Steve Mould is a British science presenter. In this 2016 video he plays a square metal plate with a violin bow, the classic method Ernst Chladni used in the 1780s, and uses couscous rather than sand so the grains are large enough to see jumping.
The explanation is the reason to watch. Mould frames the figures as two-dimensional standing waves, the same physics as a vibrating string but on a surface, and makes the point that the grains are not attracted to the nodal lines so much as thrown off everywhere else. That distinction matters when you start using liquids or finer powders, which behave differently.
The bow method is worth trying if you have only ever used a speaker: the bow picks out modes by where you touch the edge, which is a different kind of control from a frequency sweep.