For sound reflection at the interface of two media to occur, what condition must be met?

• A. Oblique incidence
• B. Temperature differences
• C. A difference in the acoustic impedance
• D. Different media diameters

Answer: (C)

When sound hits a boundary between two media, whether part of it reflects depends on how similarly the two media respond to pressure, quantified by acoustic impedance. Acoustic impedance Z equals the product of density and sound speed (Z = ρc). If the impedances of the two media differ (Z1 ≠ Z2), the boundary cannot transfer the wave perfectly, so part of the energy is reflected back and part is transmitted forward. The greater the mismatch, the stronger the reflection. If the impedances are the same, the boundary presents no mismatch, and nearly all energy is transmitted with minimal reflection.

Temperature differences can change the speed of sound and density, thus affecting impedance, but they are not the primary condition for reflection by themselves. Different media diameters don’t determine reflection. Reflection can occur even at normal incidence; oblique incidence is not required for reflection to happen.