Method of characterizing the thickness or internal structure of a test piece through the use of high frequency sound waves. The frequencies, or pitch, used for ultrasonic testing are many times higher than the limit of human hearing, most commonly in the range from 500 KHz to 20 MHz.In industrial applications, ultrasonic testing is widely used on metals, plastics, composites, and ceramics. The only common engineering materials that are not suitable for ultrasonic testing with conventional equipment are wood and paper products. Ultrasonic technology is also widely used in the biomedical field for diagnostic imaging and medical research.