Acoustic perambulators: a walk with the Pipistrelles

Bats in the trees above our heads

At the end of July there was an opportunity to join a bat walk around the moors. Although bat detectors were provided I wanted to get my own and learn more about ultrasound and different forms of bat detection. As a gadget geek, I spent hours trying to find the right detector, in the end I opted for a BatBox Duet, which combines heterodyne and frequency division detection. I must point out that other bat detectors are available and that my main interest is in the sounds themselves rather than the ability to detect which bat I am listening to. I opted for the Batbox because it provided the opportunity of recording two different ‘forms’ of sound and because when I rang up to enquire about it, I spoke to David King, a sound artist who had worked on the BBC Radio 3 series Between the Ears: during my research studies at Winchester School of Art I recorded many of these programs and it was a pleasure to talk to such an interesting fellow acoustic perambulator.
The bat’s use of sound in many ways echoes my own aims for the Winnall Moors sound walk: the acoustic mapping of place. Of course the bats use sound for the echolocation of their prey, whilst sound alone is my quarry.
On the evening of the walk we set off around 8:00pm, a small group of adults and youngsters walking into the dusk with the white noise of bat detectors listening out before us like white canes tapping out the edges of an unseen world. There was something quite otherworldly in the noise of these detectors moving around the moors. At the bridge the first electronic ‘wet slap’ and ‘clicks’ stuttered into audition. Everyone fell silent, the group twisting around trying to tune into the same frequency and discover the bat in their hand.  It was like a scene from some optimistic science-fiction film, where humans encounter an alien species and try to communicate with them.
As an observer I could see the bats and witness their ultrasonic approach as they dextrously swooped through the trees and around the heads of the people listening. Later I swapped from acoustic recording via microphone, to the in-line ultrasonic audition of my bat detector. The sound emitted by the bats is astounding, similar to early electronic instruments such as the Theremin or Moog synthesizer and reminiscent of electronic music pioneers such as Karlheinz Stockhausen, Delia Derbyshire and Daphne Oram.
As a child of the seventies, television was full of science-fiction dramas such as Dr Who and Tomorrow’s People. The sound of space, the future and our encounter with its alien inhabitants was always electronic.
The recordings above are taken from the bat detector, the first is the heterodyne recording alone and the second a mixture of heterodyne and frequency division. The sounds are irregular and intermittent and I have removed some of the white noise, which is an artefact of the bat detector. I recommend listening with headphones.
There are several methods of identifying bats, which includes not only the frequency of their sound, but also the pattern of their voice and flight. Taking all this into account  I would identify the bats you are listening to as Common Pipistrelles: but if you know better please tell me so.

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