Acoustic impedance is described as what?

• A. Resistance to Sound Propagation Through a Medium
• B. Rate of Energy Absorption
• C. Speed of Sound in the Medium
• D. Reflection Coefficient at Boundaries

Answer: (A)

Acoustic impedance describes the opposition a medium offers to the propagation of sound. It’s essentially how much pressure is required to move the medium’s particles a given amount, so it’s like resistance to the flow of acoustic energy. It’s defined as Z = p/v, where p is acoustic pressure and v is particle velocity, and it has units of Rayls (kg/(m^2·s)). In practice, many media have Z ≈ density × speed of sound, so soft tissues with similar densities and speeds have comparable impedances, while bone has a much higher impedance. This mismatch at boundaries determines how much sound is transmitted versus reflected. It’s not the rate of energy absorption (that’s attenuation), not the speed of sound itself, and not the reflection coefficient (which depends on impedance values and is derived from them).