Fig. 4
From: Formant analysis of vertebrate vocalizations: achievements, pitfalls, and promises
Pitfalls and specialized opportunities for formant tracking in high-pitched vocalizations. The formant structure is virtually invisible in the tonal part of this modulated high-pitched call, apart from slight changes in amplitude as fo crosses the first formant (yellow circles). Nonlinear phenomena (see Table 1) may help to reveal formant frequencies (here, “true” formants, labeled F1 to F4, are visible in the chaos), but caution is needed to avoid confusing formants with sidebands (resulting from modulation) or residual harmonics