Imagine a string of pearls. You can start a wave by wiggling the first pearl or the last; the waves can travel either way because each pearl is coupled equally to both neighbors. But researchers have lately become interested in “unidirectional” coupling, in which the force between neighbors only allows waves to move in one direction. This can be seen as an extreme example of anisotropic media, in which the wave speed depends on the direction.
That is from this Physical Review Focus story; it goes on to indicate the areas where this might be of relevance:
… waves in so-called anisotropic media–which are important in geology, optics, and materials science.
The news report itself is about the building of a
first mechanical medium where waves go only one-way.
There is also a nice video and an applet to go with the report; have fun!