Ultrasound is a kind of sound wave whose frequency is higher than 20,000 Hz. It has good direction, strong penetration ability, easy to obtain concentrated sound energy, and can spread far in water. It can be used for ranging, velocity measurement, cleaning, welding, gravel, sterilization and disinfection, etc. There are many applications in medicine, military, industry and agriculture. Ultrasound is named because its lower frequency limit is greater than the upper hearing limit.Scientists call the number of vibrations per second the frequency of sound in Hertz (Hz). The frequency of sound waves that our human ears can hear is 20 Hz-20 000 Hz. Therefore, we call the sound wave whose frequency is higher than 20,000 Hertz as "ultrasound". Usually used in medical diagnosis, the frequency of ultrasound is 1 MHz-30 MHz.