The Science House

Hoot Tubes
This demonstration shows how hot air rising through an open tube (stove pipe) vibrates to create a sound.

How does it work?
For a better idea, watch a short video of the burner (QuickTime, 155 KB).
The hot, vibrating air rises through tube setting up a convection current. A convection current is simply the movement of air caused by different densities. Hot air is less dense than cold air and will move upward while the cool air sinks.
When the vibrating air moves through the tube, it begins to resonate or vibrate at the natural frequency of the tube. Since our tube is open on both ends, the wave that resonates will have a wavelength 1/2 the length of the tube.
If you double the length of the tube, what happens to the resonant wavelength? How does this affect the pitch we hear? Find out by watching this video of a longer hoot tube (QuickTime, 415 KB).
Pipe organs, foghorns, the recorder and other musical instruments use this same concept to produce sounds, only the air is pushed through the pipe by other means than a hot burner.