As impedance ratio increases, transmitted energy across the interface should:

• A. Increases
• B. Is not changed
• C. May increase or decrease
• D. Decreases

Answer: (D)

When an ultrasonic wave meets an interface with different acoustic impedance, some energy is reflected and some is transmitted. The bigger the impedance difference (the impedance ratio grows), the more energy gets reflected and the less energy makes it across the boundary. This is described by the transmission of energy across the boundary, which decreases as the impedance mismatch increases (the transmission coefficient drops toward zero for large impedance contrasts). So as impedance ratio increases, transmitted energy across the interface decreases.