Ruben's Tube

Who knew sound waves could be so combustible?

The "Ruben's Tube" demonstration shown in the video gives a dramatic and visual representation of sound waves creating standing waves in a pipe. Whether or not you light the gas, once the speaker is on, there are sound waves traveling back and forth along the tube. Sound waves consist of alternating regions of high and low pressure. But by igniting the gas in the tube and allowing it to escape out of the holes cut into the top, we can see where the pressure is high and where it is low.

If you watch the video carefully you can see that at specific sound frequencies we get a static-looking wave pattern. For example, at a certain frequency/pitch we get a pattern of four wave peaks. However, watch what happens when the frequency increases. The pattern disappears for a while until the frequency rises to a (specific) higher value. Then we see five standing wave peaks spring up.

So why do these patterns form? It has to do with constructive and destructive interference of the sound wave. What's happening is that sound is continuously traveling back and forth along the tube, due to the fact that the waves reflect off of each end. The sound is continuously being produced by the speaker, so as sound traveling in one direction along the tube encounters sound traveling in the opposite direction, the waves interfere with each other. In places where two high-pressure regions overlap we get constructive interference and the wave amplitude increases. Where high pressure encounters low pressure, we get destructive interference, and the wave amplitude can drop to zero.

Now at any general frequency, there is no set pattern, as the locations of constructive and destructive interference continuously change. However, if you generate sound at the right frequencies for the length of tube that you have, you can generate standing waves, where the locations for constructive and destructive interference always occur at the same place, forming a standing pattern. The distance between the standing wave peaks is the wavelength of the sound at that frequency. By the way, musical instruments are constructed to produce standing waves at the frequency of each note.

Try making your own standing waves at home with a rope. Have your friend hold one end while you shake the other up and down. You'll find you can generate one big standing wave if you shake your rope at just the right rate, but if you double the rate you get a double standing wave, triple it and you get three, etc. It's a pleasant way to spend an afternoon!

Adam Weiner is the author of Don't Try This at Home! The Physics of Hollywood Movies.