According to ISO 345, the random-incidence sound absorption coefficient is measured in a reverberation room by determining the different reverberation times with and without the absorber inserted. The sound field in the room is assumed to be diffuse. Thus, the corresponding energy decay curves would be strictly exponential, but turn out to represent multiple decay processes. In the modal concept of the sound field, this is assigned to the existence of groups of modes, which decay according to their specific damping.

In this thesis, the use of a multiple-input multiple-output system allows a directional, time-dependent analysis while varying the source directivity. Thereby, differently decaying groups of modes are qualitatively and quantitatively identified. They are in particular discussed in terms of their reverberation times and with respect to the resulting isotropy in the room, which is a fundamental property of a diffuse sound field. Furthermore, by optimizing the source directivity with respect to a maximum isotropy, its ability in weighting the groups of modes is shown.