Found:

Quantal theory of speech

PHONETICS: The thesis of the Quantal Theory, put forward by Stevens (1972), is that certain, relatively large changes in articulator position will cause little change in the acoustic signal, while other, relatively small changes in articulator placement will cause large changes in the acoustic signal. The extent of the acoustic change appears to be related to the particular region of the vocal tract where the articulation is located. In certain critical regions, a slight adjustment of articulatory placement will cause a quantal change in sound.

LIT.	Stevens, K.N. (1972)