One of the best ways to tell bat species apart is by using a bat detector. Bats use high frequency calls normally beyond the range of human hearing to build up a sound picture of their surroundings. This echolocation system enables them to wing their way through the dark night hunting the tiniest of insects. A bat detector makes these echolocation calls audible to humans - and because different bat species hunt different prey and are different sizes, they make different calls which can help identify them.
There is a wide and often bewildering range of equipment available for detecting, recording and identifying bat species, so here is some advice to help you select the right equipment and get the most out of it. You can also watch our video guide to using bat detectors or read an article from Bat News (Autumn/Winter 2011) in which different users, ranging from beginners to experts, recommend their favourite bat detectors.
- Basic detectors
- Time expansion detectors
- Frequency division detectors
- Full spectrum, real-time sampling
- Buying a bat detector
- Digital recording with bat detectors
- Sound analysis software instructions
The most widely available bat detectors work on a principle known as heterodyning. The bat calls are picked up by an ultrasonic microphone and mixed with the output of a high frequency oscillator in the bat detector. This produces sounds that are the sum and difference of the two frequencies. Thus if the bat detector is set to 50kHz and an incoming bat call is at 49kHz then the difference is 1kHz which we can hear. The sum signal (99kHz) is ignored for obvious reasons! Clearly if the bat call is at 50kHz then we hear nothing, so the output of the bat detector is not an accurate reproduction of the original bat call. However, bats never emit a precisely steady sound, so this is not any problem in practice.
By adjusting the tuning frequency of the bat detector we can "listen" to different portions of the bat call and with practice can distinguish the calls of a number of bat species or families.
Time expansion detectors work by digitally recording a brief snatch of bat sound (usually about one second) and replaying it at a slower rate, usually ten times slower. Thus a frequency of 50 kHz is lowered to 5 kHz which is within our hearing range. The advantages are that the entire structure of the call can be heard enabling species identification using sound analysis software and the sonograms tend to be of a very high quality. The disadvantage of the time expansion system is that while the detector is playing back the slowed-down sounds it becomes "deaf" to any bats flying past. However, many models also include heterodyne or frequency division systems which can be heard on one side of your headphones, enabling you to keep on listening to "live" bat sounds and to decide when to capture another snatch of bat sound in time expansion mode. Time expansion detectors tend to be more expensive than heterodyne and frequency division detectors.
Frequency division bat detectors divide the incoming frequencies, normally by ten, thereby bringing the sounds within the human hearing range (e.g. 50 kHz becomes 5 kHz). As with heterodyne detectors the calls are heard in "real time" making the distinctive rhythms easily recognisable to those familiar with heterodyne, though differences in pitch between different calls are less discernible by ear when using this system. As with time expansion detectors the full call frequency range is captured, thus enabling identification through sound analysis, though the frequency division system captures less detail which tends to make the sonograms less clear. The advantage of frequency division over time expansion is that it captures sound continuously so bat calls are less likely to be missed, plus the detectors tend to be much cheaper.
Like frequency division and time expansion detectors, full spectrum, real-time sampling detectors detect all frequencies. They sample at very high rates to capture all signal information and output it in real-time – so you get the detail of call structure as with time expansion, but also the real-time continuous monitoring as with frequency division. Sounds can be recorded and used with sound analysis software. They enable a very detailed analysis of the sound and a clearer sonogram when compared to frequency division. They are generally more expensive than frequency division detectors.
The sounds that you get on a bat detector depend considerably on the type of detector you are using, and similar models even from the same manufacturer can sound slightly different. If you are considering the purchase of a bat detector it may be wise to attend a few bat watching events with more experienced people and see what they are using and try them out. Contact your Local Bat Group for details of such events and for any bat detector training sessions they may be arranging.
Colin Catto, former Director of the National Bat Monitoring Programme, has written a detailed paper on which mini bat detector is appropriate for different purposes.
We have also produced a Bat Detector Information Pack, which gives a summary of bat detectors, a list of those currently available on the market, their design features and approximate cost.
A guide to choosing a digital recorder by Jules Agate. This article has been updated in July 2008 and will be regularly updated to include additional details as they become available and hopefully more reviews of specific models.
The Sound Analysis Software Factsheet mentioned in the above article is now included in the Bat Detector Information Pack - see above link.
The following downloads provide step by step guides to using different sound analysis packages: