In ultrasonic testing, the region of the beam that is still converging and the diameter is decreasing is called the:

• A. Near field
• B. Far field
• C. Rear field
• D. Main lobe

Answer: (A)

In ultrasonic testing, the beam’s cross-section changes as it travels: the region where the beam is still converging and its diameter is decreasing is the near field (also known as the Fresnel zone). In this part of the path, the wave fronts are curved and the cross-sectional size can shrink with distance from the transducer, before the beam finally reaches a stable width and then begins to diverge. The length of this near-field region depends on the transducer diameter and the wavelength (roughly D^2/(4λ)); a larger aperture or higher frequency (shorter wavelength) extends the near field. Beyond this zone, the beam enters the far field, where it expands in diameter. The other terms describe different parts of the beam pattern and don’t match this converging region.