constant level signal which renders background noise and bat calls at the same high level. This causes problems with both listening and analysis. More sophisticated FD detectors such as the Batbox Duet measure the incoming volume level, limiting the noise threshold, and use this to restore the output level variations. This and other sophisticated FD detectors also include a heterodyne detector and provide a jack output so that independent outputs can be recorded for later analysis.
How it is used
With dual output FD detectors, headphones can be used to monitor both outputs simultaneously, or the loudspeaker used with the heterodyne function and the FD output recorded and analysed later. Alternatively, listening to the FD output gives an audible rendering of the bat call at 1/10 frequency. An example of a dual detector is the Ciel CDB301.Dual FD/heterodyne detectors are useful for cross country transects especially when there is a function provided for recording voice notes such as times, locations and recognised bat calls. The output or outputs are recorded on cassette tape, Minidisc or solid state recorders, downloaded to a computer, and analysed using custom software. Calls missed by the heterodyne function, if present, can be seen and measured on the analysis.
Pros and cons
Advantages, As with a heterodyne detector, an FD detector works in real time with or without a heterodyne function. Bat calls can be heard in their entirety over their whole range rather than over a limited frequency range. Retuning with an FD detector is not required although this is done with a dual type with heterodyne. By analysing the recording later, the entire call frequency range and the call pattern can be measured.A serious disadvantage with real time listening is that the speed of a bat call remains fast, often too fast for the species to be recognised. The frequency changes of CF calls are not exaggerated as with a heterodyne detector and so are less noticeable. Also with some species such as the Lesser Horseshoe bat with a call around 110 kHz, the resulting frequency is still quite high although it can be recorded. The synthesising of the call means that only one bat call can be reproduced at a time and a muddle is caused by simultaneous calls. Surprisingly, this is not a great disadvantage when analysing a recording later
Time expansion
Time expansion detectors work by digitising the bat calls at a high sampling rate using an analog-to-digital converter and storing the digitised signal in an on-board memory.TE detectors are "real time" devices in that they can be monitored at the time of recording, but there is an inevitable delay while the high speed sampled extract is slowed down and replayed.
How it is used
In real time mode, with or without an associated heterodyne or FD detector, the slowed down calls can be heard as a drawn-out bat call at audible frequencies. Therefore, fast FM calls can be heard as a descending note instead of a click. Thus it is possible to hear the difference between FM calls which just sound like clicks on the other types of detector.After downloading an audio recording to a computer, the original calls are analysed as if they were still at the original non-expanded rate.