Over the past week I’ve been constructing a Rubens’ Tube at work. It’s basically a metal tube filled with flammable gas, with lots of holes drilled in the top, with a drum head on one end. If you put a speaker next to the drum head and light the holes on fire, you can see sound waves.
You can see sound waves on fire.
The other day I did a dry run to see if it would work, and I recorded it on my shitty webcam. The first half shows more science, but about halfway in is where it gets really cool.