This paper analyzes vocal tract resonance (VTR) frequency trajectories and their relationship to formants from a new point of view. Considering abrupt/continuous changes in the physical geometry of vocal tract, VTR may change in number, suddenly change their positions or may leak to some regions where they usually do not exist. We define the visible VTR (VVTR) as VTR that can be seen from the spectrogram. So we propose an algorithm, based on Kalman filtering, that can handle all these changes in VVTR. The suggested properties of VVTR trajectories and the performance of the algorithm are demonstrated on several examples.