I had not heard of it either, until I ran across an Instructible describing how to build one. A Rubens’ tube (named for Heinrich Rubens) is a device that demonstrates acoustic standing waves in a pipe with fire. It consists of a long tube that has many small holes drilled along its length. One end is connected to a source of flammable gas, such as a propane tank, and the other end is connected to a speaker. In the Instructible the speaker connection consists of some PVC bushings and a big wad of masking tape.
Inside the tube, certain audio frequencies will set up a standing wave pattern inside the tube. The frequencies that produce a pattern depend on the length of the tube. The standing wave has areas of high pressure (anti-node) and low pressure (node). The change in pressure results in more or less gas leaving from the small holes in the tube, and therefore higher or lower flames that indicate the internal pressure.
The first Reubens’ tube was built in 1904, but this version brings the device into the 21st century by incorporating a WiiMote, which is used to change the sound frequency and switch the sound between the left and right channels, which are each connected to their own tube.
In my first class on electromagnetics I remember using a slotted waveguide to measure the wavelength and SWR of electromagnetic waves. It was interesting, but a stereo Rubens’ tube would have been much more so!