Question

what is ultrasound? explain how defects in a metal block can be detected using ultrasound

Answer 1

Ultrasound is sound waves with frequencies higher than the upper audible limit of human hearing. Ultrasound is no different from 'normal' sound in its physical properties, except in that humans cannot hear it. 

Answer 2

Ultrasound is the type of sound ranging above 20,000Hz. That humans can't hear as it is above the audible range ...

Ultrasound detects defects in a metal block because:-
1) When the ultrasound is produced by the transmitter, sound waves reflect back from the surface and is received by the reviewer.
2) If there is a defect, sound waves reflect from the surface either slowly or fastly.
3) In this process, we can understand the defect.