Explain how defects in a metal block can be detected using ultrasound.

Hint
Ultrasound is the sound waves of high frequencies which are used in many fields such as construction, medical field, manufacturing weapons, communication etc.

Complete step by step answer
Ultrasonic sounds are sound waves of high frequency i.e. higher than 20 KHz. It does not have different properties. It has the same properties of normal sound but has a high frequency. These high frequency sounds are used for measuring distance, cleaning, sonography etc.
Ultrasounds can be used to detect the defects in the metal blocks. It is used to detect the cracks and flaws in metal blocks. Metallic blocks are generally used in construction of big structures like buildings, bridges, machines etc.
There might be cracks or holes inside the metal blocks. Those cracks and holes are invisible to the outside. It reduces the strength and quality of the metal blocks. If we use those damaged metal blocks to build a grand structure it reduces the strength of the building.
These damages must be detected. This is where ultrasonic sounds are used.
The Ultrasonic waves are allowed to pass through the metal blocks. There are detectors placed on the other side of the metal block which are used to detect the transmitted waves.
If there is even a small defect, the ultrasonic sound gets reflected at that damaged point. If the ultrasound gets reflected back, there will be less number of output waves than number of input waves detected by the detector. It shows the presence of the flaw or defect.

Note
Ordinary sounds of wavelengths cannot be used for such purposes as it will bend around the corners of the defective location and enter the detector, because ordinary sound waves have long wavelengths where the ultrasonic sound waves have short wavelength and high frequency.