Call Types
There are 3 basic types of call structure;
-
CF Constant Frequency
A signal whose frequency remains unchanged over its entire duration.
Note: In reality Rhinolophus (Horseshoe) bats, the only UK genus to produce CF calls, start and end their calls with short FM sweeps. The correct terminology should therefore be ‘FM/CF/FM’ calls but for this guide we refer to them as CF.
Example of the CF call of a Greater Horseshoe bat.
(Displayed within a 60ms window, FFT 2048 Hanning).
-
QCF Quasi-constant Frequency
A signal whose frequency only sweeps through a narrow frequency band (<5 kHz).
Example of a QCF, Type A call of a Noctule bat.
(Displayed within a 30ms window, FFT 2048 Hanning).
-
FM Frequency Modulated
A signal where the frequency varies over a range of frequencies (>5 kHz).
Example of a FM call of a Bechstein’s bat.
Displayed within a 10ms window. (FFT 2048 Hanning).
Note: In some publications FM calls are further separated into ‘Wide Band FM’ and ‘Narrow Band FM’
Some species of bat will however adapt these 3 basic types to produce a more complex call structure;
-
FM/QCF
A signal which starts with a descending FM component but then transitions into a QCF component.
On the spectrogram this translates as a ‘break’ in the slope, which starts steeply and then levels out
Example of a FM/QCF call of a Common Pipistrelle bat.
Displayed within a 10ms window. (FFT 2048 Hanning).
-
QCF/FM
A signal which starts with a QCF component but then transitions into a descending FM sweep.
On the spectrogram this translates as a ‘break’ in the slope which starts almost level then descends steeply.
Example of a QCF/FM call of a Daubenton’s bat.
It is thought that these calls may have a dual function; Social and Echolocation.
Displayed within a 10ms window. (FFT 2048 Hanning).
-
FM/Flat End
A descending FM sweep which has the appearance of an even curve with no visible ‘breaking’ point.
Example of a FM/Flat End, Type B call of a Noctule bat.
Displayed within a 20ms window. (FFT 2048 Hanning).