C2 -- Perturbation Theory

C2 -- Perturbation Theory
Perturbation theory says that a slight constriction in a uniform tube will cause a shift in formant frequency. A formant that has a volume velocity maximum near the constriction will be shifted down in frequency.

Figure from Stevens, Acoustic Phonetics p. 151
Therefore, all closures in the front half of the vocal tract will cause a decrease in F1 frequency. This includes all English consonants.
Prediction: Every interconsonantal vowel should have a peak in F1.