why salt/sand produces patterns when put on a vibrating surface,

Question

We all know the experiment right? putting fine grains on a flat surface and then pass a frequency through that surface. But why does it produce complex patterns? and why particular patterns at certain frequencies?

why do 2 dimensional sound waves manifest themselves as these complex shapes and patterns?

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EDIT:spelling

In: 2

Answer 1

Because those specific frequencies produce things called “standing waves” where some parts of the surface are vibrating quickly, while at other parts, the vibrations cancel out completely. So the sand/salt gets bumped around, until it reaches one of those cancelation points (called nodes), where it stops.

And the specific orientation of nodes ends up being a pretty complex thing, where the material, thickness, and other properties all matter.