Longitudinal standing waves in rods
Practical Activity for 14-16
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
- Wooden blocks
- Rosin (for metals)
- Alcohol (for glass)
Health & Safety and Technical Notes
- 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?
- 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.
- 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:
This experiment has yet to undergo a health and safety check.