Several numerical methods are used to describe the effects of diffraction by two kinds of screens in a room: a perfectly rigid screen and a vibrating screen. This study has two main applications in acoustics: In room acoustics, the aim is to describe the influence of a vibrating screen on the acoustical characteristics of a room in which screens made of light materials are suspended from the ceiling; in structural acoustics, it is to describe the effect of a vibrating structure in a cavity. An application in active noise control is also presented. First, the sound pressure in the room is computed by using a boundary element method. Then, a perturbation method is used in the case of the vibrating screen. Comparisons are made between the two methods and a finite-element method for two typical geometries. The numerical examples presented correspond to a 2-D problem and a rectangular room. From the acoustics point of view, it is shown that the effect of a vibrating screen compared with a rigid screen is bigger at low frequencies and for light materials. This difference also depends on the acoustical properties of the room (i.e., the reflection coefficients on the walls).