Being able to predict the reflected pressure from a diffuser enables efficient evaluation, design and characterization of the performance. Currently, this is usually done by considering the surface in the free field in isolation of other objects and boundaries. The prediction techniques could in theory be used to predict the sound for whole rooms or in outdoor environments such as street canyons (if the effects of weather are ignored). At the moment, however, long computation times and storage limitations mean that algorithms dealing with large spaces are forced to use relatively crude representations of actual scattering processes. Consequently, when predicting the responses in rooms and semi-enclosed spaces, such as street canyons or pavilions, it is more common to use geometric models. The issue of modelling scattering in geometric models is discussed in Chapter 12.