Urban background noise mapping: The multiple-reflection correction term

Mapping of road traffic noise in urban areas according to standardized engineering calculation methods systematically results in an underestimation of noise levels at areas shielded from direct exposure to noise, such as inner yards. In most engineering methods, road traffic lanes are represented by point sources and noise levels are computed utilizing point-to-point propagation paths. For a better prediction of noise levels in shielded urban areas, an extension of engineering methods by an attenuation term Acan has been proposed, including multiple reflections of the urban environment both in the source and in the receiver area. The present work has two main contributions for the ease of computing Acan. Firstly, it is shown by numerical calculations that Acan may be divided into independent source and receiver environment terms, As and Ar. Based on an equivalent free field analogy, the distance dependence of these terms may moreover be expressed analytically. Secondly, an analytical expression is proposed to compute As and Ar for 3D configurations from using 2D configurations only. The expression includes dependence of the street width-to-height ratio, the difference in building heights and the percentage of facade openings in the horizontal plane. For the expression to be valid, the source should be separated from the receiver environment by at least four times the street width. © S. Hirzel Verlag EAA

Reduction of the performance of a noise screen due to screen-induced wind-speed gradients: numerical computations and wind-tunnel experiments

Downwind sound propagation over a noise screen is investigated by numerical computations and scale model experiments in a wind tunnel. For the computations, the parabolic equation method is used, with a range-dependent sound-speed profile based on wind-speed profiles measured in the wind tunnel and wind-speed profiles computed with computational fluid dynamics (CFD). It is found that large screen-induced wind-speed gradients in the region behind the screen are responsible for a considerable reduction of the performance of the screen, for receivers near the boundary of the shadow region behind the screen. The screen-induced wind-speed gradients cause a considerable reduction of the size of the shadow region. If the screen-induced wind-speed gradients are taken into account, computed sound-pressure levels near the shadow boundary are in reasonable agreement with levels measured in the wind tunnel. In contrast, computed levels are considerably lower, up to 10 dB, if the screen-induced wind-speed gradients are ignored. This implies that the performance of a screen can be considerably improved if the screen-induced wind-speed gradients can be suppressed, e.g., by the use of 'vented' screens. The reduction of screen attenuation by RESWING (refraction by screen-induced wind speed gradients) for receivers is studied. The sound propagation over a screen in a refracting atmosphere is computed by parabolic equation method. The wind-speed profiles measured in the wind tunnel and wind-speed profiles are computed by computational fluid dynamics. The computer sound-pressure levels in the shadow of the screen are in good agreement with the measured levels

Caustic diffraction fields in a downward refracting atmosphere

A fast and accurate ray model is developed for sound propagation in a downward refracting atmosphere. The model employs a new approach to the classification and computation of ray paths and caustic curves. The approach is valid for a large set of smooth sound-speed profiles, including realistic, nonlinear profiles such as the logarithmic profile. An ordered series of rays and caustics is found in this case, including caustics with a cusp. The sound-pressure field is computed by combining geometrical acoustics and the theory of caustics. The field on the illuminated side of a caustic is computed by modifying the geometrical-acoustics solution. The field on the shadow side of a caustic, i.e., the caustic diffraction field, is computed by extrapolation of various quantities into the shadow region. Different approaches to this extrapolation are considered. It is found that horizontal extrapolation gives the best results. The accuracy of the ray model is demonstrated by comparison with numerical solutions of the one-way wave equation. If caustic diffraction fields are ignored, discontinuities of more than 10 dB occur in the sound-pressure field

Urban traffic noise and the relation to urban density, form, and traffic elasticity

Traffic noise in cities has serious effects on the inhabitants. Well-known effects are annoyance and sleep disturbance, but long-term health effects such as cardiovascular disease have also been related to traffic noise. The spatial distribution of traffic noise in a city is related to the distributions of traffic volume and urban density, and also to urban form. This relation is investigated by means of numerical calculations for two cities, Amsterdam and Rotterdam, and for various idealized urban fabrics. The concept of urban traffic elasticity is introduced to relate local population density to local vehicle kilometers driven on the urban road network. The concept of Spacematrix is used to represent urban density and urban form. For the two cities it is found that the average sound level in an urban area decreases with increasing population and building density. The results for idealized urban fabrics show that the shape of buildings blocks has a large effect on the sound level at the least-exposed façade (quiet façade) of a building, and a smaller effect on the sound level at the most-exposed façade. Sound levels at quiet facades are in general lower for closed building blocks than for open blocks such as strips. © 2012 Elsevier B.V

Evaluation of exposure to traffic noise in an urban recreational area

Environmental noise annoyance due to transportation noise in the home environment has been widely studied, and exposure-response relationships have been established previously for the expected percentage of annoyed residents with the most exposed façade exposure level as a determinant. However, relatively little is known about the evaluation of transportation noise when residents are residing outdoors, for instance when seeking relaxation or restoration in urban recreational areas. In an urban park in the Netherlands, 52 participants walked either in an area with a high level of road trafftc noise due to a nearby highway, or in a more quiet area further away from the highway. Noise exposure was individually monitored during the walk, and the evaluation of the acoustic environment was assessed immediately after the walk. The individual variance in exposure (L,q",r, Lto and Ls5) was high enough to derive exposure-response relationships for annoyance by road traffic noise, interference with experience of natural quiet, perceived quietness and perceived soundscape quality. The results may have implications for urban planning concerning levels of transportation noise in outdoor urban recreational areas

Engineering modeling of traffic noise in shielded areas in cities

A computational study of road traffic noise in cities is presented. Based on numerical boundary-element calculations of canyon-to-canyon propagation, an efficient engineering algorithm is developed to calculate the effect of multiple reflections in street canyons. The algorithm is supported by a room-acoustical analysis of the reverberant sound fields in the source and receiver canyons. Using the algorithm, a simple model for traffic noise in cities is developed. Noise maps and exposure distributions of the city of Amsterdam are calculated with the model, and for comparison also with an engineering model that is currently used for traffic noise impact assessments in cities. Considerable differences between the two model predictions are found for shielded buildings with day-evening-night levels of 40-60 dB at the facades. Further, an analysis is presented of level differences between the most and the least exposed facades of buildings. Large level differences are found for buildings directly exposed to traffic noise from nearby roads. It is shown that by a redistribution of traffic flow around these buildings, one can achieve low sound levels at quiet sides and a corresponding reduction in the percentage of highly annoyed inhabitants from typically 23% to 18%. © 2009 Acoustical Society of America