Do we still need diffuse field theory?

More than twenty years after Murray Hodgson’s “When is diffuse field theory applicable?” paper we have gathered more and more evidence that diffuse field theory is mostly a chimera. If we consider the two most important implications of the diffuse field model, i.e. uniform distribution of sound pressure level and reverberation time invariance, it is quite easy to say that such conditions are hardly ever found, based on actual measurements in a number of different spaces. Ideal sound diffusion requires ergodic and mixing conditions, which do not necessarily occur, particularly when sound absorption is unevenly distributed or rooms are not proportionate. Thus, apparently, diffuse field theory might be dismissed in favour of more accurate approaches capable of taking into account the specific nature of each space. Nowadays we have several instruments spanning from the many variations of the ray-tracing algorithm to the numerical solution of the wave equation. However, such methods rely on the measurement or estimation of other coefficients that, if not properly made, may introduce even greater inaccuracies. A critical analysis is presented here, mostly based on the author’s research experience, showing that diffuse field theory still represents an important way to understand sound propagation in enclosed spaces.