The effect on vowel directivity patterns of higher order propagation modes

Measurements of speech directivity patterns show that it differs according to the phoneme pronounced. In the case of the vowels, these differences can be attributed to variations of the vocal tract shape. The multimodal method, which takes into account higher order propagation modes, is used to simulate directivity patterns (2 kHz-15 kHz) for simplified vocal tract geometries of the vowels [a], [e], [i], [o] and [u]. The directivity patterns of a simplified and a realistic replica of the vowel [a] are measured experimentally. The comparison of the experimental data with the simulations shows a good agreement (average difference of 1.7 dB). It is observed that the amplitude, orientation and number of lobes can change significantly for some small frequency intervals, of the order of 100 Hz; these are shown to be caused by higher order modes. These changes can occur as low as 3 kHz if a wide cavity is present near the mouth exit. A small mouth exit limits the effect to specific frequency intervals, and a narrow channel limits the transmission of the higher order mode effect to the mouth exit. The comparison of the directivity measured on a realistic replica corresponding to the vowel [a] with simulations performed on two simplified geometries shows that a fully asymmetric shape (for which the centers of the cross-sectional contours are not aligned) is qualitatively more realistic than a partially asymmetric shape (when the centers of the cross-sectional contours lie along the same axis).