Which method of fixed focusing uses a curved active element?

• A. External Focusing
• B. Dynamic Focusing
• C. Internal Focusing
• D. Acoustic Lens Focusing

Answer: (C)

Fixed focusing relies on shaping the emitted wavefront so it converges to a single point. When the active element itself is curved, its surface acts like a built‑in lens: the curvature imposes a varying phase across the aperture so all parts of the wavefront travel toward the same focal point. The result is a converging beam with a fixed focal depth determined by the element’s curvature. This built‑in curvature is why this method is called Internal Focusing. External focusing would require a separate lens in front of the element, and acoustic lens or dynamic focusing involve additional components or electronic delay, respectively.