Weeks 11-12 · repertoire · internal-key retention
Eight beats. No drone. Land back on the tonic.
The rule
pass: tonic ±35¢ + pulse drift ≤ 8% on 5 of 8
Left to right: the arpeggio sets the key, two count-in clicks, then 8 beats of unaccompanied singing. The emerald trace is your pitch over time; the band on the right is the ±35¢ tonic landing zone. The classic failure here is key drift— by beat 6 the remembered tonic has shifted a quarter-tone and your final note lands “in tune” with a key that’s no longer the one you started in.
Hear it first
The arpeggio prime, then a phrase that resolves cleanly, then one that drifts flat by the final beat. Listen for how the second example feels “almost right” but doesn’t close — that’s key drift.
What’s happening. A cappella singing reveals whether your internal pitch reference is stable or merely relative. With no drone, no click, no guide tone, your auditory cortex has to hold the tonic across 8 beats — about 8 seconds at 60 BPM. Untrained adults drift 30–80¢ over that span; trained folk and choral singers stay within 20¢. ±35¢ on the final note is the boundary for “resolved in the original key.”
Why two scoring axes (pitch + pulse). A phrase that lands the tonic on the wrong beat doesn’t count as “resolved.” A phrase that holds the pulse but flatlines a quarter-tone below doesn’t either. Real a cappella performance requires both — that’s why we pulled in the 8% pulse-drift gate from the rhythm family. This is the only place in the curriculum where both grids fire at once.
Why this is the capstone. Every earlier lesson scaffolded one axis at a time — pitch with a drone, pulse with a click, melody with a guide. This is the first where you carry it all yourself. If you can pass this, you can sing in real-world a cappella settings (campfires, choir warm-ups, recording without a click) without losing the key.
Session ahead
8 trials · 1-3-5-3-1 arpeggio primes the key · 2-beat count-in, then 8 unaccompanied beats at 60 BPM · land tonic ±35¢ + pulse drift ≤ 8% · 5/8 promotes.