In electromagnetics a cavity resonator is a resonator composed of a space that is usually surrounded by a dielectric or metal which allows standing wave patterns at particular frequencies, eigenmodes, correlated to their size, shape, and material.
Most musical instruments have resonators. They are the parts which makes the sound louder. A vibraphone, for example, has long tubes underneath the keys. It also has a special effect: a mechanism which, when it is turned on, opens and shuts the resonators very quickly so that a trembling sound is heard.