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. The frequency of sound waves that our human ears can hear is 20HZ～20000Hz. Therefore, when the vibration of the object exceeds a certain frequency, that is, higher than the upper limit of human hearing threshold, people will not be able to hear it. Such sound waves are called "ultrasound." The frequency of ultrasound usually used for medical diagnosis is 1 to 5 MHz.

       Although humans cannot hear ultrasound, many animals have this ability. They can use ultrasound to "navigate", hunt down food, or avoid dangerous objects. You may have seen many bats flying back and forth in the courtyard on summer nights. Why do they fly without light without losing their way? The reason is that bats can emit 20,000 to 100,000 Hz ultrasonic waves, which is like an activity. "Radar Station". Bats use this "radar" to determine whether there is an insect or an obstacle in front of the flight. We humans didn't learn to use ultrasound until the World War. This is to use the principle of "sonar" to detect targets and their states in the water, such as the position of submarines. At this time, people send a series of ultrasonic waves of different frequencies into the water, and then record and process the reflected echo. From the characteristics of the echo, we can estimate the distance, shape and dynamic changes of the detected object. Ultrasound was used very early in medicine in 1942, when the Austrian doctor Dusick used ultrasound technology to scan the brain structure for the first time; later in the 1960s, doctors began to use ultrasound to detect the abdomen. Nowadays, ultrasound scanning technology has become an indispensable tool for modern medical diagnosis.

       The working principle of medical ultrasound is similar to that of sonar, that is, ultrasound is emitted into the human body. When it encounters an interface in the body, it will be reflected and refracted, and may be absorbed and attenuated in human tissues. Because the shape and structure of various tissues of the human body are different, the degree of reflection, refraction, and absorption of ultrasound are also different. Doctors distinguish it by the characteristics of the wave, curve, or image reflected by the instrument. they. In addition, combined with anatomical knowledge, normal and pathological changes, it is possible to diagnose whether the examined body is diseased.

       Sound waves with a frequency higher than 20000 Hz (Hertz). 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 devices that generate ultrasonic waves include mechanical ultrasonic generators (such as gas whistles, steam whistles, and liquid whistles, etc.), electric ultrasonic generators made using the principle of electromagnetic induction and electromagnetic action, and the electrostrictive effect and ferromagnetism of piezoelectric crystals. Electro-acoustic transducer made by the magnetostrictive effect of the substance, etc.