At a boundary between two layers with acoustic impedances Z1 (upper) and Z2 (lower), the normal-incidence reflection coefficient is R = (Z2−Z1)/(Z2+Z1), while the transmission coefficient (for amplitude) is T = 2Z1/(Z1+Z2) (or an analogous expression depending on convention). Interchanging Z1 and Z2 reverses the sign of the numerator in R (since Z2−Z1 becomes Z1−Z2 = −(Z2−Z1)), while the transmission coefficient formula's structure does not produce a sign reversal upon swapping, since both terms in its numerator and denominator remain positive quantities.
Reflection coefficient: R = (Z2−Z1)/(Z2+Z1). If Z1 and Z2 are interchanged, R' = (Z1−Z2)/(Z1+Z2) = −(Z2−Z1)/(Z1+Z2) = −R, i.e., the reflection coefficient exactly reverses sign...
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