Sound Flames: Rubens Tube
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Height of flames from holes in a tube shows sound nodes and antinodes.
This apparatus, called a Rubens Tube, illustrates that sound in air is related to pressure variations in the air. Relationship between wavelength, frequency, and wave speed. The speed of sound is higher in a lighter gas (methane) than in air.
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Gas is connected to the tube at one end and a speaker is connected at the other. A match is held to the gas jets, lighting them. When the speaker is turned on variations are seen in the height of the flames indicating pressure variations.
The demonstration makes the point that sound involves pressure variations in a very dramatic way. Students can measure the wavelength and use the signal generator frequency to calculate the wave speed from:
A detailed analysis of the experiment is non-trivial. The maximums in the flame height may be associated with pressure nodes (displacement antinodes) or pressure antinodes (displacement nodes) depending on the pressure, flow rate, and sound amplitude. Analysis of this experiment has been the subject of several published papers. See references below.
The tube can get quite hot and the flames can get quite high. Be CAREFUL. Also watch for gas leaks. We recommend teachers use the video rather than constructing an apparatus.
Descriptions of multiple versions of the apparatus are available here.
A good, comprehensive analysis is available in the American Journal of Physics: AJP 51(9), 848.
This teaching resource was developed with support from