One of the most important NDT techniques for finding interior discontinuities is ultrasound testing. substance is penetrated by an ultrasonic beam, revealing discontinuities on an oscilloscope. High frequency sound waves that cannot be heard by human ears are known as ultrasonic waves. The range of frequencies that the human ear can hear is from 20 cycles per second to 20,000 cycles per second. For the testing of engineering materials, ultrasonic waves typically range in frequency from 0.5 MHz to 10 MHz. The production of such high frequency waves is made possible by the piezoelectric effect. Essentially, in ultrasonic testing, we inject ultrasonic waves into a specimen. Due to the material’s characteristics and the presence of discontinuities, these waves encounter changes in their propagation. Testing equipment may detect these changes, which are then shown as an indicator. Acceptance criteria are used to evaluate an indication.