Where does beam divergence occur?

• A. Near field
• B. Far field
• C. At the crystal
• D. None of the above

Answer: (B)

Beam divergence comes from the way the radiated wave diffracts as it travels away from the transducer. The finite size of the transducer aperture acts like a diffraction aperture, so the wavefronts spread out as distance increases. This spreading becomes a stable, measurable cone in the far field, where the beam angle can be described by diffraction theory (roughly sin theta ≈ 1.22 λ / D for a circular element, with λ the wavelength and D the aperture diameter). In the near field, the pattern is more complex and does not exhibit a simple divergence angle. The divergence is therefore a far-field phenomenon, not something that happens right at the crystal.