In his book "Discoveries on the Theory of Sound", published in 1787, Ernst Florens Friedrich Chladni describes the following experiment to visualize sound modes. Sand grains are distributed over a metal plate, which is then stimulated using a violin bow. Given the correct frequency, the sand grains form so called Chaldni figures. They emerge in the time-independent zero set of a standing wave resulting as solution to the oscillation equation.

This principal can also be applied to 3D: In the article, published at the Bridges conference (see here) all pictures are generated with computer experiments, still, a physical experiment could be performed. It would have to consist of light particles moving in a viscose fluid that is stimulated e.g. by speakers. Further information can be found in the paper, see link above.

One image from the paper has now been featured as cover-image of the German Mathematical Society and their Facebook page. See the page here.