The devices used in the ultrasonic examination method work according to two principles. These principles; transmission reflection principle and resonance principle.

Transmission Reflection Method:

With the transmission and reflection method, high frequency sound waves are sent from a point to the part to be inspected. These waves are detected by the generating head (probe) and seen as echoes on the oscilloscope screen, or the sound waves emitted by the generating head are captured by the receiving head and seen as echoes on the oscilloscope screen. By looking at the echoes, it is determined whether there is a defect in the part.

The waves detected by the probe are converted into electrical signals (by piezoelectric effect) and appear on the cathode ray tube screen as echoes that are a precursor of the material internal structure. The positions and amplitudes of the echoes observed on the screen provide information about the location and size of the discontinuity.

Resonance Method:

In examination with the resonance method, the frequency sound waves sent to the part are not constant. When the natural frequency of the material and the frequency of the sound waves coming from the transmitter are the same, the width increases. The increase in width can be understood from the distance between wavelengths by looking at the echo on the oscilloscope screen. This method is also used to measure the thickness of parallelepiped materials.

High frequency sound waves sent into the material are reflected if they hit an obstacle in the sound path. Depending on the angle of impact, the reflected signal may or may not reach the receiving probe (Figure 15). The reflected signal reaching the receiving probe creates an echo symptom on the screen of the ultrasonic inspection device. Depending on the position of the echo, the coordinates of the reflector within the examination piece can be calculated. Additionally, the height of the echo gives an idea about the size of the reflector. It may be possible to make a comment about the type of reflector by looking at the shape of the echo signal.