If you've never heard of the Chladni plate experiment, the principle is very simple. Sound frequencies create patterns. If you can make a metal at certain frequencies, acoustic resonance will cause those plates to vibrate with those patterns.
Normally, you can't actually see this -- unless you sprinkle something granular over the top. Which is exactly what YouTube user Brusspup has done in his latest video. Using a metal plate attached to a speaker, he drove frequencies through it using a tone generator to create intricate configurations of sand.
"Certain frequencies vibrate the metal plate in such a way that it creates areas where there is no vibration," he said. "The sand 'falls' into those areas, creating beautiful geometric patterns. As the frequency increases in pitch, the patterns become more complex."
The patterns, called Chladni figures, are fascinating to watch as Brusspup changes the frequency; and, if you have the equipment, this one is absolutely safe to try at home.