To focus a sound beam relatively far away from the transducer, it is advantageous to:

• A. Increase thickness of the element
• B. Increase the diameter of the element
• C. Decrease the width of the element
• D. Decrease the frequency of the element

Answer: (B)

A larger aperture helps focus the beam farther from the transducer because diffraction spreads the beam, and the amount of spread is inversely related to the aperture size. By increasing the element diameter, you create a bigger opening for the sound waves, so the beam remains narrow longer and you can form a focus at a greater distance. This effectively lowers the f-number (focal length divided by aperture), improving lateral resolution at that deeper location.

Increasing thickness doesn’t directly set how far you can focus; it mainly affects electrical impedance and bandwidth. Narrowing the width reduces the aperture, which increases beam divergence and worsens far-field focusing. Lowering the frequency increases the wavelength, making diffraction more pronounced and the beam wider, which also makes far-field focusing harder.