Hydraulic performance of Low Crested Structures have been widely studied both theoretically and through experimental analyses; yet, only few indications are available on the interaction between the main transmission modes (overtopping, filtration) and wave diffraction. Starting from the analysis of hydraulic model tests conducted at the 3D wave basin of Delft University of Technology, the paper investigates the spatial distribution of the transmitted wave energy due to both wave overtopping and diffraction, and discusses the corresponding change of the power spectrum functions