A classic physics experiment called "Rubens' Tube" involves sound, a tube of propane, and fire. 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. The pipe is filled with the gas, and the gas leaking from the perforations is lit. If a constant frequency is used, a standing wave can form within the tube. When the speaker is turned on, the standing wave will create higher and lower pressure points along the tube. Where there is higher pressure due to the sound waves, more gas will escape from the perforations in the tube, and the flames will be higher at those points. In these cases, it is possible to determine the wavelength by simply measuring with a ruler.