The diameter of a sound beam is smallest at the end of the near zone. Which term describes this end zone?

• A. Near field
• B. Focal zone
• C. Far field
• D. Transducer face

Answer: (A)

In ultrasound, the beam evolves as it travels away from the transducer. Right after emission, waves from every part of the transducer interfere so the beam narrows as it moves through the near field (the Fresnel zone). The diameter is smallest at the far end of this near field, and beyond that point the beam begins to spread again in the far field (Fraunhofer zone). So the end zone where the beam is at its smallest diameter is the near field. The far field describes what happens after that, where the beam diverges; the focal zone refers to the region around a focus where the beam is narrow, and the transducer face is just the emitting surface rather than a zone description. The near field length depends on the transducer diameter and wavelength, with an approximate rule of L ≈ D^2 / (4λ).