Equipment:

• hoot tubes (labeled singing pipes) - shelf 8E
• propane torch and lighter - shelf 6E

Theory: The tube has a fundamental frequency dependent on its length. Heating the mesh at the base of the tube establishes a convective flow of air up the tube, exciting the fundamental.

Description: The end of the tube with the stainless steel mesh is held over the burner for about 15 seconds. It should glow orange. When the tube is removed from the burner and held vertically (mesh end down) it will emit a loud hoot. If the tube is then turned horizontally, the noise will stop, returning again when the tube is put back in a vertical position. If you like, you can create the illusion of pouring the sound out of the tube into a container, and then pouring the sound back into the container again.

There are two tubes of slightly different lengths. When sounding simultaneously, beats can be heard.

Setup time: 5 minutes

References: (available in the demo room)

• Sutton, "Demonstration Experiments in Physics", S-62, p. 159.
• Freier and Anderson, "A demonstration handbook for physics", Se-5.