Probably the coolest way to see sound waves is with fire. In 1905, German physicist Heinrich Rubens invented the Rubens’ tube, or the standing wave flame tube. The Rubens’ tube demonstrates acoustic standing waves through fire.
The Ruben’s tube is a length of pipe that has evenly-spaced holes along the top and is sealed at both ends. One seal is attached to a speaker or frequency generator while the other seal is attached to a propane tank. The tube is filled with gas, and the gas that comes out of the holes is lit on fire. You can now see the sound waves, etched into the air by way of fire.
If a constant frequency is used, then the inside of the tube will have a standing wave, and will create points with oscillating pressure and points with constant pressure. The flames will be lower at the points of oscillating pressure because less gas will escape through the holes in the tube. Naturally, the flames will be higher at the points of constant pressure.
Having a standing wave is cool, but not nearly as much fun as playing music into the Rubens’ tube. Take a look, it’s really quite enjoyable: