Tube of rubens

A Rubens' Tube setup
The Rubens' tube, also known as the Standing wave flame tube, or simply flame tube, is a physics experiment demonstrating a standing wave. It shows the relationship betweensound waves and air pressure.

Overview
A length of pipe is perforated along the top and sealed at both ends - one seal is attached to a small speaker or frequency generator, the other to a supply of aflammable gas (propane tank). The pipe is filled with the gas, and the gas leaking from the perforations is lit. If a suitable constant frequency is used, a standing wave can form within the tube.When the speaker is turned on, the standing wave will create points with oscillating (higher and lower) pressure and points with constant pressure (pressure nodes) along the tube. Where there isoscillating pressure due to the sound waves, less gas will escape from the perforations in the tube, and the flames will be lower at those points. At the pressure nodes, the flames are higher. At the end ofthe tube gas molecule velocity is zero and oscillating pressure is maximal, thus low flames are observed. It is possible to determine the wavelength from the flame minima and maxima by simplymeasuring with a ruler.
[edit] Explanation
Since the time averaged pressure is equal at all points of the tube, it is not straightforward to explain the different flame heights. The flame height isproportional to the gas flow as shown in the figure. Based on Bernoulli's principle, the gas flow is proportional to the square root of the pressure difference between the inside and outside of the tube. Thisis shown in the figure for a tube without standing sound wave. Based on this argument, the flame height depends non-linearly on the local, time-dependent pressure. The time average of the flow isreduced at the points with oscillating pressure and thus flames are lower.[1]


History
John Le Conte discovered in 1858 that flames were sensitive to sound. In 1862 Rudolph Koenig showed that...