What is Fraunhofer zone also known as, and what region does it describe?

• A. Near field: region just beyond transducer surface.
• B. The region where wave fronts are curved.
• C. Boundary layer where reflections occur.
• D. Far zone: region starting at focal point and extending deeper.

Answer: (D)

The Fraunhofer zone is the far-field region of the ultrasound beam. Beyond the near field, the wavefronts become essentially planar as they propagate, and the beam pattern is governed by diffraction from the transducer aperture. This is why it’s called the far zone—the region deeper from the transducer where the beam has settled into a more predictable, plane-front form. The starting point is often described using the approximate distance 2D^2/λ (with D being the aperture diameter and λ the wavelength); the focal point is not the defining boundary, though the far field occurs after the beam has passed the near field and its curvature has diminished. The region near the transducer, where fronts are still strongly curved and the pattern is changing, is not the Fraunhofer zone.