If you want to visualize sound waves, you reach for your oscilloscope, right? That wasn’t an option in 1905 so physicist [Heinrich Rubens] came up with another way involving flames. [Luke Guigliano] and [Will Peterson] built one of these tubes — known as a Rubens’ tube — and will show you how you can, too. You can see a video of their results, below. Just in case a flame oscilloscope isn’t enough to attract your interest, they are driving the thing with a theremin for extra nerd points.
The guys show a short flame run and one with tall flames. The results are surprising, especially with the short flames. Of course, the time base is the length of the tube, so that limits your measurements. The tube has many gas jets along the length and with a sound source, the height of the flames correspond to the air pressure from the sound inside the tube.
According to their plans, the tube is a 2 inch tube, six feet long. They used a #42 drill bit to create the gas jet holes an inch apart although they mention if they did it again they’d go smaller and space them closer. The working gas is propane and if you want to exactly duplicate their build, you’ll need to weld. They mention, though, that you could probably build it without welding. Total cost? About $350.
You can extend the idea of a Rubens’ tube to a square — we hate to call it a Rubens’ cube. Or you can shrink it down to a single point. Either way, it is fire, so you want to be careful, but there is a certain appeal to it, too. It always amazes us how resourceful people can be when they have to be. The invention of the Rubens’ tube is an example of that, although there were many other ways people made up for not having oscilloscopes.