Sound amplification by stimulated emission of radiation phenomenon, known as the Larsen effect, is the result of a positive feedback. In general, every oscillator consists of three main parts. These are the power source or pump, the amplifier and the positive feedback leading to the output. The corresponding parts in a SASER device are the excitation or pumping mechanism, the active (amplifying) medium, and the feedback leading to acoustic radiation. Pumping can be performed, for instance, with an alternating electric field or with some mechanical vibrations of resonators. The active medium should be a material in which sound amplification can be induced. An example of a feedback
Seismic refraction Seismic refraction Seismic refraction is a geophysical principle (see refraction) governed by Snell's Law. Used in the fields of engineering geology, geotechnical engineering and exploration geophysics, seismic refraction traverses (seismic lines) are performed using a seismograph(s) and/or geophone(s), in an array and an energy source. The seismic refraction method utilizes the refraction of seismic waves on geologic layers and rock/soil units in order to characterize the subsurface geologic conditions and geologic structure. The methods depend on the fact that seismic waves have differing velocities in different types of soil (or rock): in addition, the waves are refracted when they cross