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

If you stretch a string tight and pluck it, you see it moving up and down and make a noise. Depending on frequency, and string length, the string might vibrate in two places. The middle will looks like it is still. Translate that into two dimensions. The flat surface will have internal reflections from the edges, depending on the frequency of the vibrations. These reflections cause constructive and destructive interference, like dropping two rocks in a. pond. Some places the ripples will be higher, other places they will be flat.