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 air pressure.
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. When the speaker is turned on, the pressure changes caused by the sound waves will cause the flames to heighten in some areas and to lower in others.
If a constant frequency is used, it is possible to determine the wavelength by simply measuring with a ruler.