The condition of the surface through which a sound beam enters a material affects ultrasonic measurements. Increased roughness reduces the transmitted energy of the sound beam and this, in turn, reduces the amplitude of the received signal, leading to difficultly in measuring the size of the discontinuity.
The defect signal amplitude decreases with increases in surface roughness (Ra). Higher frequency signals are more affected by surface roughness. As a general rule:
- for Ra 25 m, no major effect on signal amplitude
- for Ra between 25-50 m, moderate effect on signal amplitude
- for Ra between 50-100 m, major effect on signal amplitude
- for Ra above 125 m, no ultrasonic inspection possible
In addition, frequencies above 10 MHz are usually not recommended for Ra above 25 m. The inability to detect an ultrasonic defect signal with rough surfaces is due to the general scattering losses of the ultrasonic signal at the rough front surface and to the difficulty to discriminate due to the noisy backscattered signal in the region of the rough front surface.