Sound and shaking

The speaker only shakes the sheet, and its code never mentions salt

Sieve salt over the page, pick up the speaker and hold it in the middle. You hear a steady note. Within seconds the salt runs together into lines and curls, a symmetrical figure like the ones Ernst Chladni made famous with sand on a vibrating plate.

The surprising part is that the speaker's code knows nothing about salt. It does one thing: it sets the sheet shaking, hard in some places and not at all along certain lines. Everything lying on the paper reads that shaking in its own way. A salt grain gets bounced off the parts that move and comes to rest where the sheet is still. No line of code describes the figure. It's what's left once every grain has hopped away from the shaking.

A test holds the code to this. It reads the speaker's source file and fails if it ever mentions grains, foam, soap bubbles, water, ripples or particles. It also reads ten simulation files, among them the grains, the foam, the puddles and the shake map itself, and fails if any of them mentions the speaker. A second test sets the sheet ringing with no speaker on the page at all, and glitter still drifts to the still lines.

The rule each grain follows is written by hand, and it's short: when the sheet under you shakes hard enough, take a step toward where it shakes less. Salt doesn't fly through the air in the game, and it doesn't find the lines by chance. The figure is the only part that nobody wrote.

Sand and fine powder go opposite ways, as they do on a real plate

Sieve cocoa instead of salt and the picture turns inside out. The cocoa gathers where the sheet shakes hardest and leaves the still lines bare. Flour does the same, but white on white paper it hardly shows. Real powders do this. Félix Savart noticed it, and Michael Faraday explained it in 1831: very fine powder is swept along by the air the vibrating plate stirs up, while heavier grains bounce until they land where the plate is still.

The game doesn't compute that air. It sorts each material by grain size. Coarse grains (sand, salt, yeast, chia seeds) step down the shake map. Fine powders (flour, cocoa, icing sugar, baking soda, cornflour) step up it, more slowly. That's a written rule, and it's in the table at the bottom of this page. What isn't written anywhere is the figure each one draws.

The volume knob sets how thin the lines are. A grain only bounces where the sheet shakes harder than a set threshold, so the quiet strip along each still line is where grains come to rest. Turn it up and that strip narrows. But shake too hard and the grains on the most shaken parts are thrown a few cells in any direction, and the figure turns messy. Too quiet, and nothing lifts at all.

Sand piles its lines only as steep as sand can stand. Bouncing heaps grains up, the slope rule slides them down, and an automated test checks that after ten seconds almost no cell is steeper than sand can hold. Wet the sand first and it stays where it is.

Every sound effect is made by code at the moment it happens

There isn't a single recorded sound effect in the game. A splat is a burst of filtered noise with a falling tone under it. A bonk is a tone gliding down with a short click on top. Fire is noise through a filter, and it gets louder as more of the page burns. The flies' buzz wobbles and grows a little with every fly, up to a quiet ceiling, so a swarm hums instead of roaring. The toy car's motor rises in pitch with its speed. The only audio files are the six songs and the professor's spoken lines.

A sound is also something that happens in the world. When the game plays a bang at a place, the bang writes a patch of noise that creatures hear, as loud as that kind of sound is: a splash is loud, a clink much quieter, a pencil scribble hardly there. The game does this before it checks whether you've muted it, so with the sound off a cat nearby still hears the bang. A dog doesn't startle at its own bark, because each creature takes the noise it made itself off what it hears. The pigeon nearby does startle, by its own hearing, and no code tells it the dog barked. An automated test checks it.

The professor's mouth opens and shuts with the loudness of his voice as it plays. That's two pictures, mouth shut and mouth open, swapped on loudness and each held a moment so they don't flicker. It isn't lip-sync. While he talks, the music dips.

Figures appear the moment the note plays, and a few classic shapes are left out, to fit a tablet

A real Chladni plate is a hard problem in mechanics. A child with a tablet needs the figure to appear, change with the note and the press point, and sort sand from flour, within a few milliseconds a frame. Here is what we keep and what we let go.