Fig. 3
From: Formant analysis of vertebrate vocalizations: achievements, pitfalls, and promises
Pseudo-formants caused by frequency modulation or amplitude modulation. A tonal sound at an fo of 150 Hz is modulated at 20 Hz. Both frequency modulation (vibrato one semitone in depth) and amplitude modulation (non-sinusoidal oscillator at half the amplitude of the carrier wave) produce sidebands around harmonics, which can resemble formants in the spectrum or on narrowband spectrograms. Diagnostics: modulation can typically be perceived by ear. Frequency modulation (FM) can be detected visually in the broadband spectrogram as vibrato-like frequency oscillation at 20 Hz, especially in the upper harmonics, while amplitude modulation (AM) produces noticeable beats at 20 Hz in the oscillogram (bottom panel beneath each spectrogram)