A situation in which acoustic energy that propagates in one direction is confined in the other two directions as, for example, a mode confined to an interface between two different materials or within the borehole. The Stoneley wave, tube wave and flexural mode have important applications in formation evaluation, while most of the others, such as the Rayleigh wave and the various guided borehole modes (normal mode, leaky mode and hybrid mode), are considered interference that must be filtered out. In y slow formations, leaky modes can help determine formation compressional slowness.