Reflection and diffraction in room-acoustic modeling

Abstract

Many programs for room-acoustic modeling are based on the mirror image source concept. This way, only specular reflections are taken into account such that the resulting wave field is incomplete. Nonspecular reflections, or diffraction phenomena, can be added using an algorithm based on Pierce’s theory. Then, the true wave field is significantly better approximated. However, comparison of both results reveals that, averaged over time and space, the energy is equal in both cases: the added diffraction field so strongly interferes with (i.e., is so strongly correlated with) the specular reflections that no energy is added. This intuitively surprising result can be argued with a thought experiment. In this context, taking into account that acoustic perception is described with energetic criteria (clarity, definition, lateral energy fraction, etc.), it should be critically reconsidered as to how far the physically incomplete mirror image source programs are adequate for the prediction of the global acoustic quality of a hall.