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.
Direction, position, or indication sign sign category into direction signs, which are only those giving distances or directions to a given location; road identification signs (also known as "reassurance signs"), which repeat the name or number of the road, and place identification signs, which give the name of a landmark, such as a town, river or border. Unlike the other classes of sign, direction signs remain broadly undefined by the convention; the only restrictions given are that direction signs must be either a rectangle or an arrow shaped pentagon, and that they may not contain placenames in more than two languages. Additionally, direction signs on