Vowels are just resonant peaks in the harmonic spectrum. Shape six partials and make the drone sing "ah" and "ee" like a vocal tract in miniature.
In the world
Your vocal tract is an additive filter: the larynx supplies a buzz rich in harmonics, the mouth and throat boost certain frequency regions (formants) that your ear decodes as "ah" or "ee".
A formant is a fixed peak in the spectrum. Vowels differ only in where their peaks sit. Say "ah" and the first two peaks land near 700 Hz and 1100 Hz. Say "ee" and the second peak jumps up past 2300 Hz while the first drops. The pitch is irrelevant; the spectral shape carries the vowel.
With six tuned VCOs you can fake a formant by boosting the partials nearest the target peak. It is a coarse vowel, but unmistakably "ah" or "ee".
Did you know?
Babies can distinguish all the vowel sounds of every human language. By 6 months they start losing sensitivity to vowels outside their native language - their brains literally prune the formant detectors they do not need.
Explore
A morphing vocal tract tunnel stretches on the right. Move your cursor left/right and up/down to steer through vowel space. The tunnel walls reshape to match the formant pattern - wide for "ah", narrow for "ee". Glowing formant rings mark the resonant peaks and shift as you move.