Stationary waves in an air column

Demonstration

This experiment, using what is often referred to as Kundt's tube, demonstrates vividly how sound waves make air vibrate in a tube.

Apparatus and Materials

• Loudspeaker
• Signal generator
• Polystyrene beads, small
• Filter funnel
• Tube, transparent plastic or glass (1.0m to 1.5m long)
• Rubber bung, or similar, to seal one end of tube

Health & Safety and Technical Notes

Read our standard health & safety guidance

Place a thin layer of polystyrene beads along the length of the tube. Alternatively, you could use lycopodium powder or cork dust. These materials are best inserted by first sprinkling them along a metre rule, placing the rule inside the tube and then tipping them off.

Tape the loudspeaker in place. If the loudspeaker and tube have different diameters, join them with a paper cone or plastic cup.

If you don't have a long tube, use a large measuring cylinder instead.

A video showing how to use a signal generator is available at the National STEM Centre eLibrary:

video

Photographs courtesy of Mike Vetterlein

Procedure

1. Switch on the signal generator. Vary the frequency of the sound coming out of the loudspeaker, through the range 1 kHz to 10 kHz, keeping the amplitude constant. Ask students to observe what effect this has on the vibrations of the beads.
2. Vary the amplitude of the sound coming out of the loudspeaker, keeping the frequency constant. Again, ask students to observe what effect this has on the vibrations of the beads.

Teaching Notes

• At some particular frequencies, a standing wave pattern will be formed, with the beads settling into heaps (piles) at certain positions along the tube. At the simplest level, the experiment demonstrates that sounds from the loudspeaker produce vibrations in air.
• At more advanced level, you could measure the average distance between adjacent nodes (where the beads settle). This will be half a wavelength.
• Using wave speed = frequency x wavelength you could go on to make an estimate of the speed of sound in air at the temperature of the room.
• You could also explore the relationship between frequency and wavelength, for frequencies that produce a standing wave pattern (they should be inversely proportional). Or you could compare the speed of sound in open air with the speed of sound in a tube, then ask whether there an appreciable difference.
• A version of this experiment, using polystyrene beads, was shown by Pascal Daman on the Luxemburg stand at Physics on Stage 3 in November 2003.

This experiment was safety-tested in February 2006