Time expansion

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 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.

How time expansion bat detection works:

The sound produced by a bat is not constant, but changes in both pitch and loudness.  Unfortunately these changes 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 frequencies present in a bat call, and changes in their pitch and loudness can be shown on a diagram called a sonogram, and it is claimed these can be used to aid identification of the bat species.  Here you can see a call oscillogram and spectrogram of a call from a Daubenton's bat recorded with a time expansion of 10 (I think!)  The upper chart shows amplitude against a time axis; while the lower shows the changes in freqencies present in the call against time. These results were obtained using the free program SoundRuler (see links page)

 

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)