A widening of the front surface indication when testing a rough surface is caused by?

• A. Diffraction from surface roughness
• B. Partial reflection of ultrasonic beam side-lobe energy
• C. Attenuation due to rough surface
• D. Scattering from subsurface features

Answer: (B)

Widening of the front surface indication on a rough surface comes from partial reflection of energy in the beam’s side lobes. In ultrasonic testing, the transducer emits not just a single main beam but also weaker side lobes at nearby angles. On a smooth surface, most energy is reflected back from the very top surface in a narrow time window, giving a sharp front-surface echo. When the surface is rough, those side-lobe components can strike different points along the uneven surface and reflect back with slightly different travel times. The result is a spread in arrival times, which appears as a wider front-surface indication on the display. Attenuation would just reduce signal strength, subsurface scattering would show echoes below the surface, and diffraction-related effects don’t account for the specific time-spread pattern of the near-surface echo caused by side-lobe reflections.