What is a consequence of impedance mismatch between two media?

• A. Increased reflection at boundary
• B. Increased transmission into second medium
• C. No change
• D. Faster propagation

Answer: (A)

When an ultrasound wave meets a boundary between two media with different acoustic impedances, the boundary conditions require that both pressure and particle velocity be satisfied across the interface. If the impedances are different, the wave cannot continue wholly into the second medium without some energy being reflected back into the first. The result is a portion of the incident wave reflecting and a portion transmitting into the next medium.

The amount of reflection depends on the impedance difference. The amplitude reflection coefficient is (Z2 − Z1)/(Z2 + Z1), so unless Z1 and Z2 are exactly equal, this value is nonzero, and the reflected energy is nonzero (its fraction is the square of that ratio). Therefore, impedance mismatch produces increased reflection at the boundary. In contrast, when impedances are matched, almost all energy transmits with minimal reflection; if there were no difference at all, there would be no reflected wave. The wave’s propagation speed itself is determined by the medium’s properties and isn’t directly caused by the mismatch, though the transmitted wave adopts the second medium’s speed.