Sound waves are the form of propagation of an object's mechanical vibration state (or energy). The so-called vibration refers to the round-trip motion of the mass point of the substance near its equilibrium position. For example, after the drum surface is knocked, it vibrates up and down. This vibration state is transmitted through the air medium in all directions, which is sound waves. Ultrasonic means that the vibration frequency is greater than 20KHz, and the number of vibrations per second (frequency) is very high, which exceeds the upper limit of the human ear (20000Hz). People call this inaudible sound wave ultrasonic. Ultrasound and audible sound are essentially identical. They have a common point of mechanical vibration, which is usually propagated in an elastic medium in the form of a longitudinal wave. It is a form of energy propagation, and the difference is that the ultrasonic frequency is high. The wavelength is short, and it has good beam ability and directivity along a straight line within a certain distance. At present, the frequency range used for abdominal ultrasound imaging is between 2∽5MHz, usually 3∽3.5MHz (1Hz per second is 1Hz), 1MHz = 10 ^ 6Hz, that is, 1 million vibrations per second, the frequency of audible waves is between 16-20,000HZ).