At depths more than two near zone lengths the beam is ______ the active element.

At depths more than two near zone lengths the beam is ______ the active element.

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

Answer: (C)

Understanding how an ultrasound beam behaves from a finite transducer helps explain this. A transducer acts like a small aperture, so the sound wave diffracts as it travels. In the near-field region, the waves from across the active face interfere in such a way that the beam stays relatively narrow. As you move farther away, diffraction causes the beam to spread. After roughly twice the near-field length, the spreading accumulates enough that the cross‑section of the beam becomes larger than the physical size of the active element. In other words, the beam footprint at depths beyond about two near-field lengths is wider than the transducer itself. This widening is governed by the transducer diameter and the ultrasound wavelength in the medium (the near-field length is about D^2/(4λ), and the far-field spread is related to λ/D), but the key idea is that beyond that distance the beam grows wider than the element.

Understanding how an ultrasound beam behaves from a finite transducer helps explain this. A transducer acts like a small aperture, so the sound wave diffracts as it travels. In the near-field region, the waves from across the active face interfere in such a way that the beam stays relatively narrow. As you move farther away, diffraction causes the beam to spread. After roughly twice the near-field length, the spreading accumulates enough that the cross‑section of the beam becomes larger than the physical size of the active element. In other words, the beam footprint at depths beyond about two near-field lengths is wider than the transducer itself. This widening is governed by the transducer diameter and the ultrasound wavelength in the medium (the near-field length is about D^2/(4λ), and the far-field spread is related to λ/D), but the key idea is that beyond that distance the beam grows wider than the element.