The mechanism by which baleen whales (Mysticeti) produce sounds has remained largely unknown, due in part to our limited knowledge of the relationship between the sound-producing anatomy and the vocal characteristics of calls. Recent studies on mysticete anatomy indicate that the laryngeal vocal folds are the sound source, and the surrounding air spaces may play an important role in airflow, and sound modification or transduction. This current study offers a theoretical model to describe the mysticete vocal production system, which is much more complex than that of typical terrestrial mammal species. Metric data from laryngeal structures and air space volumes are combined with frequency and duration ranges defined by recordings of humpback whales off the coast of Madagascar. The resulting model delivers a prediction of sound unit durations and frequency formants that are constrained by the measurements of the trachea, laryngeal sac, and nasal cavities. Results predicted by the model are comparable to those obtained from real recordings. Errors between the frequencies of real vocalizations and the frequencies estimated using our theoretical model are less than 60 Hz for the low frequency band. Then, this new model should hopefully advance our growing understanding of sound generation in humpback whales, and mysticetes in general.