Evidence for Nonlinear Sound Production Mechanisms in the Vocal Tract

Abstract

Much of what speech scientists believe about the mechanisms of speech production and hearing rests less on an experimental base than on a centuries-old faith in linear mathematics. Based on experimental evidence we believe that the momentum waves, or the interactions of the inertia-laden flows leading to various modes of oscillation, within the vocal tract are neither passive nor acoustic. Measurements of flow within the vocal tract indicate that acoustic impedance, or the pressure-flow ratio, is violated. The pressure across any cross section of the tract is constant and does not exhibit the differentials expected from the markedly different separated flows across that same cross section. There has been little proof that the ear is primarily a frequency analyzer, or any solid explanation given for its extraordinary sensitivity. Finally, nonlinear processing techniques, that are less prey to Fourier artifacts, are described.