The time expansion system is based on recording the signal from the bat electronically and playing it back more slowly than it was recorded. The graph that follows shows what happens when the original signal is time dilated by a factor of ten.
The result of stretching out the signal means that the duration of
the bat call is also stretched out. This provides a very true likeness of the pitch and
loudness variation in the signal from the bat, and gives you more time to hear the
changes; but it does give a rather false impression of the duration of the bat call.
The sound produced by a bat is not constant, but changes in both pitch and loudness. These changes can be shown on a diagram called a sonagram, and can be used to aid identification of the bat species. Unfortunately they take place too rapidly to be detected by the unaided ear. The time expansion detector by stretching the signal out can let us hear these changes. The sound you are hearing is a recording of a Daubenton's bat, time extended by ten times. As Daubenton's bats produce very short calls (about 2ms) you can't normally appreciate the pitch, and they just sound like clicks on a heterodyne detector.
The time expansion principle can only be applied to short bursts of sound, so it cannot be used for continuous observations. However it can easily be combined with a recorder or data logging unit that will allow later detailed analysis of the calls recorded. (a bit like an answerphone)