Limitations of the scattering coefficient for simulating early reflections

The introduction of the scattering coefficient has enabled geometrically-based acoustic simulations to produce full-length impulse responses of fairly high accuracy. However, the scattering coefficient is a random-incidence quantity that can be applied only when considering a statistical energy average over different directions. This model is inaccurate if a single reflection by a surface element is considered for which the scattered part of the sound energy depends strongly on the angle of incidence. The resulting deviation can be particularly relevant for first-order reflections as these reflections have significant influence on acoustic parameters that make use of early-energy estimates, such as D, C80 or LF. In this paper, the uncertainty of the specularly reflected energy is estimated and discussed for a number of typical surface structures and scenarios. The corresponding impact on the accuracy of predicted early-energy parameters is also discussed. Possible solutions and extensions such as an angle-dependent scattering model are outlined.