Acoustics of the avian vocal tract

Abstract

The avian vocal tract can appropriately be modeled on a source-filter basis, with the pressure-controlled syrinx valve providing the wideband harmonic source and the vocal tract the linear passive filter. This filter consists of four parts: the bronchial tubes above the syrinx valves, the trachea, the mouth with included tongue, and the beak. Each of these can be modeled and its behavior calculated quantitatively using standard methods [Fletcher, Acoustic Systems in Biology (Oxford U. P., New York, 1992)]. The long trachea defines the main resonance frequencies, upon which the bronchial resonances impose formant bands. The mouth, the cross section of which can be varied by changing the tongue position, influences particularly the frequencies of the first two tracheal resonances, but also the harmonicity of the upper system resonances. The beak behaves as a slotted tapered transmission duct, and contributes an end-correction to the system, the magnitude of which is a strong function of both beak gape and frequency. This paper reports the numerical results of such a theoretical analysis, compares these with experimental measurements upon appropriate model systems, and discusses the implications of the results for the understanding of avian vocalization.