For her 5th grade science fair project, my daughter Samantha decided to do her report on "Visualizing Sound Waves." To do this, one of the things we built was a Rubens' Tube. A Rubens' Tube is a long tube, sealed at both ends, with closely-spaced holes drilled in a line down one side. One end of the tube is sealed with a sheet of latex rubber, and a speaker attached to the outside, so that sound sent through the speaker will enter the tube. Here's a picture of what ours looks like:
The tube is filled with a flammable gas (propane), and the gas escaping from the holes is lit. Then, by playing sounds of different frequencies through the tube, standing waves can be created. The standing waves will create areas of higher and lower pressure within the tube, and these areas will be visible outside the tube as flames of different heights. You can easily determine the wavelength of the source sounds (or the standing wave) by measuring with a ruler:
Our Rubens' Tube was built in a manner similar to the one described here by Nik Vaughn, with some modifications and parts substitutions. Here is the complete parts list. Our tube is 60 inches long with 101 1/16" holes drilled at 1/2" intervals, and five inches of "buffer" on each end. Gas is fed into tube via two inlets at the rear of the tube (spaced a third of the way in from either end) from a 20 lb. propane tank (the kind you use with a barbecue grill). Sound was delivered from an iPhone to a pair of computer speakers; one speaker was at the end of the tube, and the other was for the audience. We found that converting the MP3s to monaural (as opposed to stereo) helped, since only one channel was feeding the tube.
Samantha worked very hard to learn all the physics behind how the Rubens' Tube works, and she practiced her presentation several times. All her hard work paid off; she tied for First Place - 5th Grade at the school science fair.
The videos below were included on the DVD that we made as part of Samantha's project. Note that the music chosen here was selected because it created lots of standing waves within the tube. You can run other stuff through the tube as well, and make the flames "dance" and otherwise go crazy (Styx' Come Sail Away and Dire Traits' Sultans of Swing are good examples), but that was not the point of the project. With the exception of the signal generator video, I have remixed the stereo MP3s back onto the videos; this sounds much better than the monaural audio played through a crappy computer speaker and recorded by the camcorder microphone.