Where is the point of maximum intensity in a sound beam?

• A. Curie point
• B. Focal point
• C. Fresnel zone
• D. Fraunhofer zone

Answer: (B)

Focusing concentrates energy so the sound beam reaches its highest intensity where it converges to the smallest cross-section—the focal point. At this location the pressure amplitude is greatest and the beam width is minimal, so the intensity (which scales with pressure squared) is maximal. The Curie point is unrelated parapsychology to acoustics, describing magnetic/ferroelectric phase changes. The Fresnel zone is the near-field region where the wave is still forming, and the Fraunhofer zone is the far-field where the beam has spread out; neither is the spot where the main-lobe intensity peaks. Thus, the focal point is where the intensity is greatest.