Decay Processes in Rooms with Non-Diffuse Sound Fields Part I: Ceiling Treatment with Absorbing Material

The decay process in rooms with non-diffuse sound fields was the subject of this investigation. A rectangular room with high absorption at the ceiling and low absorption at the walls and floor constituted the basic configuration for the study. Several alterations of the room were carried out, including changes of room dimensions and the type of ceiling absorber, also additional treatment with absorbent material. The effects of these alterations on the decay curves were analysed. In order to predict the gross features of the decay curves, a two-system Statistical Energy Analysis model was developed. The sound field was subdivided into a grazing and nongrazing part. The grazing part comprised modes with propagation direction almost parallel to the ‘principal’ surface (generally the absorbent ceiling). The nongrazing part comprised the oblique modes, of importance for the decay process. The angle dependent absorbent properties of the principal surface were taken into account when subdividing the sound field into a grazing and nongrazing part. Measured and calculated decay curves for different room configurations were compared.