Sound propagation over irregular terrain with complex meteorological effects using the parabolic equation model

Sound impact of road and railway infrastructures are more and more severely regulated by European laws: acceptable thresholds in emission and reception are decreasing. This implies to develop propagation models able to take many phenomena into account at the same time (meteorology, uneven ground, impedances discontinuities...). The parabolic equation (PE) is one of those numerical methods. Its main purpose is to predict long-range sound propagation under range-dependant environment. Despite of its efficiency, this method shows a number of limitations in complex outdoor situations. This paper aims at presenting ATMOS (Advanced Theorical Models for Outdoor Sound propagation). This GFPE (Green’s Function Parabolic Equation method) based calculation code is dedicated to complex outdoor situations which can not be solve with a classical PE approach. Usually PE neglects backscattering and complex topography can not be considered. ATMOS takes those phenomena into account by new several techniques: complementary Kirchhoff approximation, GFPE-BEM hybrid method, referential rotation. The atmosphere properties are included into the range dependant sound speed profile, integrating atmospheric attenuation, density fluctuation, wind, temperature gradients and turbulence… All those parameters may be varying with height and distance along the propagation. Numerical examples of road traffic configurations that illustrate those combined effects are presented and compared to scale model measurements

Simple and multi-reflections using the PE method with a complementary Kirchhoff approximation

Sound impact of road and railway infrastructures are more and more severely regulated by European laws: acceptable thresholds in emission and reception are decreasing. This implies to develop propagation models able to take many phenomena into account at the same time (meteorology, uneven ground, impedances discontinuities...). The parabolic equation (PE) is one of the numerical methods used for sound propagation simulation in complex outdoor situations. It neglects backscattering. Even if this assumption is effective in many configurations, it does not allow to use PE for studies of acoustic wave propagation between a source and a receiver when an obstacle (rigid barrier, building) is located just before the source, or just behind the receiver. In those cases, energy reflected by obstacle is not negligible and results obtained with PE may be incorrect. This paper aims at presenting a new method able to integrate backscattering in GFPE (Green’s Function Parabolic Equation method). In this approach a complementary Kirchhoff approximation is used by setting to zero the sound pressure above the vertical obstacle. Thus, new configuration as the multi-reflections can be studied with this new method. In order to point out the role played by backscattering, we first study a barrier located just behind a source. Then, comparison with BEM (Boundary Element Method) calculations is presented in the case of a simple reflection in homogeneous and inhomogeneous atmosphere. A more complex road traffic noise configuration made with two parallel barriers and meteorological effects is also studied. Results show that the complementary Kirchhoff approach seems to be promising

Outdoor sound propagation: comparisons between calculations performed with atmos, a pe-based model, and wind tunnel experiments

Noise impact of road and railway infrastructures are more and more severely regulated by national laws: acceptable thresholds and reception levels are decreasing. It becomes necessary to predict more and more finely meteorology and its interactions with boundaries effects in current sound prediction models. ATMOS (Advanced Theoretical Models for Outdoor Sound propagation), a PE (Parabolic Equation) based calculation code dedicated to complex outdoor situations, has been developed to fulfil this need. In order to validate it, a measurement campaign has been performed in the wind tunnel of CSTB, Nantes (France). Such measurements present many advantages compared to outdoor experimentations. The main one is the possibility to control precisely many parameters such as temperature, wind speed profile and wind direction. Aerodynamic measurements as well as computational fluid dynamic simulations with FLUENT have also been undertaken in parallel to acoustical studies. Their results have been used to perform excess attenuation calculations with ATMOS. Comparisons between measurements and numerical simulations for realistic complex traffic noise configurations are presented here for a few cases (flat ground, impedance jump, noise barrier, embankment)

Comparison of low-frequency background noise cancellation systems for room acoustics

Low frequency background noise (below 200 Hz) is perceptible in many situations such as in room acoustics or industrials issues. Generate by numerous type of source (traffic noise, aircraft, industrial plants, electricity transformer,… ), this type of noise is under evaluate by regulations laws due to the used of the A-weighted decibel (dB(A)). In spite of that low frequency background noise can reach high levels which produce poor speech intelligibility and annoyance feeling. Unfortunately traditional acoustic treatments have little impact to decrease this kind of disturbance. New approaches have to be developed to give new efficient techniques in order to overcome the problem. Three noise cancellation systems dedicated to low frequencies are studied here. The first one is based on an electromechanical transducers loaded passively to get an optimal damping around the resonance frequency of the disposal. In the second system the passive load is substituted by an active control to enhance acoustic properties of device. The last strategy consists on a control of the first modal frequencies of a room to decrease the low frequency background noise level. This paper aims at presenting those three noise cancelation systems. Their advantages and drawbacks will also be discussed

Une approche hybride pour la propagation du son en milieu extérieur complexe

In this paper a new hybrid method which allows coupling between different complex outdoor sound propagation models is presented. The aim is to develop a powerful tool able to take complex topographies and range dependant meteorological profile into account. The work is achieved by coupling two outdoor propagation models and using the advantages of each of them for a given setup

Wind tunnel experiments for the validation of numerical models for outdoor sound propagation. in Inter-noise

Since regulations concerning traffic noise are becoming more and more demanding, meteorological effects (turbulence, temperature and wind speed gradients) can no more be neglected in noise propagation prediction at long ranges. To validate numerical models such as Parabolic Equation approach or Boundary Elements Methods with modified Green’s functions, it is of high interest to collect experimental data obtained under controlled atmosphere. A measurement campaign has been performed in the wind tunnel of CSTB, Nantes (France). The objective of this experiment was to characterize the aerodynamic flow and the acoustic pressure during the sound propagation. A number of geometrical configurations (flat ground, embankment, with or without complex noise barrier, the ground surface being absorbing or not) and various wind profiles and turbulence intensities have been tested. A combination of traditional (hot wire probe) and recent (Particle Image Velocimetry, PIV) measurement techniques has been used in order to describe precisely the 2D wind speed field along the propagation as well as in the vicinity of the barrier where recirculation phenomenon occurs. For acousticians, experiments in wind tunnel are a good mean to improve their knowledge on outdoor sound propagation and to develop novel barrier shapes

Accuracy of outdoor sound propagation prediction in a complex environment using some reference numerical models

The paper presents principles which can be used in reference numerical models to make easy calculations for predicting long-range outdoor sound propagation under complex environment. Limits, assumptions as well as approximations used are discussed here in terms of accuracy for typical road traffic configurations, depending on range of frequency, geometry of the site and atmospheric conditions. Part of this work has been achieved during the European Project Harmonoise

Coupling BEM and GFPE for complex outdoor sound propagation

In complex outdoor sound propagation meteorological effects as well as various absorbing properties and shapes of the boundaries need to be accounted for. This paper presents a hybrid GFPE-BEM method relying on the power of the BEM near obstacles and uneven topographies in order to compute the starting field which is then propagated thanks to the GFPE. The approach is firstly described. Then some numerical results are given for typical road traffic noise configurations with and without meteorological effects. The results show that this hybrid GFPE-BEM model can predict accurately the sound pressure levels in complex outdoor configurations

Propagation acoustique en milieu extérieur : Application de l'équation parabolique rapide au couplage d'effets météorologiques et de topographies complexes Outdoor Sound Propagation : Application of the Green's Function Parabolic Equation to Study Coupling Effects of Meteorology and Complex Topographies

Noise impact of road and railway infrastructures are more and more severely regulated by national laws: acceptable thresholds are decreasing. This implies noise prediction for longer distance of propagation. It becomes necessary to predict very finely meteorology and their connectivity with boundaries effects at the same time. Based on the Green's Function Parabolic Equation (GFPE), ATMOS (Advanced Theoretical Model for Outdoor Sound propagation) is a numerical code where new developments have been implemented to take complex topographies, sophisticated anti-noise protections with backscattering effects in the same way as relief, obstacle effects on wind speed profiles into account and to allow calculations of the temporal evolution of received signal. Comparisons with results given by different numerical methods used in outdoor sound propagation and for more sophisticated cases, by measurements accomplished on scale model, have been done to validate this new tool