Acoustic wave is a form of propagation of the mechanical vibration state (or energy) of an object. The so-called vibration refers to the reciprocating motion of the material particle near its equilibrium position. For example, after the drum head is struck, it vibrates up and down. This vibration state propagates in all directions through the air medium, which is the sound wave.
We know that when an object vibrates, it makes a sound. Scientists call the number of vibrations per second the frequency of sound, and its unit is hertz (Hz). The frequency of sound waves that our human ears can hear is 16 to 20,000 Hz. Therefore, when the vibration of an object exceeds a certain frequency, that is, above the upper limit of the human hearing threshold, people cannot hear it. Such a sound wave is called "ultrasound".
It is generally accepted that sound waves with frequencies below 16-20 Hz are called infrasound or subsonic waves, and sound waves with frequencies above 20 kHz are called ultrasound waves.
   
Ultrasound and audible sound are essentially the same. Their common point is a mechanical vibration, which usually propagates in an elastic medium in the form of longitudinal waves. It is a form of energy propagation. The difference is that the ultrasonic frequency is high. The wavelength is short, and it travels in a straight line within a certain distance with good beam and directivity. The branch of acoustics that studies the generation, propagation, and reception of ultrasound, as well as various ultrasound effects and applications, is called ultrasound. The ultrasonic generating devices include mechanical ultrasonic generators (such as air whistle, whistle and liquid whistle, etc.), electric ultrasonic generators made using the principle of electromagnetic induction and electromagnetic action, and the electrostrictive effect and ferromagnetic using piezoelectric crystals Electroacoustic transducers made of the magnetostrictive effect of substances.