The amplitude of a boundary reflection depends on what property of the two media?

• A. Frequency determines reflection amplitude
• B. Gain determines reflection amplitude
• C. Depth determines reflection amplitude
• D. Acoustic impedance mismatch governs reflection amplitude

Answer: (D)

The amount of ultrasound energy that reflects at a boundary is determined by how different the acoustic impedances of the two media are. Acoustic impedance, Z, is the product of density and sound speed (Z = ρc). The amplitude of the reflected wave relative to the incident wave is described by the reflection coefficient r = (Z2 − Z1)/(Z2 + Z1). A larger mismatch between Z1 and Z2 produces a larger reflection; if the impedances are matched, there is essentially no reflection. If the impedances differ greatly, a strong reflection occurs.

Frequency, depth, and gain don’t set this intrinsic boundary reflection. Frequency affects wavelength and attenuation, depth changes travel time and attenuation with distance, and gain is a processing factor that scales the detected signal but does not change the boundary’s inherent reflectivity.