Longitudinal standing waves in rods

Demonstration

With this experiment you can show standing waves, either transverse or longitudinal, in a rod or rods.

Apparatus and Materials

• Glass, steel, or brass rod, about 10 mm diameter and about 1.5 m long
• G-clamp
• Wooden blocks
• Cloth
• Rosin (for metals)
• Alcohol (for glass)

Health & Safety and Technical Notes

Read our standard health & safety guidance

Procedure

1. Hold the rod fairly firmly with rosined fingers, or a dampened cloth, and stroke it. What is the wavelength of the standing wave being produced?
2. If you can find a way of estimating the frequency of the sound emitted, then go on to estimate the speed of sound in the rod.

Teaching Notes

• In its fundamental mode of oscillation, the rod will have a node at the fixed end and an antinode at its free end. Thus the wavelength will be twice the length of the rod. The standing wave is audible as a sound of pure frequency.
• By displacing the free end slightly and then releasing it, you might also show an equivalent transverse wave, which is visible.
• Alternatively, the experiment could be used as one station in a circus of class experiments.
• A more dramatic version of this demonstration, often called the ‘singing rod’, uses an aluminium rod as much as 2 m in length. See, for example, this video:

  Watch video

This experiment has yet to undergo a health and safety check.