(a) Amplitude and (b) phase of the axial particle velocity simulated on the mouth exit plane of the vocal tract geometry without lips of the vowel /a/ of participant 2 at 5157 Hz and (c) the corresponding simulated directivity half-balloon. (d) Directivity balloon measured at 5039 Hz on the HATS equipped with the vocal tract replica of the vowel /a/ of participant 2 without lips and without a torso. These frequencies correspond to the vertical streaks visible around 5 kHz in Figures 9a and 9b.