MP Board · Class 9 · Science · SoundWhat is Ultrasound? Explain briefly how ultrasound is used to detect cracks and flaws in metal blocks.

Ultrasound refers to sound waves with frequencies higher than $20,000 \text{ Hz}$ ($20 \text{ kHz}$), which are beyond the upper audible limit of human hearing.

Detection of cracks in metal blocks:

1. High-frequency ultrasound waves are passed through the metal block being tested.
2. Detectors are placed on the opposite side to receive the transmitted waves.
3. If there is even a small crack, void, or defect inside the block, the ultrasound waves get reflected back from that defect location rather than passing through.
4. As a result, the waves do not reach the detector at that point, indicating the exact presence and position of an internal defect without damaging the metal block.