[SciShow intro plays]

Olivia: Cymatics are visual patterns that combine three ingredients: thin plate, a pinch of loose material, like sand or salt, and a heavy dose of music. They let us actually see sound... and it’s kind of beautiful. Sound is a vibration through a medium, like air, that eventually vibrates our eardrums, which triggers an electrical signal to our brain that we interpret as a noise.

But these waves can vibrate more than just our eardrums — they vibrate everything in their path. If you yell loudly enough, for example, the sound waves are strong enough to vibrate through a whole wall, which is why you might get complaints from your neighbors. Sometimes you can even see these vibrations, like when your rock ‘n roll garage band shakes and rattles everything in the house. Also an activity that might attract angry neighbors.

These vibrations may seem like mini-earthquakes, but every single one follows the laws of physics. And it’s easy to see these laws in action when sound waves go through something visible, like sand. One way to make cymatics is to spread sand over a thin plate and then produce sound waves right in the center. The sound waves spread out from the center, which makes the plate and the sand vibrate.

One of the principles that helps create cymatics is the idea that sound waves are longitudinal waves. That means the particles that the wave is moving through are vibrating in the same direction as the wave itself. The particles move back and forth so that there are some areas of compression, where all the particles have moved really close to one another; and some areas of expansion, where all the particles have moved away from one another.

But there’s more: to make your beautiful artwork, you have to make something called a standing wave. A standing wave is pretty much what it sounds like: a wave that looks like it’s standing still. It happens when the sound wave travels to the edge of the plate and breaks into two parts. One part of the wave will keep going out into the air, but the other part of the wave will reflect, which means it turns back around and starts moving back toward where it came from.

These reflected waves then overlap with new incoming waves. When two waves interfere with each other, their effects either add or subtract from one another. Like if you look at just one grain of sand, and both waves are trying to vibrate it to the left, the sand will move farther to the left. That’s constructive interference.

But if one wave is vibrating the sand to the left while the other wave is vibrating it to the right, they’ll cancel out each other -- so the sand won’t move at all! That’s called destructive interference. These spots, where the sand stays still, are called nodes. At certain frequencies, these nodes will be permanent because all the waves will keep canceling each other out.

Meanwhile, other spots will have as much constructive interference as possible -- so the sand is moving super fast and much less noticeably. It might look like there’s no wave traveling through the sand at all, but really it’s two waves moving in opposite directions. Different frequencies can make lots of different standing waves — and therefore different patterns. In fact, the higher the frequency, the more cycles of the wave you can fit on one plate, and the more intricate the image becomes.

We’ve been just been talking about flat plates so far, but sound waves actually move in all directions. So, you can actually make these patterns in 3D. It’s pretty tricky to do, because you have to account for the fact that gravity is trying to pull the sand — or whatever else you’re using to create the pattern — to the ground. But at the University of Tokyo, a group of researchers were up for the challenge.

In 2014, they announced that they’d used ultrasound waves, tiny Styrofoam balls, and the right interference pattern to make art float in midair! Just like in the plate, the sound waves had constructive and destructive interference.

At the nodes, this effect was strong enough to overcome gravity and allow the Styrofoam to stay perfectly still. They’d created floating art — and in the process, discovered a new way to make things levitate.

Thanks for watching this episode of SciShow, which was brought to you by our patrons on Patreon. If you want to help support this show, just go to Patreon.com/SciShow. And don’t forget to go to YouTube.com/SciShow and subscribe!