Ultrasonic energy is mechanical energy. Its transmission is dependent upon the elastic properties and the densities of the media through which it is propagated. The stresses associated with the propagation of ultrasonic waves are the basic cause of the numerous mechanical effects attributable to applying ultrasonic energy. The stresses may operate directly or may be converted into thermal energy by absorption or into chemical energy by their effects upon the molecular conditions of the materials. Examples of the direct effect of ultrasonic stresses are breaking particles down into smaller particles, emulsifi cation,
degassing of liquids, drying and dewatering of materials, ultrasonic machining, atomization of liquids, and metal forming. Examples of thermal effects of ultrasonic irradiation are ultrasonic welding of polymers and metals. Chemical factors include ultrasonic cleaning involving assisting chemical attack of contaminants, acceleration of chemical reactions by mixing and curing epoxy materials, and the production of new products by accelerating reactions. The objective of this chapter is to review the mechanisms of several ultrasonic operations.