Diffusion Coefficient

The diffusion coefficient (d) measures how uniformly a surface scatters reflected sound across different directions, ranging from 0 (all energy reflected in one direction) to 1 (energy equally distributed in all directions). Defined in ISO 17497-2:2012, it is measured in a large anechoic or hemi-anechoic space using a semicircular microphone array to capture the polar response of the reflected sound. Unlike the scattering coefficient (which measures total non-specular energy), the diffusion coefficient evaluates the directional uniformity of scattering. A surface can have a high scattering coefficient but low diffusion coefficient if it scatters energy into a few preferred directions rather than uniformly. The diffusion coefficient is particularly important for evaluating the performance of purpose-built diffusers (QRD, Schroeder, skyline types) and is used in concert hall and studio design where spatial uniformity of the reflected sound field is a design objective.