The predicted effects of a barrier in an urban environment

Abstract

In an urban community, high-rise buildings can be found almost everywhere. They line up along both sides of a road. With traffic operating closely to the buildings, acoustic measures such as noise barriers are required so that noise sensitive receivers are protected from excessive noise. However, it is well-known that many phenomena for sound propagation outdoors in open space will be quite different from those observed in dense high-rise cities. In the present study, some typical situations of sound propagation in urban environments are investigated in details. A study of the facade effects due to the presence of an impedance ground has been investigated. Over the years, an empirical correction factor of 2.5 or 3 dB has been used for the calculation of noise levels 1 m in front of a reflecting facade. However, it has been shown in this study that when a noise source is located close to a facade (within 4 m from the facade surface) or a receiver position is raised high above an impedance ground (more than 4 m), the accuracy of using the correction factor is in doubt in the noise prediction schemes. From the findings, it is deduced that a modified and accurate prediction method is required in future. Investigation of the sound fields in front of a roadside barrier, which is an extension of the previous facade effect problem, has been conducted. The sound fields in front of a barrier have been studied when both the source and receiver are on the same side. Diffraction formed at the top edge of the barrier is found not to be significant to the overall sound field. However, from the simulation results, it is found that due to the diffraction effect, an addition of about 1 dB to the empirical correction factor of 2.5 or 3 dB is required when a listener is close to the barrier top edge at 1 m in front of it. The predicted barrier effects in the vicinity of building facades have been carried out. Distribution of sound fields between the facade and barrier is investigated. The effects due to an insertion of a barrier in front of a building are discussed. It is found that sound pressure level attenuation behind the barrier can reach to a level of 30 dB(A). The insertion loss degradation due to the erection of a building facade is examined. From the numerical simulations, a degradation of insertion loss about 5-8 dB(A) is found above the height of barrier and inside the shadow zone. The study concludes with the work on the acoustic performance of parallel barriers. Different sound field regions around the barriers are identified and discussed. The effect of double diffraction in parallel barriers is significant especially in the low frequency region. The insertion loss degradation due to the construction of a far side barrier is studied. The degradation is found to be the greatest along the line of reflection path formed by the source on the far side barrier surface, attaining a value of about 15 dB(A). A ray tracing technique and the Pierce model for diffracted sound fields are used as the main numerical model of this research. Indoor experiments in an anechoic chamber are conducted to validate the theoretical predictions. In the numerical analysis, a Boundary Element Method model is used to benchmark some of the numerical models proposed in this thesis. The aspects of sound propagation in a complex environment are elucidated in the present study. The configurations chosen are the ones that are normally found in urban situations. It is essential to understand how sound propagates in such an urban environment so that noise and its related problems can be considered and minimised at the planning stages of new developments. On the other hand, the knowledge of sound propagation can assist in the design of noise abatement measures which help to create a better acoustic environment.