In this work we study how the head position in relation to that of the cerebral vertebrae affects phona-tion of five vowels of French. Our aim is to examine how this affects acoustic parameters, draw conclusions to adapt acoustic simulations and vocal tract geometric models for articulatory synthesis, verify whether the effect of posture is observable and evaluate the changes in the formant frequencies. We use MRI data to capture the shape of the vocal tract in three different positions per vowel, with simultaneous speech signal recording. We use acoustic simulations with no underlying plane wave hypothesis to see how the acoustic wave propagates. We simulate three head postures (up, middle/natural and down) for every vowel, compare the results with the original speech signal and validate the existence of difference both in the acoustic domain in terms of for-mant frequencies and in the articulatory domain, especially in the pharyngeal cavity.