2016 年 11 巻 4 号 p. 16-00414
We previously described an analytical model regarding how human falsetto voice is produced upon interaction between the respiratory airflow and vocal fold motion. This theory highlights the role of an unsteady flow effect―or specifically, convective acceleration of wall motion-induced flow―in inducing a Hopf bifurcation or aerodynamic flutter of vocal folds, reminiscent of falsetto voice production. Importantly, the mucosal wave motion and glottal closure of the vocal folds―typically observed in human modal voice production but absent in falsetto vocalization of high voice pitch―are dispensable in this analytical model. Thus, given its rigorous applicability to high-pitched vocalization, our model may function as a universal physical mechanism underlying the vocalization of not only humans but also other diverse vertebrate animals that share a basic anatomical design. Here we show that the relationship between the vocal frequency and animal body size and mass, derived from the present model, captures the actual features reported elsewhere, thus suggesting that the allometric scaling of animal vocalization is explained theoretically. Moreover, the critical biomechanical conditions that induce the vocalization are rewritten to highlight an intriguing consequence from our model that the voice pitch can be controlled simply and extensively with the mechanical tension in vocal membranes. Furthermore, several dimensionless numbers that characterize the aerodynamic flutter are introduced to shed light on the physical essence of the possible universal mechanism underlying vertebrate animal vocalization: whether the animal prefers to falsetto or modal vocalization is determined by its communication frequency.