acoustics lesson - sound waves and salt

If you've never seen this before, you should check out how salt reacts to sound waves. The sound produces a standing wave in the metallic plate, causing the salt to form a pattern. As the pitch changes, the standing wave does too, when the pitch matches one of the plate's natural resonances. Notice how the patterns change more rapidly as the experiment goes on. This has to do with what's called the harmonic series -- the plate's (and any object's) natural resonance points get closer together the higher the pitch.