In high school physics we all learn that sound travels in waves. But you can’t actually see sound waves, so for the most part we take their existence on faith. But there are ways to see the effects of sound waves. One of the most impressive visual demonstrations of sound waves involves a set-up called a Rubens’ Tube, which was invented in 1904 by, well, Rubens, Heinrich Rubens, a German physicist. It’s a long tube with a series of evenly spaced holes drilled along the top. One end of the tube is plugged with a speaker and the other end is sealed. The tube is filled with a flammable gas like propane, and the gas escaping from the holes is lit on fire, creating a line of flames. When a tone is played through the speaker, a standing sound wave can be created in the tube and the height of the flames mirrors the peaks and troughs of the sound wave. It’s easier to see than describe, so here’s a picture:

image:New Zealand Physics Teachers’ Resource Bank

Here’s how it works. Sound waves are pressure waves, which means they are composed of regions of compression (high pressure), and regions of rarefaction (low pressure). So, as a sound wave travels through a Rubens’ Tube it creates localized pressure differences. Regions of high pressure force more gas through the holes, creating higher flames, while regions of low pressure deprive the flames of gas, creating shorter flames.

When a single tone is played through a Rubens’ Tube it creates a standing, or stationary, wave. Sound waves travel outward from the speaker, bounce off the sealed end of the tube and then travel back down the tube in the opposite direction. When two equal waves traveling in opposite directions interact they form a standing wave that doesn’t move at all. So when a standing sound wave is created in a Rubens’ Tube, by definition it just kind of sits there, not doing anything too interesting. But playing music through the tube provides a more exciting way to visualize sound as the sound waves are constantly shifting. Check out the video below to see how a Rubens’ Tube at the University of Portland responds to music from Journey, Beethoven, and The Phantom of the Opera.

Sound seems like such an ephemeral thing, but the Rubens’ Tube demonstrates that music, and all sound, is just bursts of compressed air.

  One Response to “Sound in Flames”

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