The shape of the vocal tract and its dynamic transitions convey an essential part of the phonetic information in speech. Therefore the way to represent this shape is important for the quality of speech coding or speech synthesis. In this paper, we first study the influence of the spatial sampling of the area function upon the acoustic characteristics of the vocal tract, and determine an optimal sampling step minimizing the computational cost without perceptual degradation. In a second part, we deal with the problem of continuous variation of the vocal tract length for a KELLY-LOCHBAUM reflexion line analog simulation: we propose a method of temporal sampling frequency conversion.