Time expansion (TE) detectors are broadband detectors, i.e. they detect all frequencies across the frequency range within which bats are likely to be calling.
They 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.