For a disc shaped crystal the width of the sound beam at the focus is

For a disc shaped crystal the width of the sound beam at the focus is

• A. The full width of the beam at focus
• B. Equal to the transducer width
• C. One half the width of the beam as it leaves the transducer
• D. Twice the transducer width

Answer: (C)

Focusing a disc-shaped transducer narrows the lateral extent of the beam at the focal point. The beam starts with a width roughly set by the transducer’s diameter as it leaves the surface, but the focusing action concentrates energy in the central lobe and reduces that width at the focus. For a typical disc-shaped crystal, this focused width is about half of the width that exits the transducer. So the beam at the focus is approximately half as wide as the beam leaving the transducer, which is why the correct description is that the width at the focus is about one half the exit width. The other options would imply either no narrowing, excessive widening, or the full exit width at the focus, which isn’t consistent with how focusing tightens the beam.

Focusing a disc-shaped transducer narrows the lateral extent of the beam at the focal point. The beam starts with a width roughly set by the transducer’s diameter as it leaves the surface, but the focusing action concentrates energy in the central lobe and reduces that width at the focus. For a typical disc-shaped crystal, this focused width is about half of the width that exits the transducer. So the beam at the focus is approximately half as wide as the beam leaving the transducer, which is why the correct description is that the width at the focus is about one half the exit width. The other options would imply either no narrowing, excessive widening, or the full exit width at the focus, which isn’t consistent with how focusing tightens the beam.