The sound beam can be focused by all of the following except:

• A. Acoustic mirror
• B. Acoustic lens
• C. Matching layer
• D. Electronic phasing

Answer: (C)

Beam focusing comes from shaping the wavefront so energy converges at a point. An acoustic lens bends the wavefront to create a focal region, an acoustic mirror redirects or concentrates energy to a focus, and electronic phasing uses precise timing across array elements so the waves arrive in phase at the desired spot. A matching layer sits between the transducer and tissue to maximize energy transfer by impedance matching and to broaden bandwidth; it does not bend or time the waves to form a focus. Its job is to improve transmission efficiency, not to create a focal point, so it’s the one that does not focus the beam.