How to read a sonogram

Bat calls are very high pitched and therefore above human hearing range. In order to listen to the calls, we use a bat detector to convert the ultrasound into something we can perceive.

Some bat detectors only provide us with sound i.e. heterodyne detectors like Magenta. Other detectors are broadband systems (frequency division, time expansion and full spectrum) that can simultaneously record all bat calls. This makes the recordings suitable for sonogram analysis, using software like SonoBat, BatSound, or BatScan.

Sonogram (also known as a spectrogram) is a picture of the noises that bats make, a graphical image of a sound wave. It provides a snapshot of a bat pass at a particular location at a specific time. It shows the pitch of each sound, how it varies and the length and intensity of each call. It also shows the shape, as different bat species emit distinctive echolocation pulses. (see Different Call Shapes blog).

With the SonoBat software, which we use to analyse our bat recordings, calls are shown in two ways. The green line at the bottom of the frame is called the oscillogram. This displays time against sound pressure (amplitude). When the green line is wider, this means that the call is getting louder and when the line in thinner it is softer.

Time is shown as milliseconds on the bottom horizontal ruler and will give you an idea regarding the length of the pass.

The vertical ruler on the left side shows the frequency of sound in kHz (kilohertz). The higher up the blue shapes are, the greater the frequency of the calls.

The middle section of the frame where the blue shapes can be seen is the sonogram. This displays time against frequency and amplitude is represented by colour intensity. The red colouration that you might see indicates that the call is getting louder and this should correlate with the oscillogram below.