Acoustic wave is the mechanical vibration of an object state (or energy) forms of communication. The so-called refers to the movement of the vibration material particles were in the vicinity of the equilibrium position. For example, when the drum is struck, it moves up and down, and the vibration spreads through the air medium, which is the sound wave. Ultrasonic wave refers to the vibration frequency more than 20000Hz, its per second vibration frequency (frequency) is very high, beyond the upper limit of human hearing (20000Hz), people will not hear the sound wave is called ultrasonic. Ultrasonic and audible is essentially the same, they have in common is a mechanical vibration, usually in the elastic media in longitudinal way, energy is a form of communication, the difference is that the ultrasonic high frequency and short wavelength, within a certain distance to travel along a straight line with a beam of and the good direction, the abdominal ultrasound imaging is used in the frequency range between 2 to 5 megabytes of Hz, commonly used for 3 ~ 3 trillion and 500 billion Hz (1 vibrations per second 1Hz, 1 MHz Hz=10^6Hz, namely 1 million vibrations per second, can be heard in the wave frequency between 16 and 20000HZ).

By using the method of ultrasonic detection instrument to detect the called ultrasonic flaw detector. It is the principle of ultrasonic detection in materials dissemination, influence propagation change material acoustic properties and the internal organization of the ultrasonic, understanding the properties and structure of materials by detecting the change degree and status influence on the ultrasonic technology called ultrasonic detection. Ultrasonic detection methods usually have penetration method, pulse reflection method, tandem method etc..