Noise barriers are one of the most effective ways to mitigate road traffic noise. They are often found along highly trafficked roads close to urban areas. However, compared to other known ways to mitigate traffic noise, noise barriers are among the least cost-effective, i.e. less noise reduction is achieved per price unit. There is therefore a need to study noise barriers thoroughly and try to optimise their acoustic performance in order to achieve more noise reduction per invested unit of money. The present report reflects the first phase of the project “Optimised Noise Barriers”, which intends to develop better and more cost-effective noise barriers. The goal of this first phase was to investigate which types of noise barriers can be found on the market, what the international state of research in this area is and what could be the solutions to study and optimise in the next phase of the project. This report is therefore a state-of-the-art on optimised noise barriers. The introduction presents some basic physical principles of noise barriers. These are necessary to better understand how the sound behaves around noise barriers and also to identify where an optimisation of such barriers is possible. The first type of barrier to be presented is regular noise barriers, i.e. flat (or vertical) barriers. The next chapter presents noise barriers with improved performance. Usually these barriers are modified versions of the regular barrier. These barriers can be absorbing, tilted, dispersive or active. Some have specially formed tops such as random edges or forming a T-, L-, Y- or Q-shape. The last type of noise barrier presented is green noise barriers, i.e. made up of vegetation (earth berm, tree belt…). For each of the above mentioned barrier types, a summary of the study is presented and the barrier performance expressed in dB. The final chapter of this state-of-the-art report deals with remaining important considerations that have to be taken into account in order to benefit from any noise barrier. These are for example the location of the barrier, its length, its cost and the influence of the atmospheric conditions and the neighbours’ perception of the barrier. In conclusion different types of noise barriers are found suitable for further research and full scale testing in Denmark. These are barriers with T-top, random-edge top and Watts type (three-panel top). A suggestion is given for the procedure to follow in order to test the different noise barriers. This includes finding a location where the noise barrier will be tested in full scale, simulating the sound propagation with the help of a software (possibly using the boundary element method), and choosing which of the different barrier shapes provides the best noise reduction for that specific location. Following this first phase in the project is to determine a location in Denmark where noise barriers with modified top can be tested. (Author/publisher)
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