While it is interesting and exciting to be out in the dark listening and watching bats, its great to be able to share your experience with someone who was not there. Perhaps you are unsure about the species you were hearing, especially if you could not see them well. Maybe the calls they made weren't typical of the species. Recording the sounds for later playback or analysis can bring a new dimension to your interest, and give you greater insight into the amazing capabilities of bat sonar. It can also be used (with a considerable degree of caution) to HELP identify or discriminate between different species.
Perhaps the best way of recording bat calls is to record the original bat
sound. However this requires equipment that is prohibitively expensive and
unsuited to use in the field. More practical approaches are based on
recording the output from your bat detector. Any bat detector with a
signal output can be connected to a sound recorder to record signals. (see
sound recorders below.)
Heterodyne detectors are not suitable for this kind of work as they lose the
most important part, the frequency of the original signal; however time
expansion detectors and frequency division detectors change the signal in a very
precise and predictable way that supports detailed analysis.
Using a time expansion detector allows very good quality recording and faithful analysis of the signal. A professional quality set up would typically include a Petterson Time expansion detector, such as the D240x (£1085). To go along with this you would need a recorder; The Edirol R-09 recorder records at high quality onto SD cards up to 8G: Price £300 inc 1G SD card. You would then need a sound analysis package, and the software recommended for the D240x is the BatSound package, priced at £250. (All available from Alana Ecology.)
Detectors such as the BatBox
Duet ( £275 ) can also be used to good effect with a sound recorder.
To keep costs down and because the signal quality is not so good you can use a
cheaper recorder such as an MP3 recorder or MiniDisk. The Sony MiniDisc
transfers signals to the computer via a USB cable. Sony has a program called
‘Connect’ which you can download from their website FOC. This enables a fairly
smooth extraction of the data from the disc and stores it into a library. The
data can then be converted to WAV format for analysis.
For sound analysis you could choose
BatScan (about £100) or
WaveSurfer which is a
free open-source program.
Many MP3 players offer speech recording via a built-in microphone. The quality of this is nowhere near good enough for bat recordings. BE WARNED! Only use an MP3 that can record from a line in socket to produce an mp3 file. Another factor to consider is whether the device can be set to record from a single button press. My MuVo 200 takes ages to set up so is no good in the dark!
Sooner or later you will want to do some editing - to remove hiss or wind noise and clean up your bat call recordings. Audacity is a great free tool for doing these jobs, and can also record directly and change from one format to another.
Advances in digital signal processing are making new approaches possible. In particular wavelet transforms are a powerful strategy recently discovered for use in digital signal processing. Another new technique, Waveform Similarity Overlap Addressing (WSOLA) allows data reduction by recognising that most sound signals don't change much between one wave and the next. WSOLA allows a representation of the original signal to be built using fewer waves. IF the original signal can be represented by one tenth as many waves, and if these waves are then 'stretched out' by a factor of ten we get a true representation of the original signal, but at one tenth the frequency. This would allows us to convert bat sounds at 20 - 120kHz down to the human hearing range at 2 - 12 kHz.
CSE bat detectors (made by Armin Lenck and based in Germany) now offer a stereo bat detector. Naturally this uses headphones. Stereo brings a new dimension to detecting bats. By giving directional information to your ears it helps you to tell where the sound is coming from, and makes it much easier to spot them. Armin's design uses a heterodyne principle but has a wider bandwidth than most of this type which gives a wonderfully clear and open sound - a delight to use.