Noise barriers in a complex urban environment

Abstract

With the rapid advancement of civilization and tremendous expansion of human population, transportation noise is one of the major environmental issues which have to be tackled. To reduce the impact from the noise pollution, various mitigation measures have been investigated and implemented. Roadside barriers are the most commonly mitigation measures adopted. By alternating the propagation of sound, the barriers manage to significantly attenuate the noise levels in the protected regions. Numerical studies on the shielding performance of the barriers have been conducted. However, there are relatively few studies taking the influence of tall buildings in urban areas into consideration to account for the efficiency of the barrier. In our study, an investigation of the acoustic performance of noise barriers in conditions commonly found in metropolitan areas is conducted. Our study begins with the development of a measurement methodology for the acoustic impedance of a plane surface. In contrast to those standardized laboratory method, the methodology developed is in-situ in nature. The method is based on the classical spherical wave propagation theory and numerical minimization techniques. Actual quantity of impedance is obtained at the location of measurement. With the improved measurement techniques and minimization algorithm, the in-situ method developed is proved to be robust and successful. Noise prediction model founded on the image source method for the evaluation of barriers in an urban environment is then developed. The first scenario examined consists of a pair of parallel barriers standing aside a road which is aligned parallel to a building facade. Based on the image source method, a prediction model of the sound field of this configuration is developed. The flexibility of adopting acoustic absorptive surface is included in the model for more general applications. For the validation of the model, the predicted results are compared with the results from the boundary element method (BEM) and experimental measurement. These comparisons conclude good agreements and the accuracy of the image source model is proved. With the success of the previous scenario, the image source model is further extended to another commonly encountered condition in urban environment. The situation taking into consideration consists of a pair of parallel barriers standing aside a road which is semi-enclosed by sky-rise buildings on both sides. Due to the additional facade surface, the multiple reflections of sound among the surfaces are much more complicated in which the complexity is almost double that of the previous case. Nevertheless, a study of the sound field in this spatial condition is processed and a complete hierarchy of the source images is outlined. In comparisons with the BEM and the laboratory measurements, the image source model demonstrates a fairly good validity. These two computationally efficient image source models successfully manage to provide a quantitative justification of the construction of roadside barriers in the planning stage.