Which of the following is proportional to the square of the amplitude?

• A. Period
• B. Attenuation
• C. Power
• D. Frequency

Answer: (C)

Power is tied to the energy carried by an ultrasound wave, and that energy scales with the square of the amplitude. In tissue, the intensity (and thus the power flowing through a unit area) is proportional to the square of the pressure amplitude (I ∝ p^2/(ρc)). So doubling the amplitude makes the power increase by a factor of four. Period and frequency describe how often the wave repeats and are not determined by the amplitude in this simple relation. Attenuation describes how amplitude decreases with distance, but the quantity that varies with the square of the amplitude at a point is the power (or intensity).