The branch of acoustics that studies the generation, propagation, reception, and various ultrasonic effects and applications of ultrasound is called ultrasonics. The devices that generate ultrasonic waves include mechanical ultrasonic generators (such as gas whistle, whistle, and liquid whistle), electric ultrasonic generators made using electromagnetic induction and electromagnetic action principles, and electroacoustic transducers made using the electrostriction effect of piezoelectric crystals and the magnetostriction effect of ferromagnetic materials.

Ultrasound effect: When ultrasound propagates in a medium, the interaction between ultrasound and the medium causes physical and chemical changes in the medium, resulting in a series of mechanical, thermal, electromagnetic, and chemical ultrasound effects, including the following four effects:

① Mechanical effects. The mechanical action of ultrasound can promote the emulsification of liquid, the liquefaction of gel and the dispersion of solid. When standing waves are formed in ultrasonic fluid media, small particles suspended in the fluid condense at the wave nodes due to mechanical forces, forming periodic accumulations in space. When ultrasound propagates in piezoelectric and magnetostrictive materials, induced polarization and magnetization are caused by the mechanical action of ultrasound (see dielectric physics and magnetostriction) Cavitation effect. When ultrasound acts on a liquid, it can generate a large number of small bubbles. One reason is that local tensile stress occurs within the liquid, resulting in negative pressure. The decrease in pressure causes the gas that was originally dissolved in the liquid to supersaturate and escape from the liquid, becoming small bubbles. Another reason is that the strong tensile stress tears the liquid apart into a cavity, known as cavitation. The cavity contains liquid vapor or another gas dissolved in the liquid, and may even be a vacuum. Small bubbles formed by cavitation will continuously move, grow, or suddenly burst with the vibration of the surrounding medium. When it bursts, the surrounding liquid suddenly rushes into the bubbles, producing high temperature and pressure, as well as shock waves. The internal friction accompanying cavitation can form charges and produce luminescence within the bubble due to discharge. The technology of ultrasonic treatment in liquids is mostly related to cavitation. ③ Thermal effect. Due to the high frequency and energy of ultrasound, significant thermal effects can be generated when absorbed by the medium. ④ Chemical effects. The effect of ultrasound can promote or accelerate certain chemical reactions. For example, pure distilled water produces hydrogen peroxide after ultrasonic treatment; Water dissolved in nitrogen produces nitrite after ultrasonic treatment; The aqueous solution of dyes will change color or fade after ultrasonic treatment. The occurrence of these phenomena is always accompanied by cavitation. Ultrasound can also accelerate the hydrolysis, decomposition, and polymerization processes of many chemical substances. Ultrasound also has a significant impact on photochemical and electrochemical processes. After ultrasonic treatment, the characteristic absorption spectral bands of various amino acids and other organic substances in aqueous solutions disappear and exhibit uniform general absorption, indicating that cavitation has altered the molecular structure.