On the prediction of sound fields in large empty rooms

This article discusses the prediction of sound propagation and sound decay in large nominally empty rooms of parallelepipedic shapes. The results of sound propagation and sound decay measurements made in an empty 1:50 scale model in 12 shape configurations, and in 14 parallelepipedic, nominally empty, full-size gymnasiums and factories, are compared with predictions by the Eyring and method-of-image models. The Eyring model is shown only to be accurate for quasicubic rooms; the method of images is shown to account for arbitrary shape. Using method-of-image prediction, the diffuse-field absorption coefficient or that derived from the measured sound decay using the Eyring formula, is found to give, within the limitations of the prediction model and experimental uncertainties, a best fit with the experimental results. In the case of the scale model, improved prediction accuracy is obtained when the angular variation of the surface absorption, this due to the surface boundary layer and porosity, is considered. The results further suggest that significant volume and/or surface scattering occurred in the test rooms, and that this must be considered in prediction. It is alternatively hypothesized that it may be necessary to consider wave sphericity in order to obtain accurate predictions.