The Rayleigh Disk

Author: N.H. Crowhurst

The principal difficulty in measuring the intensity arises due to the very small amount of energy in a sound wave. One method, discovered by Lord Rayleigh, uses the Rayleigh disk. It is still used today.

The action of the Rayleigh disk

When a small disk is suspended in air, any air movement striking it tends to set the disk "head on" to stop the air. This happens whichever side the movement comes from. In a sound wave, the movement comes alternately from opposite sides, as air particles move back and forth. Both movements tend to turn the disk the same way, hence the torsion on the disk will depend on the amount of air particle movement, not its direction or frequency.

To use this delicate instrument, the disk is carefully hung at a definite angle to the approach of the sound wave to be measured, and shielded from any other air currents. Then the thread by which it is suspended is twisted by a calibrated instrument, to see how much torsion is needed to maintain the same angular position against the force caused by the wave. This information can then be used to calculate the air particle movement in the wave by measuring the torsion on the disk when it is held in a steady stream of air that is moving at carefully controlled velocity.