Reflection of an ultrasound wave depends upon __________.

• A. The average speed of sound
• B. The difference in acoustic impedances of the two media at a boundary
• C. The sum of densities
• D. The difference in densities

Answer: (B)

Reflection occurs at a boundary when the medium presents a different acoustic impedance to the incoming wave. Acoustic impedance is the product of density and the speed of sound in the medium (Z = ρc). If the impedances on either side of the boundary are different, the wave cannot perfectly match the boundary, so part of the energy is reflected back toward the source. The greater the mismatch in impedance, the stronger the reflected signal; if the impedances are the same, little to no reflection occurs and nearly all energy transmits, though the speed may still change.

This is why the difference in acoustic impedances at the boundary determines reflection. The average speed of sound alone isn’t sufficient to predict reflection, and looking only at density or the sum of densities misses the key factor—the impedance contrast that combines density and speed.