The noise reduction of a (low) barrier can be enhanced by using an additional element with quarter-wavelength resonators with varying depths. A so-called Whiswall or WHIStop deflects sound upwards for specific frequencies. Measurements for a 1.1 meter high Whiswall and for a 1.1m barrier are compared in a separate paper. The enhanced barrier effect is measured at a short distance behind the barrier, for several situations. In this paper these measurements are compared with the results of a numerical finite element model (FEM) to validate this model. Next, the noise reduction is calculated at long ranges, up to 600 meters, for different point-to-point scenarios representative for road and rail traffic. A numerical parabolic equation method (PE) is coupled to the FEM model and a representative downwind condition is taken into account. The results at longer distance are used to design an engineering method for the enhanced barrier effect that can be used in standard noise calculation models, such as the Dutch national calculation model (SRM2) or the ISO 9613-2 standard.