Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

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

Diagram of secure way of clamping rod

Procedure


a 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?

b 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


1 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.

2 Alternatively, the experiment could be used as one station in a circus of class experiments.

3 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 https://www.youtube.com/watch?v=_bfT4kUFbIQ

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

 

Relating experiment


Standing waves on a rubber cord

Standing waves with a variable wavelength

Stationary waves in an air column

Vibrations of circular wire rings

Longitudinal standing waves

Vibrations in a rubber sheet

Vibrations on a loudspeaker cone

Ring of standing waves

Musical instruments