Physical basis of ultrasonic flaw detector

Ripple and Wave Classification
All the particles of the medium are connected with each other by elastic force. A particle vibrating in the medium can excite the vibration of surrounding particles. The propagation process of vibration in elastic medium is called wave. Waves include electromagnetic waves (electric waves and light waves) and sound waves (or mechanical waves).
Sound Waves
Sound waves are elastic waves that can propagate in gases, liquids, and solids. It can be divided into fractionated sound waves, audible sound waves, ultrasonic waves and ultra-sonic waves. The sound waves that can be heard by the human ear are between 20-20000 Hz. The sound waves with a frequency exceeding 20,000 Hz, which cannot be heard by the human ear, are called ultrasonic waves. The higher the frequency of the sound wave, the more similar it is to certain optical properties (such as reflection and refraction laws).