What is the term for the point at which the ultrasonic beam begins to diverge, located a distance of (D^2)/(4λ) from the transducer face?

What is the term for the point at which the ultrasonic beam begins to diverge, located a distance of (D^2)/(4λ) from the transducer face?

• A. Near Field Boundary
• B. Focal Point or Yo Point
• C. Focusing Node
• D. Far Field Limit

Answer: (B)

In ultrasonics, the beam from a circular transducer has a near-field (Fresnel) region where the wavefronts are still complex and the pattern can vary with distance, followed by a far-field (Fraunhofer) region where the beam spreads more predictably. The distance D^2/(4λ) marks the boundary between these two zones for a circular aperture. At that point the beam transitions from the near-field pattern to a region where divergence begins, so this spot is called the near-field boundary (the Fresnel boundary). The focal point refers to a specific depth where the beam is intentionally focused, which is not fixed by this general boundary distance, and the far-field limit describes the region beyond the boundary rather than the boundary itself.

In ultrasonics, the beam from a circular transducer has a near-field (Fresnel) region where the wavefronts are still complex and the pattern can vary with distance, followed by a far-field (Fraunhofer) region where the beam spreads more predictably. The distance D^2/(4λ) marks the boundary between these two zones for a circular aperture. At that point the beam transitions from the near-field pattern to a region where divergence begins, so this spot is called the near-field boundary (the Fresnel boundary). The focal point refers to a specific depth where the beam is intentionally focused, which is not fixed by this general boundary distance, and the far-field limit describes the region beyond the boundary rather than the boundary itself.