Defective metal blocks will not allow ultrasound to pass through it and reflect it back. This technique is used in detecting defects in metal blocks. Make a set up as shown in the figure with ultrasound being passed through one end and detectors placed on the other end of a metal block. Since the defective part of the metal block does not allow ultrasound to pass through it.it will not be detected by the detector. In this way, defects in metal blocks can be detected with the help of ultrasound.