http://gizmodo.com/watch-audio-frequencies-create-amazing-visual-patterns-511804949 Yep, those are the different vibrational modes of that plate when constrained in the center by the screw. If memory serves - make the plate thicker or thinner (or change the material), and the frequencies at which the modes occur will shift. Constrain it differently (e.g. put screws in the four corners), and the modes will change, causing the salt lines to look different. The mode shown in the image below would (if I'm looking at it correctly) have an "X" pattern. Yes, yes, I know..."I see breasts!" :D