In order to determine how to accurately predict the reverberation time and sound propagation (the variation of steady‐state level with distance from a source) in rooms, predictions have been compared with measurements for an empty scale‐model room and in various nominally empty factories. In the case of rooms with disproportionate dimensions, predictions by the method of images, which accounts for room shape but assumes specular surface reflection, were found to deviate in a consistent way from experiment. The reverberation time is always too high; the short‐distance sound propagation tends to be slightly low and the large‐distance sound propagation is always too high. This result suggests that diffuse surface reflections were occurring. To test this hypothesis, further predictions were mad.e using a ray‐tracing model that accounts for diffuse surface reflections. It is found that excellent prediction accuracy is obtained as follows: (a) scale model—if all surfaces are 10%–10% diffusely reflecting; (b) real factories—if the ceiling and walls are 70%–90% diffusely reflecting.