Which property is highest in the center of the sound beam and falls off toward the periphery?

• A. Intensity
• B. Frequency
• C. Wavelength
• D. Phase

Answer: (A)

The central idea is how ultrasound energy is distributed within the beam. The energy is most concentrated along the central axis, so the intensity—power per unit area—is highest at the center. As you move away from the axis toward the edges, the beam spreads out due to diffraction and focusing, so the same amount of energy covers a larger area and the intensity decreases. Frequency and wavelength are inherent properties of the wave set by the transducer and medium and don’t inherently peak at the center, and phase can vary across the beam but isn’t defined by being greatest at the center. So the property that is highest in the center and falls off toward the periphery is intensity.