For a non-focused transducer, the region between the transducer element and the point at which the sound beam becomes narrowest is called the:

For a non-focused transducer, the region between the transducer element and the point at which the sound beam becomes narrowest is called the:

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

Answer: (C)

In ultrasound, the beam has a near field (Fresnel zone) where it is initially converging, and a far field (Fraunhofer zone) beyond it. For a non-focused transducer, the region from the transducer face up to the point where the beam is narrowest is the near field. The distance to that narrowest point—the end of the near field—is the near zone length. This length depends on the transducer diameter and the wavelength, approximately NZL ≈ D^2 / (4λ). Beyond this point the beam begins to diverge, entering the far field. A focal distance only applies to focused transducers with a defined focal point.

In ultrasound, the beam has a near field (Fresnel zone) where it is initially converging, and a far field (Fraunhofer zone) beyond it. For a non-focused transducer, the region from the transducer face up to the point where the beam is narrowest is the near field. The distance to that narrowest point—the end of the near field—is the near zone length. This length depends on the transducer diameter and the wavelength, approximately NZL ≈ D^2 / (4λ). Beyond this point the beam begins to diverge, entering the far field. A focal distance only applies to focused transducers with a defined focal point.