The Flame Tube, or Rubens Tube, is a classic device for showing the wave structure of sound. It was named for German physicist Heinrich Rubens, who developed it around the turn of the last century.
You can see our own flame tube demonstration in action in this video starring Prof. Norbert Linke
The device is, in essence, and oscilloscope without the electronics – it transforms a sound wave into a visible trace by allowing the changes in pressure in the gas in the tube to drive flames to different heights along the top. In this way, you can see that sound does form a wave, and can show a standing wave and measure its wavelength.
To learn more about the Rubens’ Tube, explore these links: