Spectrograms visualise the time-frequency content of a signal. They are commonly used to analyse animal vocalisations. Here, we analyse how far we can deduce the mechanical origin of sound generation and modulation from the spectrogram. We investigate the relationship between simple mathematical events such as transients, harmonics, amplitude- and frequency modulation and the resulting structures in spectrograms. This approach yields not only convenient statistical description, but also aids in formulating hypotheses about the underlying mathematical mechanisms. We then discuss to what extent it is possible to invert our analysis and relate structures in spectrograms back to the underlying mathematical and mechanical events using two exemplary approaches: (a) we analyse the spectrogram of a vocalisation of the Bearded Vulture and postulate hypotheses on the mathematical origin of the signal. Furthermore, we synthesise the signal using the simple mathematical principles presented earlier; (b) we use a simple mechanical model to generate sounds and relate experimentally observed mechanical events to characteristics of the spectrogram. We conclude that although knowledge of sound producing systems increases the explanatory power of a spectrogram, a spectrogram per se cannot present unambiguous evidence about the underlying mechanical origin of the sound signal.
Bioacoustics, biomechanics, Fourier analysis, Gypaetus barbatus, Duffing equation