1.2 How it works
Three rules. Each bird steers away from neighbours that come too close (separation), toward the heading of its neighbours (alignment) and toward their centre (cohesion), each within its own radius. The three sliders set these radii. At 0 the birds ignore each other and the swarm is silent; as the radii grow, flocks form, merge and split (2.2).
Measuring. Between the motion and the sound lies a measuring layer. It finds the flocks, follows each one from birth through merging and splitting to its end, and measures its position, its order (how aligned its birds are), its size and its shape. It also notices encounters: two birds coming close (2.3).
Every flock sings. The four largest flocks each get a voice, a drone of four sine partials. Its pitch follows the flock's height on the screen, its place in the stereo field the flock's left and right, its loudness the flock's size, its brightness the flock's shape. Its harmony follows the flock's order: bell-like while the birds fly in all directions, the pure harmonic series once they fly as one. When a flock dissolves, its voice fades out (2.4.1).
Every encounter rings. Each meeting of two birds plays one short sine note: height sets the register, left and right the panning, the pair's speed the loudness. Inside a flock the notes take the flock's pitch, free while it is disordered and locked to the harmonics of its drone when it aligns (2.4.2, 2.4.4).
Trembling crackles. Birds that stay where they are and tremble, pushed back and forth by their neighbours, crackle instead of ringing: every time a bird is turned back, a tiny tick sounds, and the faster it trembles, the faster and louder the crackle (2.4.3).
The beat. An invisible grid lies over the stage. When the centre of a flock crosses a line, a short hit sounds on the flock's own partials. A flock flying straight crosses evenly spaced lines at even intervals, so a steady beat grows out of the flight itself, with no clock. A finer or coarser grid changes the tempo (2.4.5).
Only sine waves. Every sound is a sine wave, the plainest electronic sound there is: no samples, no noise. And nothing is left to chance: the same score with the same seed gives the same performance, down to the last note (2.7).