To achieve greater depth of penetration in coarse-grained materials, which frequency should be used?

• A. Lower frequencies
• B. Higher frequencies
• C. Medium frequencies
• D. Very high frequencies

Answer: (A)

Lower frequencies provide greater depth of penetration because attenuation and scattering increase with frequency, and coarse-grained materials amplify this effect. Grain boundaries in coarse metals scatter high-frequency waves more strongly, causing energy to be lost quickly and reducing the signal from deeper regions. Using a lower frequency reduces these losses, allowing the ultrasonic wave to travel farther and the echoes from deeper flaws to be detected. The trade-off is lower resolution, but for deep penetration in coarse-grained materials, depth is the priority.