Bat Detectors
Heterodyne detectors
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.
Pipistrelle echolocation calls heard on a heterodyne detector
Pipistrelle social calls heard on a heterodyne detector
Noctule echolocation calls heard on a heterodyne detector
Time expansion detectors
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.
Pipistrelle echolocation calls heard on a time expansion detector
Pipistrelle social call heard on a time expansion detector
Noctule echolocation calls heard on a time expansion detector
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.
Pipistrelle echolocation calls heard on a frequency division detector
Noctule echolocation calls heard on a frequency division detector
Buying a bat detector
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.
Detectors are available to buy at the Bat Bazaar.
Digital recording with bat detectors
A guide to choosing a digital recorder by Jules Agate. This article will be regularly updated to include additional details as they become available and hopefully more reviews of specific models.
Click below to download the following documents which are referred to in the article:
Sound Analysis Software Factsheet
Review of Edirol R-1 solid state recorder by Richard Fox