Bat Calls: Analysis

The recorded sound from a bat detector contains a lot of information. The bats brain is able to take the reflected sound and use it to create a picture of its surroundings. Our hearing and our brains are not set up to make very much sense of it, and are very much more capable when dealing with images. We can use technology to help us get more information about the bat call by portraying the sound signal as a picture. Lets look at some very simple sound signals.

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Here is a single frequency wave plotted as sound pressure against time. The maximum value of this wave is 1. We say it has an "amplitude" of 1.


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Now we add a second wave. The second wave is changing faster - it has a higher "frequency". It also has a smaller amplitude - only 0.6.


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In the chart above we see the result of combining the two single frequency waves. This way of showing the signal is no longer very helpful, because it isn't helping us to make sense of what is happening. Our ears would still hear this as two notes of different pitch and loudness.


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This chart shows the same two waves but plotted in a different and (for now) more useful way - as a "spectrogram". This clearly shows the signal is composed of two waves of different frequency (pitch) and amplitude (loudness). However this plot does not show us how this changes with time.


A Sonogram or sound spectrogram aims to show how the relative strength of the various frequency components in a signal changes with time. This is done by plotting frequency on a vertical axis and time on a horizontal axis. The relative strength of each frequency is then indicated by the brightness, darkness or colour at that point.

This spectrogram is generated by Soundruler from a recording of a soprano pipistrelle using a frequency division detector. The call oscillogram shows clearly how the amplitude (loudness) varies with time; this is called the "envelope". The "wiggle" that follows the call is a wave created by the signal starting and stopping - a bit like the wave you get when you drop a stone into water. Just ignore it.

Below the call oscillogram you can see from the spectrogram that the call starts near 60kHz and comes down to 55kHz. The player below plays this section of the sound.

By default Soundruler uses a black on white colour scheme and to change it you need to use Options - General, not Options - Spectrogram. However you will need to use Options - Spectrogram to set the frequency range to suit your recording.

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Here you can play the sound used to make the sonogram above.
It's repeated 3 times at 0.1 sec intervals.