Recording Bat Calls

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.

Some alternative techniques

1: Direct recording of bat call

Perhaps the best way of recording bat calls is to record the original bat sound. Avisoft BioAcoustics, Wildlife Acoustics and others make ultrasound recording equipment that records to an internal SD type card, or links directly to a computer or tablet, enabling the actual sound from the bat to be recorded. This professional standard equipment is expensive (£2000 upwards) and you would need to consider the practicality of its use in the field.

A more economical alternative is the Dodotronic Ultramic which also records direct to a computer or tablet via USB.

Less expensive approaches are based on recording the output from your bat detector.

2: recording from bat detector direct to computer.

A good solution because it allows immediate analysis in the field. The audio (headphone) output from your bat detector is connected through a wire to your tablet or laptop computer, either through a "line in" or "ext mic" connector, or if necessary through an external USB sound card. You need to ensure the sampling rate is sufficient - many modern USB sound adaptors offer 96kHz, but 48kHz is also fine.

3: recording from bat detector to sound recorder for later transfer to a computer.

Any bat detector with a signal output can be connected to a sound recorder to record signals. Heterodyne detectors are not ideally suited to 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.

3a: Using a time expansion detector

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 (£950).  To go along with this you would need a recorder; A ZOOM or ROLAND like this will cost around £150-250.  You would then need a sound analysis package, and the software recommended for the D240x is the BatSound package, priced at £280.  (All available from NHBS.)  Digital recorders have the advantage of allowing you to transfer the recorded and encoded signal directly to your computer.

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3b: "Budget" using a frequency division detector

Detectors such as the BatBox Duet ( £275 ) and even cheaper, the BATON can be used to good effect with a sound recorder. 

BCT have an excellent paper on recording calls here.

Sound recorder quality

To get the best from your recording set-up you need a sound input (for the computer) or a recorder that can cope with the wide range of frequencies forund in bat calls.

Typically, bats like Bechsteins use call frequencies in the range 130kHz - 29kHz, and noctules 52kHz - 15kHz. Using a detector like the BATON or a *10 time expansion results in sounds from your detector which can range from 1.5kHz to 13kHz. To accurately record this your recorder needs to have a frequency response extending to well above 15kHz.

Many "dictaphone style" recorders have a frequency response that falls off above 3kHz, because no higher frequencies are needed to convey speech. They are completely unsuited to making recordings of bat calls. However there are recorders (and some MP3 players) that can record in high quality, direct to a memory card in mp3 format.

As a minimum you should record at 16 bit resolution, with a sampling rate of 44.1 kHz and a bit rate for mp3 conversion of better than 128kbps.

Faulty sampling with a low quality recorder

This file shows what happens when you record with an unsuitable recorder. The first sound is a chirp from 12kHz to 440Hz. The next sound is the same chirp after resampling at 3.1kHz.

 

 

Sound Editing

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.  If you want to save the sound file you need to "Export" it. To use mp3 you will also need to download the (free) LAME encoder. Audacity can also provide simple spectrograms from your recorded signal.

Signal processing

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 in real time.  A similar result is produced by "frequency compression" where the signal is analysed and a similar signal synthesized at a lower frequency.  This is currently used in DSP hearing aids!