Noise reduction potential of absorptive low height noise screens and trench like segments; estimating insertion loss from urban traffic scenarios

Abstract

Urban planning is facing growing difficulties due to increasing traffic noise in the cities. Low-height noise screens (LHNS) could be considered to be implemented instead of the common tall noise screens as noise reducing devices (NRD). An investigation of implementing multiple absorptive LHNS combined with trench-like road segments, i.e. as effective NRD in urban area was conducted. Trench-like segments are defined as strips adjacent to the road surface filled with sound-absorbing ground materials. In order to evaluate the effect of multiple absorptive LHNS and trench-like segments, three traffic cases were defined and evaluated. The absorptive LHNS, with a height of 1.1 meters, were tested for six types of absorbing materials and for two variations of trench-like segments. Two noise prediction tools; Boundary Element Method (BEM) combined with analytical calculations and a commercial noise mapping software SoundPLAN using the Nord2000 model, were applied to obtain the insertion loss in terms of A-weighted equivalent levels (IL) for the different cases. The IL was evaluated for two different heights, 1.5 and 4 meters, as well as for three distances, 7.5, 15 and 30 meters. One case was applied to a realistic urban scenario at Gibraltarvallen in SoundPLAN. The results from both BEM and SoundPLAN show that absorptive LHNS materials, such as Vitrumit, mineral wool and Wood wool cement board, consistently provide higher IL than reflective hard screens across all traffic cases. Meanwhile, trench-like segments, regardless of tested ground materials, have little to no effect on the IL. For all three cases, removing the trench, enabling a narrower placement of the screens, was found to improve the performance of absorptive LHNS. The BEM results show a highest IL of 14.6 dB at the 1.5 meter receiver height for one of the traffic cases, while SoundPLAN yields 7.5 dB for the same case. At the 4 meter receiver position, BEM shows a highest IL of 12.5 dB while SoundPLAN gives 10.6 dB for the same case. BEM consistently displays higher insertion loss than SoundPLAN across all cases, indicating that BEM predicts a stronger noise reduction effect. Insertion loss for maximum levels were also obtained for SoundPLAN, however since no clear trends were found, no further investigation was conducted. The case applied at Gibraltarvallen consisting of two absorptive LHNS on either side of the road, showed an IL of 5–8 dB on the ground floor, 2–4 dB on the middle floors and, 0–1 dB on the top floors. Two LHNS showed an improvement of 1 dB compared to a single LHNS. The sound propagation in the area shows that placing multiple absorptive LHNS along the road reduces the A-weighted equivalent sound level by up to 10 dB