The heights 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.
NCEA & Science Curriculum
Download (right-click and “save as”): Wavesonfire.m4v
This is a nice compilation of the four videos below by Denis Burchill, Senior Lecturer in Science and Technology, University of Auckland Faculty of Education. The 768 Hz sound was added as the original video had background noise.
Download (right-click and “save as”):
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.
This is a potentially dangerous experiment. 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.
Individual teachers are responsible for safety in their own classes. Even familiar demonstrations should be practised and safety-checked by individual teachers before they are used in a classroom.
Would you like to contribute lesson suggestions? Contact us.
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 demonstration was conducted and filmed at Hutt International Boys School (HIBS) in collaboration with Mr. Gerald McIlhone.
The “Waves on Fire” video was developed by Denis Burchill, University of Auckland Faculty of Education.
This teaching resource was developed with support from