The concept of a diffuse sound field is widely used in the analysis of sound in enclosures. The diffuse sound field is generally described as composed of plane waves with random phases, which wave number vectors are uniformly distributed over all angles of incidence. In this study, an interpretation in the spatial frequency domain is discussed, with the prospect of evaluating the diffuse field conditions in non-anechoic enclosures. This work examines how theoretical considerations compare with experimental results obtained in rooms with various diffuse field conditions. In addition, the paper investigates how the results relate to the modal theory of room acoustics, based on the conception that any mode, also in non-rectangular rooms, can be expanded into a number of propagating waves.