Notes · Speech from first principles
Formant
also: resonance · F1 / F2 / F3
A formant is a resonance of the vocal tract: a frequency at which the tube of throat and mouth sustains standing pressure waves, and so a peak in the spectral envelope of the sound leaving the lips. For a uniform tube closed at the glottis and open at the lips they sit at
\[F_n = \frac{(2n-1)\,c}{4L},\]which for a 17.5 cm adult tract gives roughly 500, 1500, 2500 Hz.
What makes formants the load-bearing object in speech is that they belong to the filter, not the source: move your tongue and they move; change your pitch and they stay put. The first two, F1 and F2, separate most vowels on their own — which is why a recogniser’s first job is to discard the pitch comb and keep the formant envelope.