u/Correct-Apartment618 asked r/Cymatic for an affordable device, and for something that could identify the frequency of a sound. The replies are the most practical short answer to that question we have found in public.
The substance: for sand patterns you do not need a dedicated instrument, a single speaker will do; a phone app will identify frequencies, though the poster reports mixed opinions on apps versus hardware; a balloon stretched over an empty cylinder with sand on top and a tube underneath lets you use your own voice; and a metal plate can be driven by a speaker, a bowed edge or a DC motor — anything that vibrates.
That last point is the one worth taking away. The plate, its material and how it is clamped decide the patterns. The driver is almost incidental until you want repeatability.