This part of ISO 17497 specifies a method of measuring the directional diffusion coefficient of surfaces. The diffusion coefficient characterizes the sound reflected from a surface in terms of the uniformity of the reflected polar distribution. The diffusion coefficient is a measure of quality designed to inform producers and users of surfaces that, either deliberately or accidentally, diffuse sound. It can also inform developers and users of geometric room acoustic models. The diffusion coefficient is not suitable for direct use as an input to current diffusion algorithms in geometric room acoustic models. This part of ISO 17497 details a free-field characterization method.