Standing Wave

When a wave propagates in one direction, and another wave propagates in the opposite direction (a reflected wave with the same amplitude and the same frequency), it is possible that the superposition of the two waves together forms a "not-moving" wave motion. This is called a standing wave. Let's take a look at this phenomenon in the following video clip.

In this video, both ends are not fixed (i.e., free). Such standing waves can be illustrated as follows:

In the next video clip, one end is free and the other end is fixed. These standing waves can be illustrated as follows:

In these examples, we can observe two types of points: one is a point with no displacement, and the other is a point with maximal displacement. The point with no displacement is called a node, and the point with maximal displacement is called an anti-node.

Free end reflection

Fixed end reflection

Fletcher, N. N. H. and Rossing, T. D., The Physics of Musical Instruments, Springer-Verlag, 1998.