Sound waves with frequencies higher than 210 kHz. 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.

The propagation laws of ultrasound in media, such as reflection, refraction, diffraction, and scattering, are not fundamentally different from those of audible sound waves. But the wavelength of ultrasound is very short, only a few centimeters, even a few thousandths of a millimeter. Compared with audible sound waves, ultrasound has many peculiar characteristics: propagation characteristics - the wavelength of ultrasound is very short, and the size of typical obstacles is many times larger than that of ultrasound, so the diffraction ability of ultrasound is poor. It can propagate in a straight direction in a uniform medium, and the shorter the wavelength of ultrasound, the more significant this characteristic is. Power characteristics - when sound propagates in the air, it drives particles in the air to vibrate back and forth, doing work on the particles. Sound wave power is a physical quantity that represents the speed at which sound waves do work. At the same intensity, the higher the frequency of sound waves, the greater their power.