If waves are confined by boundaries, they may interact to establish standing waves. In a standing wave, each wave peak stays in the same place, each trough stays in the same place, and each node (place where there is zero amplitude) stays in the same place. Here standing waves are generated by a point source at the center of a circular enclosure.

Standing waves are possible only for certain frequencies, called resonant frequencies. The resonant frequencies depend on the size of the container that confines the waves. Notice in the video how the waves become less distinct as the frequency is increased and then form a new standing wave that is clearly visible.