My question is based around this video:
The video shows sand settling in vibrational nodes of a plate at certain resonance frequencies. It appears that all the patterns are radially symmetric however at 5:15 I see a pattern that is bilaterally symmetric.
My question is why this pattern occurs horizontally instead of vertically? For a rectangular plate I understand that the two situations would occur at different frequencies, but what about a square plate? Is this a random occurrence?