Hipper
Addicted to Fun and Learning
The sandbox will have to be "reset" periodically if it actually works. Sand particles will "settle" and compact after prolonged vibration/energy input.
Depends on the sand. I used to work as a groundsman (looking after sportsfields) and the best sand for winter games pitches should have rounded particles more or less the same size. If you imagine a box of table tennis balls there is only so much compaction that can occur - there will always be spaces between the particles allowing better drainage. Compare that to 'sharp' sand or builders sand which has a mix of all shapes and sizes where the smaller particles can fill the gaps between the bigger ones - more possibility of settling.
I think I'm going to introduce some 'audiophile sand' very similar to sports field sand. Of course I will have to describe how it 'drains vibrations' and discuss the quantum nature of the silicon crystals blah blah blah. I'll try and get 'Zen' in there somewhere just to please gene_stl.