Optimization of sound absorbing ceilings

The typical acoustical treatment in ordinary rooms such as classrooms, offices, conference rooms etc is an absorbing suspended ceiling. Due to the non-uniform distribution of the absorbing material the sound field at steady-state and during sound decay is far from the ideal conditions assumed in the diffuse field theory. Especially during the sound decay, the degeneration of the sound field emphasizes the shortcomings of the diffuse field assumption. On the other hand, the absorbing properties of sound absorbing ceilings are determined under diffuse field conditions according to the standard ISO 354. The outcome of this measurement in the form of an absorption coefficient is often used as input data in calculation or simulation models. This means that the absorbing properties of the ceilings are characterized under circumstances that are hardly ever encountered in practice. The directional scattering effect of furniture and other interior objects is another circumstance that has to be considered to understand the interaction between the absorbing ceiling and the sound field in rooms with ceiling treatment. In this paper the outline of a project is presented that deals with how to optimize the acoustical properties of ceilings in order to achieve certain room acoustic target values related to reverberation time, speech clarity and sound strength.