Acoustic reflections arise from impedance mismatches between neighboring medium cells

Evidence

If a cell pushes back too hard (higher impedance than its neighbor), then the neighbor doing the pushing will recoil, pushing back on its neighbor on the opposite side, causing a positive pressure pulse to propagate backward—a reflection. If the adjacent cell, on the other hand, pushes back too feebly (lower impedance than its neighbor), then the pushing neighbor will keep moving toward the weak neighbor, ultimately pulling on its neighbor on the opposite side. That neighbor pulls in turn on its neighbor on the opposite side, and so on. A rarefaction is propagating back toward the source. A positive pressure fluctuation will thus partially reflect back as a negative one if it meets reduced impedance. If the adjacent cells are impedance matched, each pushes back just enough so as not to reflect any of the pulse. — Eric J. Heller, Why You Hear What You Hear- An…, 97

Categorization

• DEFINE: Explains reflection and rarefaction as direct mechanical consequences of impedance mismatch, not just abstract coefficients.