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

Any part of the surface that vibrates sends salt flying. After a short moment, the result is that all of the salt collects in places where it won’t go flying; places where the surface doesn’t move.

These places form when multiple sound waves collide, causing regions where they cancel out. This requires that they collide at just the right timing, and so it only happens in certain places and with certain frequencies that depend on the size and shape and speed of sound of the surface.