The prediction of noise propagation in street canyons and tunnels

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The prediction of noise propagation in street canyons and tunnels


Author: Iu, King-kwong
Title: The prediction of noise propagation in street canyons and tunnels
Degree: M.Phil.
Year: 2003
Subject: Hong Kong Polytechnic University -- Dissertations
Canyons -- Noise -- Forecasting
Tunnels -- Noise -- Forecasting
Department: Dept. of Mechanical Engineering
Pages: iii, 196 leaves : ill. ; 30 cm
Language: English
InnoPac Record:
Abstract: The first part of my study addresses an important problem of predicting sound propagation in narrow street canyons with width less than 10 m, which are commonly found in a built-up urban district. Major noise sources are, for example, air conditioners installed on building facades and powered mechanical equipment for repair and construction work. Interference effects due to multiple reflections from building facades and ground surfaces are important contributions in these complex environments. Although the studies of sound transmission in urban areas can be traced back to as early as the 1960s, the resulting mathematical and numerical models are still unable to predict sound fields accurately in city streets. This is understandable because sound propagation in city streets involves many intriguing phenomena such as reflections and scattering at the building facades, diffusion effects due to recessions and protrusions of building surfaces, geometric spreading and atmospheric absorption. The development of a numerical model for the prediction of sound fields in city streets is described in this thesis. To simplify the problem, a typical city street is represented by two parallel reflecting walls and a flat impedance ground. The numerical model is based on a simple ray theory that takes account of multiple reflections from the building facades. The sound fields due to the point source and its images are summed coherently such that mutual interference effects between contributing rays can be included in the analysis. In the second part of the thesis, attention is focused on the study of sound propagation in rectangular long enclosures with geometrically reflecting boundaries both experimentally and theoretically. The current study has also extended to cover the long enclosures with impedance boundaries. Although sound attenuation and reverberant sound fields in long enclosures have been investigated in different ways for several decades, the resulting mathematical and numerical models are still relatively unsatisfactory. The image source method is often used to predict the attenuation of sound pressure levels in long enclosures in many previous studies. An infinite series of image sources is used to model the situation of multiple reflections in long enclosures. Each of these image sources is assumed to radiate sound independently. The total sound field is computed by summing the contribution from all image sources incoherently. The interference effects of the sound fields generated by all sources, which have not been modelled elsewhere, are addressed in this thesis. A numerical model has been developed which is an extension of the numerical model formulated in the first part of my study. The sound fields due to all image sources are summed coherently for predicting the attenuation of sound in long enclosures that takes into account of multiple reflections from the boundary walls. One of the applications of the developed coherent model is to predict the propagation of low frequency noise emitted from ventilation fans in the tunnels. Scale model experiments are conducted and measurement data are compared with theoretical predictions to establish the validity and usefulness of the proposed numerical models. Outdoor field measurements have also been conducted in a real built up street canyon and two road tunnels for the validation of the numerical model. The comparisons have shown that the predictions using coherent model agree reasonable well with the field data at all frequencies, with accuracy within 3 dB in most cases. While the agreements between the field data and the theoretical predictions using either the incoherent model or ASJ Prediction Model are less satisfactory, the variance with the measurement results are up to 7 dB at low frequency bands.

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