Question

Why does the defective part of the metal block not allow the ultrasound to pass through it?

Answer 1

Ultrasonic waves:

Ultrasonic waves are characterized as "inaudible sound with excessive frequency for humans," with frequencies often exceeding $20\mathrm{kHz}$.

Applications of ultrasonic waves:

1. Imperfection detection using ultrasonic waves.
2. Hard material cutting and corresponding.
3. Welding and ultrasonic soldering.
4. Flow equipment measurements

The reason why the defective part of the metal block does not allow the ultrasound to pass through it:

1. Sound is the periodic transmission of waves with ups and downs in the density of the medium, which can be solid, liquid, or gaseous. Ultrasound is similar to sound except that it has a higher frequency of ups and downs.
2. When ultrasound reaches a crack, the medium changes; previously, it was metal, but now it is occupied by a tiny layer of air or vacuum directly near the crack. As a result, the sound must decide whether to continue propagating in the same direction or to return to the same medium.
3. Most of the sounds choose both, which means some parts of the sound return back and some continue to go further.
4. But the ultrasound chooses to return back in the same medium because the difference in the density of metal and air is quite stark. Due to this property of ultrasound, it is also used in detecting metal flaws.