A Rubens Tube is a pretty flashy piece of physics demo gear that uses fire to show the acoustic standing wave pattern inside an organ-pipe like tube fitted with a loudspeaker at one end. The 48” long, 2.5” diameter tube is filled with propane, which escapes through a series of 50 0.043” diameter holes spaced by 0.9 inches all along the length of the tube. When sound is played though the speaker at one end of the tube, the flame height is modulated by the acoustic pressure from the speaker. When a single tone (sine wave) at a resonance frequency of the system is played though the speaker, the heights of the flames map out the sinusoidal shape along the length of the tube.

Check out the full blog posts: milwaukeemakerspace.org/2011/02/rubens-flame-tube/