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 there 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).