Abstract:

Acoustic waveforms produced by vocalizing humpback whales are well described by a source--filter model. This model characterizes vocalizations as the output of a time-varying signal generator (the source) passed through a time-varying resonator (the filter). For most humpback whale vocalizations, the source can be modeled as either a quasiperiodic impulse train, white noise, or a combination of these two signal types; the resonator can be modeled as an all-pole filter. The source--filter model is useful because it (a) provides a way to quantitatively characterize humpback whale vocalizations that can be automated and that allows for the ``resynthesis'' of sounds; (b) facilitates the application of advanced speech processing technologies to the analysis of humpback vocalizations; and (c) can potentially provide clues about the actual mechanisms humpback whales use to produce sounds. Currently, it is not known how humpback whales internally produce sounds, although structures in the larynx are thought to be involved. Source--filter-based analysis of humpback vocalizations revealed characteristic regions of frequency enhancement (formants) that may correspond to resonant frequencies of a physiological ``filter.'' Distribution of formants varied with pitch in a manner that appeared similar to pitch-related formant variation in human singers, suggesting that both species use analogous production mechanisms.