On the efficiency of noise barriers near sloped terrain - a numerical study

Abstract

A numerical finite-difference time-domain model of sound propagation over uneven, absorbing ground was used to simulate a selection of situations in an idealized two-dimensional cross-valley terrain profile with a valley bottom, a side slope and an elevated plateau. The distance of the source (e.g. a road on the valley bottom) from the foot of the slope and the steepness of the slope were varied. In addition, two different meteorological situations with thermally induced slope winds (upslope and downslope) were considered. The atmospheric fields (wind and temperature) were generated by precursory meteorological model runs and are consistent with the chosen topography. The power and spectral composition of the source was kept constant for all model runs. For computational reasons the source spectrum was limited to 100 – 1250 Hz. All situations were simulated with and without roadside noise barriers of uniform height and partly reflecting surfaces. The results show that the slope is exposed to sound levels that are up to 10 dB higher than for plane ground at the same range. Slope winds lead to a further variation of the sound levels on the slope and the plateau of up to 5 dB. The local insertion loss of roadside barriers depends on the steepness of the slope, the position of the source, the meteorological condition, and the properties of the barriers. The model predicts that the insertion loss is reduced over steep slopes, above all for a source position close to the foot of the slope. On the other hand a high insertion loss is predicted for nocturnal downwind situations and rather weak slope gradients.