Nonlinearities in articulatory/acoustic relations and the evolution of the human vocal tract

Abstract

The evolution of the modern human vocal tract is characterized by a progressive increase in the degree of flexion of the base of the skull. The acoustic consequences of this basicranial flexion were analyzed using a transmission line analog model and calculating variations in vocal tract transfer functions as the trailing edge of a constriction is moved anteriorly. The behavior of an unflexed tract was modeled by trading back and front cavity lengths. That of the modern human vocal tract was modeled similarly, except that after the distance of the trailing edge of the constriction tube from the larynx increased past the superior end of the pharynx, the cross-sectional area of the back cavity was increased (instead of its length) as front-cavity length decreased. The modern human vocal tract had a larger area of stability for the second formant frequency for [i]-like constrictions than that produced by the unflexed tract. It appears that the human vocal tract has evolved in the direction of increased size of quantal locations for vowel production.