At the end of the near zone the beam narrows to ________.

• A. The full width of the active element
• B. Twice the width of the active element
• C. Only one half the width of the active element
• D. The same width as the transducer

Answer: (C)

In the near field, the beam narrows as the wavelets from all parts of the transducer’s active element interfere to form a tight central lobe. At the end of the near field, this central lobe reaches its smallest width, which for many unfocused transducers is about half the width of the active element. So the beam is about one half the element’s width at that point. Beyond the near field, the beam begins to diverge again in the far field. This is a useful approximate rule of thumb—the exact value depends on the transducer’s shape and the wavelength, but half the element width is the standard takeaway.