In this work we study the effect of the velum and epiglottis on speech production of five French vowels. Our purpose is to examine whether it is possible to simplify the geometry of the vocal tract in the framework of articulatory synthesis to achieve a simpler geometric description without changing the acoustic properties. In the present study, we use MRI to acquire the 3D shape of the vocal tract with simultaneous recording of the speech signal. The geometric two-dimensional shape derived from these data was used as an input of numerical acoustic simulations. The geometrical shape was edited at the level of epiglottis and velum (with or without epiglottis, with or without a constant wall approximation at velum) and the spectra obtained via numerical acoustic simulations were compared with those obtained from audio recordings. This allows the impact of these articulators and geometrical simplifications to be assessed.