Ultrasonic nondestructive testing, also known as ultrasonic NDT or simply UT, is a method of characterizing the thickness or internal structure of a test piece using high-frequency sound waves. The frequencies, or pitch, used for ultrasonic testing typically range from 500 kHz to 20 MHz, many times higher than the limit of human hearing.

High-frequency sound waves are very directional and will travel through a medium, such as steel or plastic, until they encounter a boundary with another medium
Source: NDT