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.

The technical application of ultrasound mainly includes two aspects: (1) ultrasonic treatment. Ultrasonic treatment mainly utilizes its power characteristics and cavitation effect to change or accelerate certain physical, chemical, biological properties or states of substances. The use of strong ultrasonic waves for processing, ultrasonic cleaning, ultrasonic welding, ultrasonic emulsification, ultrasonic grinding, ultrasonic degassing, ultrasonic medical treatment, ultrasonic seed treatment, etc., has been widely used in various departments such as industry, agriculture, medical and health. In industry, strong ultrasonic waves are used to drill and cut hard objects such as steel, ceramics, gemstones, diamonds, etc. Usually, we can use hammers and steel drills to drill holes in hard rocks, and ultrasonic machining is also the same principle.

(2) Ultrasonic testing, thickness measurement, ranging, medical diagnosis, and imaging. Ultrasonic transmission method is often used in industrial production for non-destructive detection of products. The ultrasonic waves emitted by the ultrasonic generator can pass through the tested sample and be received by the opposite receiver. If there are defects inside the sample, ultrasound will reflect at the defect location. At this point, the receiver on the opposite side cannot receive or cannot receive all the ultrasonic signals emitted by the generator. In this way, it is possible to detect any defects inside the sample without damaging it. In medical diagnosis, the echo method is often used: weak ultrasound is transmitted into the human body, and when the ultrasound encounters the interface of organs, it reflects and transmits. The ultrasonic waves transmitted into the internal organs will undergo reflection and transmission again when encountering the interface, and the ultrasonic receiver specifically receives each reflected wave. Medical personnel can understand the size, location, and internal lesions of organs based on the time interval and strength of the reflected waves received. Compared with audible sound waves, ultrasound has many peculiar characteristics: 1. Cavitation. When ultrasound propagates in a liquid, small cavities are generated inside the liquid due to the violent vibration of liquid particles. These small voids rapidly expand and close, causing violent collisions between liquid particles, resulting in pressures of several thousand to tens of thousands of atmospheres. The intense interaction between particles can cause a sudden increase in the temperature of the liquid, which plays a good stirring role, causing two immiscible liquids (such as water and oil) to emulsify and accelerate the dissolution of solutes, accelerating chemical reactions. The various effects caused by the action of ultrasound in liquids are called the cavitation effect of ultrasound. 2. Power characteristics: When sound propagates in the air, it drives particles in the air to vibrate back and forth and do work on them. Sound 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. Due to the high frequency of ultrasound, its power is very high compared to general sound waves. 3. Propagation characteristics: The wavelength of ultrasound is very short, and the size of typical obstacles is many times larger than that of ultrasound. Therefore, the diffraction ability of ultrasound is poor, and it can propagate in a straight line in a uniform medium. The shorter the wavelength of ultrasound, the more significant this characteristic is.