Sound amplification by stimulated emission of radiation mechanism and an active medium. The pumping procedure can be induced for example by an alternating electric field or with some mechanical vibrations of resonators, followed by acoustic amplification in the active medium. The fact that a SASER operates on principles remarkably similar to a laser, can lead to an easier way of understanding the relevant operation circumstances. Instead of a feedback-built potent wave of electromagnetic radiation, a SASER delivers a potent sound wave. Some methods for sound amplification of GHz-THz have been proposed so far. Some have been explored only theoretically and others have been explored in non-coherent experiments.
Refraction wave vector depend on the wave speed this requires a change in direction of the wave vector. The relevant wave speed in the discussion above is the phase velocity of the wave. This is typically close to the group velocity which can be seen as the truer speed of a wave, but when they differ it is important to use the phase velocity in all calculations relating to refraction. A wave traveling perpendicular to a boundary, i.e. having its wavefronts parallel to the boundary, will not change direction even if the speed of the wave changes. Refraction of light can