Theoretical models of the avian syrinx

Abstract

The acoustic spectra of most birdsongs contain either a fundamental tone without overtones, a fundamental with harmonic overtones, or broadband noise with all frequencies present. The classic functional model of the avian syrinx suggests that vibrating medial tympaniform membranes generate these spectra. It is shown here that, while vibrating, these membranes should generate spectra that contain many partial overtones. As partials are not present in most birdsongs, the vibrating membrane model appears inadequate to explain syringeal function. Two new models are presented: the vibrating string model and the aerodynamic model. In the former, membrane-like medial tympaniform membranes are stretched into the shape of a string. Vibrating string-like medial tympaniform membranes could have acoustic spectra with either a fundamental alone or a fundamental plus harmonics. In the latter case, the medial tympaniform membranes function only to create an orifice through which respiratory air must pass. Periodic, trailing vortices are created off the downstream side of the membranes. The periodicity of vortex shedding determines the frequency of the sound. Acoustic spectra generated by this mechanism contain either a fundamental alone, a fundamental plus harmonics, or broadband noise. As the acoustic spectra postulated by the vibrating string model and the aerodynamic model correlate well with the acoustic spectra found in birdsong, they appear to be better models of avian syringeal function.