Resonograph
Hold a pitch and watch tens of thousands of sand grains crawl across a vibrating brass plate and settle onto the nodal lines of that tone's standing wave. Sing higher and the figure reorganizes into a finer lattice; the plate answers in inharmonic metal.
Sing, hum, or whistle a steady note. The plate finds the closest Chladni eigenmode and the sand settles into its figure. No microphone? A seeded pitch sweep plays it for you, and a slider lets you steer.
Best with headphones. The plate rings a metallic, inharmonic tone — smooth glow, no flashing.