How the Rubens' Tube turns music into fire

The Rubens' Tube is a lit gas jet attached via a tube to a stereo speaker. When the speaker kicks into gear, the changes in pressure along the tube make the the flames spring up or die down in waves as the music plays. You may have seen a version of this if you've attended the concerts of bands who aren't concerned with human life. Here's how it works.

The Rubens' Tube is named after an invention of Heinrich Rubens, a rock n' rolling German who built something close to the modern tube at the beginning of the last century. A tube is perforated with a row of evenly-spaced, evenly-sized holes at the top, and covered with a flexible membrane at both ends. Gas is pumped into the tube, and speakers are put at either end. When the gas is lit (along the holes at the top) there should be an even row of little jets of flame. Their height depends on the size of the holes and the pressure of the gas from within the tube. When the speakers start pouring out sound, the flames should arrange themselves into standing waves.

This is one of the less-musical demonstrations of the tube, which lets you see the waves very well. I would recommend turning off your computer speaker, though.

These waves behave in a strange way. When the sound is quiet, the parts of the wave at which the pressure varies the most, the antinodes, have jets of flame that are shorter than the jets at the nodes, the points were the pressure doesn't vary. When things get loud, the antinodes shoot up higher than the unvarying nodes. What's going on?

There are equations that reflect the flow of gas through the holes at the top. These say that the flow is proportional to the square root of the pressure difference between the gas inside the hole and the air outside. When the boost from the gas is low the square root of that difference that pressure varies at the antinodes in a peculiar way. The burst of speed that the gas at the antinodes get from the high pressure moments is less than the sudden loss of speed it gets when the pressure dips. This averages out to less than the unvarying pressure at the nodes. So in comparison to the unvarying parts of the tube, the jets of fire at the antinodes are low.

Eventually, when the sound goes up, the pressure gets so high at the antinodes that all the gas is pushed out of the tube, making the jet of flame spurt high. The gas can't be sucked back in again by the next drop in pressure, because it's already well clear of the vacuum by the time the lack of pressure hits. So for a time the tube just sucks in oxygen and any unspent gas until the next wave of high pressure gas, and the jet of fire is high. Eventually, though, the amount of fuel pushed out can't be re-supplied by the next burst of gas from inside the tubes and the antinodes sputter out. The Rubens' Tube isn't just a good demonstration of a standing wave, but of how different physical conditions can produce very different results with just one variable.

Top Image: Pihulic

Via The University of Utah.