Previous experimental work has shown the influence of a rotating diffuser upon the diffusion of a reverberant field, upon the radiation impedance of a sound source in a reverberation room, and upon the boundary absorption. Some interesting results of the latter work require a quantitative interpretation, while others suggest a need for further investigation to obtain a detailed physical insight. In an effort to gain insight into and an interpretation of the latter work, a finite‐element approach is presented. This paper is an interim report on continuing research, and it reviews an investigation of the effect of a semicircular diffuser upon a two‐dimensional sound field in a rectangular room. A numerical relationship between the resonance frequencies of the acoustical modes in the room and the diffuser orientation is given. The sound‐pressure distributions of the acoustical modes are also given as the function of the diffuser orientation. A three‐dimensional cavity is used for experimental verification. In this cavity, only the sound waves in the horizontal plane will be affected by the diffuser. The experimental and numerical results agree closely.