Bat Detectors: 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.
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.
Here the blue line represents part of a bat call; and the red line the result when it is stretched out (here by a factor of five).
This figure shows how the call is changed by time expansion to produce a sound we can hear. Using an expansion factor od 10, the bat's chirp from 55 - 45kHz with a duration of 6msec is transformed to a chirp lasting 60msec, starting at 5.5kHz and descending to 4.5kHz.
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)
The sound you can play here 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. Time expansion lets us hear the pitch variation in the call.