The beam diameter at the focus is one-half the transducer diameter.

• A. True
• B. False
• C. Cannot be determined
• D. Depends on frequency

Answer: (A)

The idea here is diffraction-limited focusing: the size of the beam at the focus is set by the transducer’s aperture and the wavelength. For a circular aperture with typical focusing, the central diffraction lobe that defines the focal spot ends up spanning about half the aperture diameter. So the beam diameter at the focal plane is roughly D/2 in common diagnostic configurations. The exact size does vary with frequency (wavelength) and how tightly the element is focused (f-number), but this 1/2 rule is a widely used practical approximation for lateral beamwidth at focus.