Influence of higher order propagation acoustic modes on speech directivity
Résumé
The acoustic of the vocal tract is often described with a plane wave assumption. In other words,it is assumed that only one dimensional acoustic waves propagate and are radiated from the vocaltract at the mouth. If this assumption is satisfying at low frequency, higher order acoustical modescan propagate above their cut-on frequency. According to the typical dimensions of the vocaltract this frequency can be estimated to lie in the range 4-5 kHz which corresponds not only to themiddle of the speech frequency range but also to the region of maximum sensitivity of the humanear. We first present experimental and numerical examples of the acoustic properties performedon simplified and realistic vocal tract shapes showing that these modes can indeed strongly affectthe pressure field above the cut-on frequency. As a consequence a strong directivity of the radiatedsound, which cannot be predicted by plane wave theory, can be expected. The influence of thehigher order modes on speech directivity is then investigated through experimental measurementsand simulations performed on simplified vocal tract replicas. The radiated pressure field is mea-sured inside an insulated room using a microphone and is compared with the expectations derivedfrom multimodal theory. It is shown that the influence on the directivity can be considerable. Theconsequences of these findings in terms of speech perception and speech synthesis are discussedand illustrated.