Efficiency factors characterizing sound reflection properties of a room ceiling

The shape of a room ceiling influences acoustics of the room dramatically. Many books give recommendations on the ceiling geometry, which has to be designed to reflect sound to the rear of the room, or to diffuse it throughout the room. But any quantitative parameters for characterizing efficiency of sound reflection from the ceiling are not applied in practice. It would be useful to specify the room ceiling by any parameters describing its reflection properties. For this purpose two efficiency factors in framework of geometrical acoustics are proposed here. The first factor is a ratio of the sound energy reflected from the ceiling toward the seats to the sound energy incident on the ceiling. The maximum value of the ratio equals 1. However, the maximum value of the ratio does not always correspond with a good ceiling shape. For this reason the second factor is also used, it is a ratio of the sound energy reflected from the ceiling towards seats to sound energy radiated by a source into the room. Proposed efficiency factors are analyzed for four classic concert halls with excellent acoustics. It provides preliminary values of the factors suitable for evaluation of acoustic properties of the ceilings.