Retro-diffraction - importance for the calculation of road traffic noise

In geometric acoustics, "retro-diffraction" describes the loss of energy when reflected at the upper edges of noise screens. By bending the sound beam against the direction of propagation under consideration, reflected energy of the mirror sound source is reduced by up to 5 dB in addition to the absorption. This effect becomes particularly effective if the sound beam is blocked by a noise screen in the direction of propagation under consideration, meaning that diffraction also occurs in this direction. Sound attenuation by retro-diffraction can be found in the French standard NF S 31-133, an alternative method to ISO 9613-2 for calculating sound propagation outdoors. It was incorporated into the French noise forecasting procedure NMPB in 2008. We also take it into account by using it in the procedure for calculating environmental noise (European: CNOSSOS; German: BUB). This paper attempts to determine the impact of considering retro-diffraction in sound propagation. First, the equations from ISO 9613-2 and BUB regarding retro diffraction are compared in a simple geometric model. The level differences calculated in commercial sound propagation software are compared with those from measurements on a 1:20 scale model. An estimate of the relevance of retro-diffraction in calculating sound propagation is given.