Ok, I'll take a whack at it.
Vibrations are introduced into the test area, in this case a thin flat sheet.
The frequency of the vibrations can be varied.
As the vibrations hit the edges of the sheet they "reflect" back into it. When they meet with vibrations coming from a different direction, "interference patterns" are created.
Those are the patterns you see. Their shape can be altered by flexing the sheet or changing the frequency or both. The salt tends to gather in the "dead zones", the clear areas are still vibrating.
"Phase shifting" uses this principle, of frequencies cancelling each other out when they're perfectly out of phase, or of modifying the waveform when they're not perfectly in phase..
You can also see it in ultrasonic cleaning tanks.
In fact you can even see it in coffee in a styrofoam cup if you slide it across a desktop at just the right speed to make the styrofoam squeak.
NOW, anybody want to learn how to make a light show?
(Hint: Shine your Heathkit home laser into the ultrasound cleaner tank and reflect that through the overhead projector) :lol: