Pure tones. Start with the volume low.
Tuning
Automatic just intonation based on the lowest note.
Try a harmony
The drawing
How much of the path remains visible behind each point. The trail builds gradually, then keeps this length.
How does it work?
With two notes, each wave moves one axis of the plane. With three, the waves move X, Y and Z: the curve lives inside a cube.
With four notes, the curve has four coordinates. To see it, we rotate the curve in 4D and project it into 3D. The fourth note also changes the drawing; the projection does not preserve all four-dimensional information.
The keyboard starts in just intonation: C–E–G automatically produces 4 : 5 : 6. The reference is the lowest selected note and changes when you add or remove the bass note. It uses a 5-limit chromatic just scale; not every interval between pairs of notes will be pure, and inversions may tune differently.
In Piano mode, you can compare equal temperament: C–E–G gives 4 : 5.04 : 5.99. Your selected mode stays active as you play. Manual lets you edit ratios; adding a key in that mode switches back to Just.
Each active key, its label and the graph share the same palette. With one note, the curve uses its color; with several, it moves through and blends the active colors. Each dot travels along the combined curve from a different position, leaving a trail in its own color. Changing the notes, tuning, phases or trail length clears the trails and starts a new drawing: first the dots, then the paths they trace. Trail length limits how much remains visible. Turning animation off freezes the drawing; turning it back on continues it. The dots are guides along the combined curve, not separate oscillations of each axis. The order in which you add notes assigns the axes. The animation uses slow, normalized time; the audio plays the actual frequencies. This is not a simulation of a vibrating plate.