At the focus, the beam diameter is ______ the transducer diameter.

• A. Equal to
• B. Half
• C. Double
• D. One-fourth

Answer: (B)

Focusing is governed by diffraction, so the energy from a circular transducer converges to a focal spot whose size is limited by the aperture. The central part of the aperture contributes most to the focused energy, while diffraction spreads energy into side lobes. In practical focusing setups, this diffraction-limited focus ends up having a beam diameter that is about half the transducer’s diameter. So the beam at the focus is significantly narrower than the full transducer width, giving a lateral resolution roughly in that half-diameter range. If the focus were equal to the transducer diameter, or larger, it would imply less control over the spot size, which diffraction and standard focusing do not support. A spot that is one-fourth of the aperture would be unrealistically small for typical transducer geometries.