As transducer diameter decreases, the beam spread:

• A. Decreases
• B. Remains the same
• C. Increases
• D. Becomes conical in shape

Answer: (C)

Beam spread in ultrasonic beams is set by diffraction from the transducer’s finite aperture. For a circular transducer, the far-field pattern’s first minimum occurs at an angle θ that roughly satisfies sinθ ≈ 1.22 λ / D. Here, λ is the wavelength and D is the transducer diameter. When the diameter D decreases, the ratio λ / D grows, so θ increases and the beam becomes more divergent. In practical terms, energy is spread over a larger area, which lowers lateral resolution and reduces signal strength at depth. The beam doesn’t suddenly take a perfect conical shape; it becomes more divergent overall. Therefore, as transducer diameter decreases, beam spread increases.