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I watch this video some time ago and I think it's Rice, not salt, any way it's amazing to see the "forms" of sound waves.

Greetings

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If it was visualizing sound, then we would see a different shape for every frequency. We are actually seeing that metal plates reaction to its resonate frequency. The main reason I believe this: you can only see shapes at certain tones.

The plates are always flexing up and down according to the sound, but it is not strong enough to move the rice. Once it hits a resonate frequency, it starts to flex in a predictable manner, causing the salt to congregate at the non-flexing sections.

This could actually turn into a really neat project. Different shapes will cause a different reaction. You could set up 5 different plates with 5 different shapes. Each one will have a different resonate frequency, and the shapes can change dynamically.

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This reminds me vaguely of the different modes of a rectangular waveguide.

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We always use beach sand for the mode shape visualization. For those that are interested, "vibration of plates" by arthur leissa has some photos of the same setup on many different shapes of plates. Once you identify the shape in an experiment, and come up with a mathematical description of it, you can tabulate the parameters for the natural frequency of that plate.

Unfortunately, sometimes sound waves have difficulties exciting some of the modes of the plate and you miss them. In addition, if the plate isn't laying flat, the sand has a tendency to vibrate off the plate.

If you use high tech materials, like graphite epoxy composites, the mode shapes get very interesting like the one in the video. Standard metals tend to be boring :-)

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