This article presents a characterisation offormant trajectories based on an active tracking of each resonance of the vocal tract. This tracking is necessary due to the presence of focal points i.e. changes of affiliations between F2 and F3 in most of transitions between back and front sounds. We show that this tracking can be easely implemented without any analysis-by-synthesis process by assuming a minimum elasticity principle (smoothness) and a monotonous relation between resonances associated with the same cavity. Our proposal is supported by intensive analysis of natural speech, articulatory simulations and perceptual experiments.

Keywords: Perception, Formants, Vocal Tract, Reso- nances.