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Ultrasound, , used in construction of big structures, like buildings, bridges, machines and, also scientific equipment. The cracks, or holes inside the metal blocks, which, are invisible from outside reduces the, strength of the structure. Ultrasonic, waves are allowed to pass through the, metal block and detectors are used to, detect the transmitted waves. If there, is even a small defect, the ultrasound, gets reflected back indicating the, presence of the flaw or defect,, , Defect or flaw, , Detectors, , , , Metal Block, , Fig 12.16: Ultrasound is reflected back from the, , defective locations inside a metal block., , SCIENCE