Figure 1

Beautiful patterns of sand (Chladni figures) are formed on black thin metal plates as a result of standing waves (Figure 1).

Materials

• Function generator + banana cables
• Mechanical vibrator
• Large aluminum square and circular plates with threaded rod through the center
• Sand

Demonstration

Figure 2

Secure the metal plate to the top of the mechanical vibrator using the threaded rod, and power the mechanical vibrator with a function generator. Standing wave patterns are visualized as the metal plate vibrates at the resonance frequencies of the plates, as shown in Figure 2.

Explanation

Standing wave patterns only occur when waves in the plates are reflected at the boundaries, occurring at characteristic resonant frequencies. The vibrations of metal plates are heard but their amplitude is too small to be seen directly. To make the standing wave patterns visible, sprinkle sand on the plate. The sand will then be jiggled away from the parts of the plates in rapid motion and tend to fall along the nodal lines. Chladni figures are admired by artists and used in violin and guitar construction.

Recommended Frequencies are shown in the image below.

Notes

• Video camera recommended for large lecture classes
• We also have a large aluminum plate which can produce various Chladni figures when excited by a cello bow

Written by Lydia Seymour.