What term describes the resistance to the propagation of sound through a medium?

• A. Acoustic Impedance
• B. Acoustic Velocity
• C. Attenuation
• D. Reflected Energy

Answer: (A)

Acoustic impedance is the measure of how much a medium resists the passage of sound. It combines the density of the medium and the speed of sound within it (Z = ρc) and has units of rayls. This resistance governs how much of an incident sound wave is transmitted versus reflected at a boundary: a large impedance mismatch between two media yields a stronger reflection and less transmission. In ultrasound practice, impedance helps explain why interfaces between tissues with different densities and compressibilities produce echoes—the boundary response is driven by this resistance to wave propagation. The velocity of sound describes how fast the wave travels, not how much it resists movement; attenuation describes loss of energy as the wave propagates, and “reflected energy” is a consequence of impedance differences rather than the resistance itself.