Sound propagates like waves. These sound waves overlap when they collide and interfere with each other. Thus, the waves amplify or cancel each other out. If two waves, which run in opposite directions in space, overlap in such a way that the resulting wave stands still (nodes) in some points and oscillates (antinodes) in others, it is called a standing wave.
Such a standing sound wave is generated in the Rubens’ tube. It consists of a tube filled with flammable gas and small holes at the top where the gas can escape and be lighted. A sound source is attached to one end and a membrane to the other. A sound with a frequency whose wavelength is a multiple of the length of the tube is played. The wave runs through the tube, is reflected by the membrane and interferes with itself to form a standing wave. Since sound corresponds to a pressure wave, there is a constant pressure at the nodes and an oscillating pressure at the antinodes. This causes more gas to escape at the nodes and a wave-like course of the flames can be observed.