Which statement best describes the beam formation in annular arrays across rings?

• A. All rings are fired simultaneously with identical delay
• B. Each ring sends a pulse with the same focal depth
• C. Each ring sends a pulse with the different focal depth
• D. Each ring sends a pulse with a different focal depth

Answer: (D)

In an annular array, each ring can be driven with its own timing so the waves from all rings interfere in a controlled way along the axis. If you set delays so that every ring aims for the same depth, you get a single focused point along the beam path. But the question describes beam formation across rings, and the best description here is that each ring is used to target a different focal depth. This creates multiple focal points along the depth, giving coverage over a range rather than a single narrow focus. That multi-focal approach is a distinctive feature of annular arrays, allowing the beam to extend its useful imaging depth by combining the contributions from rings focused at different depths.

In short, the rings can be configured to produce distinct focal depths, and doing so across rings yields a beam with multiple foci along the axis rather than a single focus.