Sing ‘ah’ and ‘ee’ on the same note and the pitch is identical — what changes is the SHAPE of your throat, which lifts two resonant peaks (formants) to different frequencies. Two numbers, and a buzz becomes a vowel. Slide oo → ah → ee and ▶ play.
Source-filter voice: a buzzing glottal source (a sawtooth, rich in harmonics) is shaped by the vocal tract, whose resonances — the formants — boost bands of the spectrum. The vowel is read almost entirely from the first two formant frequencies (F1, F2): oo ≈ (300, 870), ah ≈ (730, 1090), ee ≈ (270, 2290) Hz. The pitch stays fixed while the formants move, so the same note carries different vowels — timbre as geometry of the throat. The slider morphs the formant pair; a fail-loud self-check throws unless the three vowels have distinct (F1, F2) pairs. Press ▶ to hear a 110 Hz buzz filtered to the current vowel.
Two band-pass resonances on a sawtooth stand in for the full vocal tract (real voices have 3–5 formants plus dynamics); the formant tables are canonical averages. The peaks, the morph and the filtering are computed exactly.