They bend around small defects due to diffraction.
Explanation
The detection of flaws using sound waves relies on the principle of reflection. For a wave to reflect off an obstacle (such as a crack or void), the wavelength of the wave must be comparable to or smaller than the size of the obstacle. If the wavelength is significantly larger, the wave undergoes diffraction.
Detailed Analysis:
- They travel too slowly in metal media. is Incorrect: Sound waves actually travel significantly faster in solids (metals) than in air or liquids due to the high elasticity of the metal. The speed of the wave is not the hindrance to detection.
- They are completely absorbed by the metal. is Incorrect: Metals are good conductors of sound (elastic waves). While some energy is lost to damping, ordinary sound waves are not completely absorbed and can propagate through metal.
- They bend around small defects due to diffraction. is Correct: Ordinary audible sound has relatively low frequencies and, consequently, long wavelengths (often ranging from centimeters to meters). Flaws in metal blocks are typically minute. When the wavelength is much larger than the size of the defect, the waves bend around the defect (diffraction) rather than reflecting off it. Consequently, the flaw does not cast a "shadow" or produce a detectable echo. Ultrasound is used instead because its high frequency (short wavelength) allows it to reflect off small defects.
- They reflect back from the surface of the block. is Incorrect: While sound waves do reflect from the surface due to impedance mismatch (which is why coupling agents are used in ultrasonic testing), this is a challenge for both audible sound and ultrasound. It is not the primary reason why ordinary sound fails to resolve internal flaws once inside the medium.
Key Takeaway:
To detect a small flaw, the probing wave must have a wavelength smaller than the flaw's dimensions. Ordinary sound waves have long wavelengths and diffract around small defects, rendering them invisible to the wave.