When the beam is two near zone lengths from the transducer the beam is ______ as the active element.

• A. The same size
• B. Wider than
• C. Narrower than
• D. Twice as wide

Answer: (A)

In ultrasound beam behavior, diffraction creates a near field where the beam stays fairly confined to the size of the active element. The near-field length is determined by the transducer diameter and the wavelength, and within about this distance the beam’s cross-section remains roughly the same as the element’s width. At roughly two near-field lengths away, the main lobe has expanded to match the transducer’s width, so the beam is about the same size as the active element. This reflects how diffraction limits the beam early on: it stays narrow near the element, then gradually widens as you move farther away.