Influence of Higher Order Acoustical Propagation Modes on Variable Section Waveguide Directivity: Application to Vowel [A]
Résumé
The radiation of sound from the mouth of a speaker is often described using a plane piston assumption. This is
satisfactory as long as the wavelength is longer than the largest transverse dimension of the vocal tract. For shorter
wavelengths, higher order acoustical modes can propagate and the particle velocity field in the mouth exit plane
can be nonuniform. As a consequence, the plane piston assumption does not hold and a more accurate description
of the particle velocity distribution at the mouth exit is necessary. This can be achieved using the multimodal
theory which allows one to account for higher order mode propagation. In order to study the influence of higher
order modes on the sound radiated from the mouth, four approximations of the vocal tract shape for the vowel [A]
are studied. The eccentricity of the junctions between the different portions of these geometries is used to control
the propagation of a higher order mode. The radiated sound is simulated and compared with the measurements
performed on mechanical replicas at various angles in the horizontal plane. It is observed that the propagation
of a higher order mode can strongly modify the directivity. Varying the degree of eccentricity of the junction
between the different sections shows that a very small eccentricity can induce the propagation of a higher order
mode. Reducing the size of the open end shows that higher order modes can affect the radiated sound even if the
radiating surface is small compared to the largest transverse dimension of the geometry.