The ratio of the amplitude of a transmitted ray (A2) to that of the incident ray (A0), such that T = A2/A0. In the case of normal incidence, and in terms of the acoustic impedances (Z1 and Z2) of the two media above and below the boundary, T = 2Z1/(Z2 + Z1). Also, T = 1 – R, where R is the reflection coefficient. The transmission coefficient can also be expressed in terms of energy (T’), when T’ = 4Z1Z2/(Z2 + Z1)2.