When a sound beam strikes a tissue boundary at a 90 degree angle reflection occurs only if the media on either side of the boundary have ________.

• A. Different impedances
• B. Equal impedances
• C. Different densities
• D. Different speeds

Answer: (A)

When a sound wave hits a boundary head-on, the amount of energy that reflects depends on the impedance mismatch between the two media. Acoustic impedance is Z = ρc, the product of density and the speed of sound. If the two media have different impedances (Z1 ≠ Z2), part of the wave is reflected back at the boundary. If the impedances are the same (Z1 = Z2), there is no reflection and all energy transmits across the boundary. So, reflection at a 90-degree incidence occurs when the media on either side have different impedances. Differences in density or speed alone don’t guarantee reflection because they must produce a mismatch in impedance; it’s their product that matters.