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Introduction

A special wave is formed when two identical waves travelling in opposite directions meet and interfere. The result is a standing (or stationary) wave: a wave in which the crests stay in the same place.

Wind and string musical instruments work because of standing waves.

An oscillating system that is subject to frictional forces will lose energy, and the amplitude of oscillations will decrease; we have damping. The response of an oscillating to an external periodic force depends on the relation between the natural frequency of the system and the frequency of the external force.

When these two frequencies are equal we have resonance, and the system responds by showing oscillations with large amplitude.