Quantitative relationships are derived between the dynamic macromechanical stiffness and microparameters of planar interfaces containing distributed cracks. The derivation is based on the solution of the problem of elastic wave reflection by a plane with a continuous distribution of springs to model the cracked interface at the macrolevel. The dynamic spring stiffness is then, through averaging, related to crack-opening volumes and other microparameters. For linear springs and periodic crack distributions, numerical examples are presented for plain strain. The stiffness is shown to strongly depend on frequency.