The Rubens’ Tube standing wave experiment in action..
According to Wikipedia, “..A length of pipe is perforated along the top and sealed at both ends – one seal is attached to a small speaker, 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. 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..”
Although this was first demonstrated by August Kundt in 1866 with cork dust, it wasn’t until 1905 that Heinrich Rubens introduced the use of flammable gas, resulting in the experiment bearing his name instead. An opportunity missed, I feel..