What term is used to describe the far field of an ultrasound beam?

• A. Fresnel zone
• B. Fraunhofer zone
• C. Near zone length
• D. Focal distance

Answer: (B)

In ultrasound, the beam’s behavior changes as it travels: close to the transducer the wavefronts are curved and the pattern is affected by the aperture, but farther away the waves become essentially planar and the diffraction pattern stabilizes. That farther region is the Fraunhofer zone. It’s the zone where you can predict the angular distribution from the transducer’s size and the wavelength, and where the field behaves like a true far-field diffraction pattern.

The other terms describe different regions or concepts. The Fresnel zone refers to the near-field region where the wavefront curvature matters and the pattern isn’t yet stable. The near zone length marks how far the near-field extends. The focal distance is the distance to the focal point of a focusing transducer, not the general far-field region.

So the term for the far field is Fraunhofer zone.