Two tones a few cycles apart refuse to decide between them: the air swells and cancels on a schedule. The Chladni node is a stillness in space; the beat is a stillness in time — a silence that walks the length of the tone. Push the pair far enough apart and the difference itself becomes a third voice neither tone is singing.
Drag across the field (or use the slider) to pull voice B away from 220 Hz. Sound holds the pair; still silences it. The cinnabar columns are the silences, walking.
two voices resting in unison.