It is well known that partial contact of two rough surfaces leads to transmission of an acoustic signal across the crack, thus giving rise to a reduced probability of detection (POD). To explore the effects of such partial contact on transmission, diffraction, scattering and mode conversion of an acoustic signal, samples have been developed in which an interface simulates a true fatigue crack. Some of the effects of these interfaces on a longitudinal acoustic wave have been studied experimentally and will be reported.