Ultrasound transducers with large PZT crystals create sound beams that are shaped like ____.

• A. Hourglass
• B. Sphere
• C. Cone
• D. Rectangle

Answer: (A)

When a transducer uses a large PZT crystal, the aperture through which the sound waves emerge is big, and focusing concentrates energy toward a specific point. As the wave travels toward that focus, the beam narrows to a tight waist. After passing the focus, it begins to diverge again. If you visualize the beam's width along its path, you get a narrow region at the focal point with wider regions before and after—an hourglass shape. This contrasts with a simple cone of a nonfocused, diverging beam or other shapes that don’t reflect the converging-then-diverging pattern. The hourglass pattern best captures how a large-aperture, focused transducer concentrates energy at the focal region and then lets it spread again beyond that point.