What is the consequence of a large acoustic impedance mismatch at a tissue interface?

• A. Most energy transmitted
• B. Most energy reflected
• C. No reflection
• D. Diffraction occurs

Answer: (B)

When a sound wave meets a boundary between two tissues with very different acoustic impedances, most of the energy is reflected back rather than transmitted. The impedance of tissue is Z = ρc (density times speed of sound), and the larger the mismatch between Z values on either side of the boundary, the more the wave is reflected. Quantitatively, the fraction reflected grows toward 1 as the mismatch grows, so you get a strong echo at the interface and little energy passes into the second tissue. This is why interfaces like bone-soft tissue produce strong reflections, while well-matched tissues transmit more energy. Diffraction is a different effect and not driven by impedance mismatch.