Have you seen... [wave frequency and harmonics]

Some of the best science demos reveal something to the viewer -- making the previously unseen become visible.

In this case, a Ruben's tube is used to visualize how sound (mechanical waves) influences air pressure in a tube.  Standing waves are the easiest to visualize, but the musical dynamics and complexity are interesting too.
Plus it uses fire and rick & roll, which always makes such projects more exciting!

By the way, after the Dave Brubeck jazz, did you see the flame gap with the rock music?  I wonder if that has anything to do with how it is equalized when mastered...

Mechanical waves can create energy patterns in solids as well.  This is a clever use of salt on a vibrating table to show more complex harmonics.  The shape and composition of the table are also important here -- this table is "clean" enough to get distinct constructive and destructive effects that result in salt "mounds" and clear spaces: