Recording a Bat

Bat's produce sonar sounds that are usually inaudible to human ears, so we have to use special equipment to record it. This picture shows the basic technique:

The bat makes sounds that are picked up by a special microphone.
The microphone is connected to a computer that can record the sounds on its hard drive.
Then we analyze the sounds using computer programs like MATLAB or SIGNAL.

Here is the actual equipment I use for my recordings.

A laptop computer (the faster the better) with a large hard drive - the files take up about 200 megabytes for 5 minutes of recording, so a hard drive can fill up fast. You also want a laptop with good battery life since you can't record while it's plugged in, since that seems to cause problems with electrical noise that messes up the rcordings.

A PCMCIA card produced by Measurement Computing (I use the model PC- Card- DAS16/330).

A special connector box and cable that connects to the PCMCIA card (also from Measurement Computing)

CBDISK software provided by Engineering Design that takes the input and creates the computer files.

A custom-built amplifier (the silver box in this picture) that increases the amplitude of the signals from the bat detector.

A U30 bat detector from Ultra Sound Advice. This detector works like any bat detector producing clicks that corespond to the bat's echolocation calls, but it also can be used as a high-frequency microphone.

Obviously, this setup is not the only one that would work, it's just the one I became familiar with while working as a graduate student at Ohio State.

Once we get the sound onto the computer, the programs can make pictures of those sounds called "sonograms". This sonogram shows a typical E. fuscus sonar call recorded in the laboratory. The color of the figure tells you how loud the sound was at a given time and frequency. Dark red is very loud, while blue and green are very soft.

There are a number of characteristics to note about this call:

It is very short - this call is about 1.5 milliseconds long.
The call has very high frequencies - humans can hear up to 20 kHz, this sound starts at 25 and goes up to about 150 kHz!
The call contains 4 harmonics (shown by the four separate reddish-orange bands).

Click on the picture to hear this call after it has been slowed down on the computer to make it audible to humans. The sound will repeat five times (each time sounds like a "chirp" noise). The file is a WAV file, so it should play on any system running Windows.

Along with displaying the sonograms of these calls, the computer can automatically measure a number of variables that describe the call.

Click on the link below to see how we extract the bat's calls from the recorded files.

How can you extract recorded files?

Recording a Bat's Echolocation Calls