u/Major-Government5998 posted this to r/Cymatic in July 2025: a laser pointer bounced off a small mirror glued to a balloon stretched over a soup can, throwing sound-driven figures onto a wall. It is the classic membrane-and-mirror trick, built for nothing, and the thread is worth reading for the replies as much as the build.
The moderator u/JohnnySteet, who has made the same rig, gives the useful notes: a wider container and a more tightly stretched membrane respond to quieter sounds; an airtight vessel improves the resonance; a hole with a tube lets you sing into it directly rather than through a speaker.
The important caveat is also his. The same frequency will always give the same figure — but change the tension of the balloon and the whole pattern set changes with it. Repeatability lives in the membrane, not in the note.