Rubens’ tube
A tube filled with flammable gas, perforated along its length, with a speaker at one end. Sound waves create pressure variations inside the tube, and flames at each hole rise proportionally to the local gas pressure — making standing waves visible in fire. Adjust the frequency to explore different harmonic modes.
P(x,t) = P₀ + A·sin(nπx/L)·cos(2πft) fₙ = n·v / 2L
How it works
A Rubens' tube is a hollow pipe sealed at both ends, filled with a flammable gas (typically propane), with small holes drilled evenly along the top. Gas leaks out of each hole and is ignited, creating a row of small flames. A loudspeaker is coupled to one end of the tube. When the speaker produces sound, it creates longitudinal pressure waves inside the gas. At locations of high pressure, more gas is pushed out through the holes, creating taller flames. At pressure nodes, the flames are shorter.
The result is a direct, physical visualization of the standing wave pattern inside the tube. Unlike a string on which you see transverse displacement, here you are seeing pressure amplitude — a longitudinal quantity — rendered as flame height.
Physics
For a tube of length L with one closed end (speaker) and one effectively closed end, the resonant frequencies form a harmonic series: fₙ = nv/2L, where v is the speed of sound in the gas (~260 m/s for propane) and n is the mode number. At resonance, the standing wave pattern has n pressure antinodes and n+1 pressure nodes (including the ends).
The pressure distribution is P(x,t) = A·sin(nπx/L)·cos(2πft). The flame height at each hole is proportional to |P(x)| averaged over a short time, since gas flow responds to the time-averaged excess pressure. This is why you see the absolute value of the standing wave envelope in the flame pattern.
Historical significance
The tube was invented in 1905 by German physicist Heinrich Rubens. It provided one of the first direct visual demonstrations of standing sound waves. Before electronic oscilloscopes and modern measurement tools, the Rubens' tube gave physicists and students an intuitive way to "see" sound. It remains a popular demonstration in physics classrooms worldwide — a beautiful example of making the invisible visible.