Pressure-amplitude transmission coefficient ($t_p=2Z_2/(Z_1+Z_2)$)

The transmitted-to-incident compressional pressure or normal-stress amplitude ratio. At normal incidence, continuity of normal stress gives $t_p=1+r_p$, with pressure-amplitude reflection coefficient $r_p$. The transmitted energy fraction is $(Z_1/Z_2)t_p^2$, not $t_p^2$ alone.