Purpose:Get students to think about how a resonant cavity works by tricking them.
Description:Two identical 440 Hz tuning bars with resonant cavities are presented. When the bars are struck with a mallet, one rings for a long time like normal, but the other one does not, damping very quickly. The instructor turns the non-resonant cavity over, pushes the "resonance" button, and strikes it again. This time it works! Let the students discuss this mysterious behavior. EXPLANATION: When the tuning bar is turned over to push the "resonance" button all of the sulfur hexafluoride is poured out!