Found:

Perturbation theory

PHONETICS: A model of the acoustic consequences of vocal tract constrictions. The perturbation theory (Chiba & Kajiyama 1941) relates vocal tract constrictions to formant frequencies by taking into account the kinetic energy present at points of maximum velocity and the potential energy present at points of maximum pressure. If the vocal tract is constricted at a point of high kinetic energy (velocity maximum), air particle movement is impeded, and consequently the frequency of the movement decreases. On the other hand, if the vocal tract is constricted at a point of high potential energy (pressure maximum), air particle movement is enhanced, and consequently the frequency of the movement increases. These efffects can be summarised as follows:
1 Constriction of the vocal tract near a point of maximum velocity lowers the formant frequency.
2 Constriction of the vocal tract near a point of maximum pressure raises the formant frequency.

LIT.	Chiba, T. & Kajiyama, M. (1941)