What is 'Ultrasound'? Explain how dejects in a metal Mock can detected using ultrasound

Ultrasounds are waves having frequency greater than 20 kHz. The flaws like internal cracks, etc. the metal blocks are detected by using ultrasound. It is based on the fact that internal crack or hole in the block does not allow ultrasonic waves to pass through it. To detect flaw in metallic blocks, a source of ultrasonic sound is placed in front of one face of the block and a detector on the opposite face. The ultrasonic waves are produced and made to pass through one face of block and detected on the opposite face.
(a) If the ultrasonic waves pass uninterrupted through all parts of block, then there is no defect in block.
(b) If the ultrasonic waves do not reach the opposite face, then the block has the defect (like a crack or a hole).