Weeks 9-10 · voice · /a/ → /uh/ with ascent
As the pitch climbs, round the vowel. Resonance stays open.
The rule
pass: pitch held + centroid drops ≥ 80 Hz · 3/4
Two panels. Top: an ascending pentachord — five pitches climbing. Bottom: the vowel target for each pitch. The emerald target ellipses migrate downward as you ascend — from a bright /a/ on do to a rounded /uh/ on sol. The grey dashed line shows the failure mode: vowel stays bright at the top, the upper notes lose resonance and the voice strains. Pitch must stay within ±35¢ while the spectral centroid drops at least 80 Hz.
Hear it first
The ascending pentachord with vowels rounded properly — listen for the upper notes staying free and resonant. Then the same scale with the /a/ held bright all the way up — the top notes get pinched and edgy. The point is to hear the difference between “same vowel” (which sounds strained) and “same identity” (which preserves the line).
What’s happening. Each vowel has a characteristic pair of formant frequencies (F1, F2) — peaks in the spectrum produced by the shape of your vocal tract. /a/ has a high F1 (~700 Hz); /uh/ has a much lower F1 (~500 Hz). When you sing on an ascending scale, the fundamental gets close to F1 and the harmonic-formant mismatch causes the upper notes to lose acoustic energy. Migrating toward /uh/ lowers F1 enough that the harmonic stays inside the resonance peak. This is the aggiustamento of bel canto — and the modern passaggio modification.
Why ≥ 80 Hz centroid drop. Spectral centroid is a rough proxy for “brightness” — high centroid is a bright vowel, low centroid is a darker, more rounded one. A drop of 80Hz from note 1 to note 5 is approximately the difference between /a/ and /uh/ at the same loudness — measurable but subtle, so you can’t fake it by collapsing into a yawn. The 35¢ pitch tolerance keeps you honest: the vowel must move without the pitch drifting.
Why this is the bridge. Vowel modification is the prerequisite for bright/narrow resonance, sustained high notes, vibrato across the bridge, and most contemporary belt-pop and classical-Italian repertoire from the top fourth of your range. Without it, every high note will sound strained — or worse, will crack.