Diffraction of the sound beam is caused by what condition?

• A. Large impedance mismatch
• B. Small aperture
• C. A virtual small aperture
• D. Perpendicular incidence

Answer: (C)

Diffraction comes from the fact that the sound source has a finite size. The ultrasound beam is produced by a transducer with a limited aperture, so the edges of that aperture shape and bend the waves as they propagate. To reason about this spreading, we often model the beam as if it came from a small, virtual source behind the actual aperture. This “virtual small aperture” concept captures why the beam fans out: the smaller the effective source relative to the wavelength, the wider the diffraction pattern, and vice versa. In other words, the beam’s angular spread is governed by the ratio of wavelength to the effective aperture size.

Impedance mismatch would primarily cause reflections at interfaces, not diffraction. Perpendicular incidence affects reflection direction rather than beam spreading. While a truly small physical aperture would cause diffraction, the standard description in ultrasound is the virtual small aperture, which embodies the practical way the finite aperture limits and shapes the beam.