Anatomy to Acoustics: The Mechanism of Humpback Whale Sound Production

Abstract

Humpback whales emit complex songs comprised of successive sound units with a fundamental frequency and harmonics. It is unknown how these calls are produced, given that the respiratory tract is closed (i.e., no egress of air from the nose or mouth) during underwater vocalizations. Unlike open system of terrestrial mammals, the whale vocal tract is subject to flow cessation as pressure builds. Prior studies on cetacean laryngeal anatomy (Reidenberg and Laitman, 2007) showed a communication for airflow between the trachea and an expandable ventral diverticulum called the laryngeal sac. We propose a theoretical model of humpback whale sound production based on the source‐filter theory that accounts for a closed system constrained by the dimensions of the trachea, the nasopharynx, and the laryngeal sac. We used recordings of humpback whales from the Indian Ocean (provided by Cetamada Association, Madagascar). Results show a relationship between time (duration) and spectral features (fundamental frequencies) of these calls. As this is a closed system, duration is dependent upon laryngeal sac pliability, while frequency is determined by vocal tract length. Our investigation is a first step towards defining individual acoustic signatures. Future studies will assess whether predicted durations and frequencies, based upon known vocal tract measures, fall within the range of actual recorded sounds.