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 between sound waves and sound pressure.
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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 a flammable 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 is oscillating 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 of the 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 simply measuring with a ruler.
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 is proportional 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. This is 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 is reduced at the points with oscillating pressure and thus flames are lower.[1]
John Le Conte discovered in 1858 that flames were sensitive to sound. In 1862 Rudolph Koenig showed that the height of a flame could be affected by transmitting sound in the gas supply, and the change as time passes could be displayed with rotating mirrors. August Kundt, in 1866, demonstrated an acoustical standing wave by placing seeds of lycopodium or cork dust in a tube. When a sound was made in the tube, the material inside lined up in nodes and antinodes in line with the oscillation of the wave, creating a standing wave. Later that century, Behn showed that small flames could be used as sensitive indicators of pressure. Finally, in 1904, using these two important discoveries, Heinrich Rubens, whom this experiment is named after, took a 4-metre-long tube and drilled 200 small holes into it at 2 centimeter intervals, and filled it with a flammable gas. After lighting the gas (whose flames all rose to near-equal heights), he noted that a sound produced at one end of the tube would create a standing wave, equivalent to the wavelength of the sound being made.[2][3][4][5][6] O. Krigar-Menzel assisted Rubens with the theory. A detailed theory of resonances in Rubens' Tube is given by Gardner et al.[7]
A Rubens' Tube was on display at The Exploratory in Bristol, England until it closed in 1999, at which time the exhibit was moved to the @-Bristol museum.[8]
This display is also found in Physics departments at a number of Universities.[9] A number of physics shows also have one, such as: Rino Foundation [10](The Netherlands), Fysikshow Aarhus (Denmark), Fizika Ekspres (Croatia) and ÅA Physics show (Finland).[11][12]
The Mythbusters also included a demonstration on their "Voice Flame Extinguisher" episode in 2007.[13] The Daily Planet's The Greatest Show Ever,[14] ran a competition whereby five Canadian Science Centres competed for the best science centre's experiment/display. Edmonton's Science Centre (Telus World of Science) utilized a Rubens' Tube. In fact, the Rubens' Tube won the competition. The special was filmed on October 10, 2010.
