The intensity of a sound beam varies depending on the _______.

• A. Measurement point within the beam
• B. location of the measurement
• C. Time
• D. Angle

Answer: (B)

Intensity is the energy per unit area carried by the beam at a specific point, and a sound beam has a nonuniform spatial distribution. The energy density is greatest along the central axis and especially in the focal region, while it tapers off toward the edges. Because of this, what you measure depends on the exact spot within the beam where you measure. Measuring on-axis or near the focus will give a different intensity than measuring off-axis or away from the focus. Time or angle can influence readings in some contexts, but the primary reason for variation is the location within the beam where the measurement is taken.