Formants

Formants are the resonances of a space, for instance, the human vocal tract.¹

In speech, they exist below 5000Hz and are usually in-harmonic.²

The first formant frequency is higher for open vowels, like [a], and lower for closed vowels, like [i] or [u]. The second formant frequency is higher for front vowels, like [i], and lower for back vowels, like [u].¹

I wonder if this first and second formant frequency is responsible for complementary frequencies (see How to use complementary frequencies).

F1 corresponds to vowel height, and F2 corresponds to vowel advancement (back or forward).³

! [[ Average_vowel_formants_F1_F2.png ]]

! [[ formants-3.png ]]

More about this in chapter five of Cantabile.