Ultrasonic wave is a sound wave with a frequency higher than 20000 Hz. It has good directivity, strong penetration ability, easy to obtain concentrated acoustic energy and long propagation distance in water. It can be used for ranging, measuring speed, cleaning, welding, crushing, disinfection and so on. It has many applications in medicine, military, industry and agriculture. Ultrasound is named because its lower frequency limit is greater than the upper hearing limit of human beings.
Scientists call the number of vibrations per second the frequency of sound, in hertz. The frequency of sound waves that our ears can hear is 20Hz-20000Hz. So we call the sound wave with frequency higher than 20000 Hz "ultrasonic wave". The ultrasonic frequency usually used for medical diagnosis is 1 MHz -30 MHz.
Theoretical research shows that the vibration energy of an object is proportional to the vibration frequency at the same amplitude. When the ultrasonic wave propagates in the medium, the vibration frequency of the medium particles is very high, so the energy is very large. In the dry winter in northern China, if ultrasonic waves are put into a water tank, the violent vibration will break the water in the water tank into many small drops, and then these small drops will be blown into the room by a small fan, which can increase the indoor air humidity. This is the principle of ultrasonic humidifier. Such as pharyngolaryngitis, tracheitis and other diseases, it is difficult to use blood flow to make drugs reach the affected area. Using the principle of humidifier, the curative effect can be improved by atomizing the liquid medicine and inhaling it by the patient. The huge energy of ultrasound can also make the stones in the human body violently and forcibly vibrate and break, thus alleviating the pain and achieving the purpose of healing. Ultrasonic wave is widely used in medicine, which can sterilize articles.