Add-on elements for noise barriers offer an easy possibility to improve the acoustic efficiency of existing barriers. Using such elements leads to an additional sound level reduction due to the increased height of the barrier, on the other hand special geometries or absorbing materials allow generating enhanced insertion losses. In the framework of this study the level reduction induced by a so-called “noise spoiler” made of synthetic foam was determined and compared to the effect obtained by simply increasing the barrier height accordingly. For this purpose, model measurements in the acoustic laboratory, field measurements in the near and far field and numeric simulations of the sound propagation across the modified edge were carried out. Moreover, the durability of the absorber material against heat, cold and UV radiation is assessed. The material test with the impedance measuring tube after exposure to heat, cold and UV radiation reveals that the acoustic properties are largely preserved. No significant decrease of the degree of absorption can be detected. Regarding the acoustic efficiency the measurements illustrate that the considered add-on elements have no significant acoustic benefit compared to an equivalent elevation of the noise barrier. Values below 1 dB for the acoustic efficiency are obtained when the measurements are carried out according to the standards. When comparing the results to the case of a (lower) barrier without add-on, the installation of the “noise spoiler” yields a sound level reduction of about 3 dB – 4 dB. The numeric simulations essentially confirm the measurement results. Further analyses additionally allow separating the different influences on the sound propagation: While the absorber material itself has a positive impact on the acoustic efficiency, the geometry of the add-on elements seems to be detrimental for the achievable insertion losses. The investigation shows that, in principle, the modification of the diffraction edge has the potential for additional noise reduction. The multi-methodological evaluation allows a fair evaluation of the acoustic efficiency and will serve as valuable input for future optimization and realization processes. (Author/publisher)
Samenvatting