What are the 4 distinct alterations a sound beam undergoes when focused?

• A. Beam diameter in the near field and focal zone is reduced; Focal depth is shallower; Beam diameter in the far zone increases; Focal zone is smaller.
• B. Beam diameter in the near field and focal zone is increased; Focal depth is deeper; Beam diameter in the far zone decreases; Focal zone becomes larger.
• C. Beam diameter in the near field and focal zone is reduced; Focal depth is deeper; Beam diameter in the far zone increases; Focal zone is larger.
• D. Beam diameter in the near field increases; Focal depth is shallower; Beam diameter in the far zone increases; Focal zone is smaller.

Answer: (A)

When a sound beam is focused, the wavefronts are made to converge toward a point, so the beam narrows as it travels toward that focus. This means the beam diameter both in the near field (before the focus) and in the focal region becomes smaller, concentrating energy there. At the focus, the beam is the most tightly confined, which is why the focal zone is smaller and the intensity is higher. After passing the focus, the beam begins to diverge again, so the diameter in the far field increases. The focal depth is set by the focusing geometry and, for a given focus, lies at a shallower distance from the transducer compared with an unfocused or less-focused beam. This combination of a smaller near-field diameter, a tighter focal zone, a shallower focal depth, and a larger diameter in the far field describes how focusing alters the beam.