Coherent Prediction And Spatial Distribution Of Traffic Noise In Urban Canyon

As a typical high-density urban morphology,urban canyon is formed by urban expressways and continuous distribution of street-facing buildings.Large portion of the buildings expose to the traffic noise in the urban canyon,and the noise levels do not decrease significantly with the increase of the vertical height and the horizontal distance.Among the ray-based method,incoherent image source method and ray-tracing technique leave out phase and interference effects and make large deviations in the calculation of the spatial distribution of acoustic field.Based on coherent image source method,this paper proposes an accurate and rapid computational model for predicting acoustic field in the urban canyon to solve the problem.In addition,the spatial distribution characteristics of traffic noise in the urban canyon are studied.The paper focuses on the following three aspects:Firstly,the urban canyon is simplified into a geometric model with vertical building wall and horizontal road plane in theory.Based on an accurate coherent image source method proposed by Min et al.,a coherent computational model is proposed in the urban canyon with absorptive boundaries.Using modal expansion technique,beam-tracing method and coherent image source method,a 15 m wide and 30 m high urban canyon model is calculated under the complete absorptive sky conditions.The spatial distribution characteristics are predicted in the case of single vertical high absorptive boundary and two vertical high absorptive boundaries.The comparison with the existing ray-based computational models shows that the proposed coherent computational model has high prediction accuracy for acoustic field distribution in the urban canyon.Next,the prediction accuracy of proposed coherent computational model is verified by the 1:50 scale model experiment.The experiment designs a 300 mm wide and 400 mm high urban canyon scale model with a rigid pavement and two vertical high absorptive boundaries.The scaled dodecahedron spherical source is used as modeled source,and 10 k Hz corresponding to the full scale of 200 Hz is selected as the frequency of the scale test.The horizontal relative attenuation curves of varying height planes(110mm,150 mm,210mm)were measured.Experimental results and error analysis show that the proposed coherent computational model is in good agreement with the measured results.Finally,in view of acoustic design parameters and geometric design parameters of urban canyon,the spatial distribution characteristics and noise reduction strategies are analyzed based on the proposed coherent computational model.The results show that the local noise reduction can be achieved by adjusting the sound absorption coefficient of the building exterior surfaces and the height to width ratio of the canyon.This study can provide scientific support for improving the acoustic environment quality of urban blocks in combination with urban planning.