Which zone is defined by the property that the indication amplitude decays exponentially with distance?

Which zone is defined by the property that the indication amplitude decays exponentially with distance?

• A. Near field zone
• B. Far field zone
• C. Dead zone
• D. Fresnel zone

Answer: (B)

The key idea is that signal strength fades as the ultrasonic wave travels through material because of attenuation, which follows an exponential decrease with distance. In the far field, the beam has spread out enough that geometrical effects are less dominant and the remaining reduction in amplitude is mainly due to this attenuation along the travel path (round trip for a reflection). That makes the back-wall or flaw indication decline roughly as e^{-αx} with distance x, where α is the attenuation coefficient. In contrast, the near field and Fresnel zone have complex, interference-driven patterns from the transducer aperture and do not exhibit a simple exponential decay with distance. The dead zone isn’t about distance-based decay at all; it’s the region close to the surface where the receiver can’t detect signals right after pulsing.

The key idea is that signal strength fades as the ultrasonic wave travels through material because of attenuation, which follows an exponential decrease with distance. In the far field, the beam has spread out enough that geometrical effects are less dominant and the remaining reduction in amplitude is mainly due to this attenuation along the travel path (round trip for a reflection). That makes the back-wall or flaw indication decline roughly as e^{-αx} with distance x, where α is the attenuation coefficient. In contrast, the near field and Fresnel zone have complex, interference-driven patterns from the transducer aperture and do not exhibit a simple exponential decay with distance. The dead zone isn’t about distance-based decay at all; it’s the region close to the surface where the receiver can’t detect signals right after pulsing.