What is 'Ultrasound'? Explain how defects in a metal block can be detected using ultrasound.

(N/A) Ultrasound refers to sound waves having a frequency greater than $20 \ kHz$.
Defects like internal cracks or holes in metal blocks are detected using ultrasound. This method is based on the principle that an internal crack or hole in the block does not allow ultrasonic waves to pass through it.
To detect a flaw in metallic blocks,a source of ultrasonic sound is placed in front of one face of the block,and a detector is placed on the opposite face.
The ultrasonic waves are produced and made to pass through one face of the block and are then detected on the opposite face.
$(a)$ If the ultrasonic waves pass uninterrupted through all parts of the block,then there is no defect in the block.
$(b)$ If the ultrasonic waves do not reach the opposite face or are reflected back by the flaw,it indicates that the block has a defect (like a crack or a hole).