Which region lies between the transducer and the focus and is conical in shape?

• A. Near Zone
• B. Focal Zone
• C. Far Field
• D. Focal Point

Answer: (A)

The region between the transducer and the focus is the near field, also called the Fresnel zone. This region is conical in shape because the beam from a finite circular aperture converges as it travels, narrowing toward the focal point. The near-field length depends on transducer diameter and wavelength (Nz ≈ D^2 / 4λ), so a larger diameter or shorter wavelength extends the converging region. Beyond the focus, the beam begins to diverge into the far field. The other terms describe different parts: the far field lies beyond the focus, the focal point is the single location of smallest beam diameter, and “focal zone” isn’t the standard name for the region between the transducer and the focus.