An investigation of Rubens flame tube resonances

The Rubens flame tube is a century-old teaching demonstration that allows observers to visualize acoustic standing wave behavior [H. Rubens and O. Krigar-Menzel, (1905). Ann. Phys. 17, 149-164]. Flammable gas inside the tube flows through holes drilled along the top, and flames are then lit above. The tube is closed at one end and driven with a loudspeaker at the other end. When the tube is driven at one of its resonance frequencies, flames form a visual standing wave pattern as they vary in height according to the pressure amplitude in the tube. Although the basic performance of the tube has been explained [G. Ficken and C. Stephenson, (1979). Phys. Teach. 17, 306-310], this paper discusses a previously unreported characteristic of the tube: a shift of the tube's resonance frequencies away from those predicted by simple introductory physics. Results from an equivalent circuit model of the tube and agreement between experiments and the model suggest that the shift is caused by the presence of the holes. For teachers and educators seeking to better understand and explain the tube to students, this article serves as a resource regarding the basic phenomena affecting the behavior of the tube.