I watched the sand pattern video. These questions come to mind:
What are the dimensions of the platform that the sand vibrates on?
What is the velocity of sound for the material used in the platform?
Why a square shape? Why not triangle, hexagon, octagon, or circle?
The wavelength of the notes is affected by the frequency (obviously) and also the speed of sound in the medium. The length of the wavelength will affect where the nodes are (where the sand concentrates).
Regarding resonance, everybody's ears are unique. Therefore, each person ought to have a unique frequency that "feels" right.
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