Any change in the cross section of the vocal tract shifts the individual formant frequencies, the direction of the shift depending on just where the change in area falls along the standing wave. Constriction of the vocal tract at a place where the standing wave of a formant exhibits minimum-amplitude pressure oscillations generally causes the formant to drop in frequency; expansion of the tract at those same places raises the frequency. Three other major tools for changing the shape of the tract in such a way that the frequency of a particular formant is shifted in a particular direction are the jaw, the body of the tongue and the tip of the tongue. Moving the various articulatory organs in different ways changes the frequencies of the two lowest formants over a considerable range [18]. Johan Sundberg, "The Acoustics of the Singing Voice," SCIENTIFIC AMERICAN, March 1977.
One way to increase formant frequency is to the vocal tract at a place where the standing wave of a formant frequency exhibits minimum-amplitude pressure oscillations.
A. Stretch
B. Vibrate
C. Contract
D. Expand