Demos: 4B-13 Hoot Tubes (Resonance in Pipes)

Three glass tubes, having different lengths, are attached to a stand. A disk-shaped wire screen is attached to a handle so that the disk can be heated in the flame of a burner. When the disk is inserted into the glass tube and moved to a point about a quarter of the way up the tube, a loud resonating sound develops.

Vortices produced when the hot air rises off the grid surface creates instabilities in the air stream, resulting in low-amplitude noise. The resonant frequency of the particular tube determines which components of this noise are amplified.

Directions: CAUTION: KEEP THE BURNER AWAY FROM THE VICINITY OF THE TUBES. OTHERWISE YOU MIGHT FORGET IT IS THERE AND BURN YOURSELF WHILE HOLDING THE GRIDS IN PLACE IN THE TUBE. (I burned a hole in the sleeve of a new sport coat that way!!!) Hold the grid in the flame of the burner until it is glowing red hot. Then quickly push it up into one of the tubes until it is about one quarter of the way up. (The illustration at the right shows only one tube.) As the sound intensifies, make minor adjustments in the position to achieve maximum amplitude. Try holding two grids in the flame and then pushing them up two different tubes. Very interesting dual tones ensue!

Additional demo: Remove one of the tubes from its clamp and repeat the demo. While keeping the grid in place, slowly rotate the tube to a horizontal position. The sound will stop.

Suggestions for Presentation: Make the case that if a sound source is placed in the right position in a tube, resonance will occur if the sound is the proper frequency. But what about something that supposedly doesn’t make any sound, like a heated grid, for instance?

Note: The details of the mechanism producing the sound here are quite complicated. Just tell the students that the turbulence set up in the hot air passing through the grid generates a disturbance that can be thought of as “noise.”

Applications: Resonances in pipes.

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Last Updated: May 9, 2016 11:44 AM