Hold a flat line. Then let vibrato in. Both on purpose.

What’s happening. Vibrato is a regular periodic modulation of pitch — roughly 5–7 cycles per second, with a depth of 30–120¢ — produced by oscillation in the cricothyroid / extrinsic laryngeal muscles. Most untrained voices either have no vibrato at all (rigid muscle tone) or an irregular wobble (uncoordinated oscillation). A natural vibrato emerges when the larynx is properly supported and free; a straight tone requires conscious inhibition of that emergence.

Why 5–7 Hz, why ≤ 15¢. The vibrato rate band is the empirical range of human vocal vibrato across professional singers — below it sounds like a wobble, above it like a tremor. The straight-tone extent threshold of ≤ 15¢ is the perceptual floor at which a listener still hears the tone as “straight” rather than “subtly modulated.” You have to hit each band on purpose.

Why this is the capstone. Intentional choice between straight tone (chorale, early music, modern pop) and vibrato (bel canto, jazz, most contemporary classical) is what separates a flexible singer from a one-trick voice. Most of weeks 11-12 — dynamics, emotional contrast, a-cappella repertoire — assumes you can pick a tone-quality and hold it.