Several relationships between the articulation, the dimensions of the resonating cavities and the acoustic features of speech have been determined.  These relationahips include:
a.  the frequencies of the first two formants
b.  the dimensions of the oral and pharyngeal cavities
c.  the traditional descriptions of tongue, jaw and lip positions in the vowel quadrilateral
Look at the relationship among tongue position, cavity size, and formant frequency

F1 is correlated with tongue height, which affects pharyngeal space
a.  the area of the back or pharyngeal cavity (more pharyngeal space F1 is lower)
As the tongue moves from a high to low position, the pharyngeal cvity decreases in volume.  High vowels have more pharyngeal space which resonate at lower frequencies (pharyngeal space-raising the tongue pulls the tongue out of the pharyngeal space and increasing the pharyngeal space thereby producing a low F1.  Lowering the tongue pushes the tongue into the pharyngeal space producing a higher F1)

b.  the degree of mouth opening at the lips.
    Small mouth opening - low F1
    Large moouth opening - high F1

F2 is correlated with changes within the oral cavity, primarily tongue retraction (front vowels: higher F2, back vowels: lower F2
a. lip/jaw or tongue activity
b.  Tongue retraction- the more retracted the longer the front cavity.  The shorter the length in the front cavity the higher the F2.

With both front and back vowels the tongue becomes more retracted as the tongue loweres ( More retracted tongue produces a lower F2).
Lip rounding supplements the retraction of the tongue by elongating the oral cavity in the anterior dimension for most of the back vowels (/u/ has a lower F2 because it is retracted and has lip rounding.  Thus, the shorter oral cavity with the forward tongue position resonantes at a higher F2 frequency.  The longer the oral cavity becomes as the tongue is retracted, the lower the frequency that will be resonanted.

There will be
a.  a generally consistent lowering of F2 as the front resonating cavity is enlarged because of tongue retraction ( and lip protursion)
b. a general raising of F1 as the pharyngeal cavity size is decreased and lip aperture is increased.

F3
responsive to front versus back constriction
constriction in the back produces a high F3
constriction in the front produces a low F3