Noise attenuation by a hard wedge-shaped barrier (original) (raw)
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Open Measurements of Edge Diffraction from a Noise Barrier Scale Model
Building Acoustics, 2011
Until today, all known simulation methods for the wave phenomenon of edge diffraction are just approximations, based on either Geometrical Acoustics or the numerical solving of the wave equation. Although these methods work fine for simple test scenarios and a certain frequency range, they fail to simulate the effect of diffraction in its whole complexity. This leads to false predictions especially for more complex geometries where the influence of multiple wave diffraction and sound scattering has to be taken into account as well. Consequently, huge effort is currently put into the development of improved simulation methods. Here, a basic need is an all-embracing validation of simulation results, which also includes the comparison with real-world measurements. Unfortunately, there is a lack of such data which is the reason why the Institute of Technical Acoustics (ITA), RWTH Aachen University, Germany, and the Centre for Quantifiable Quality of Service in Communication Systems (Q2S...
Some remarks on practical methods for calculating acoustical diffraction
Applied Acoustics, 1990
A BSTRA CT Three remarks are given on the diffraction problems related to Maekawa's Chart. The first remark concerns the method o fusing Maekawa's Chart correctly. It is pointed out that two kinds of reductions in dB, AL 1 and AL2 sho,~M be calculated when a wall is built on the ground as is shown in Fig. l(c). The reduction AL in dB is given as AL 1-AL2 where ALl and AL2 are calculated from Maekawa's Chart using the value 2(A + B-d)/). and 2(A' + B"-d)/). in Fig. l(c) respectively. The second remark is related to the method for calculating the noise reduction by a wide barrier. A method proposed by the present author based on Kirchhoff's approximation theory, using Maekawa's Chart, showed the best agreement with the experimental data. The third remark is devoted to the case where a source and a receiver are above hard or soft ground. It was found that the method using only Maekawa's Chart was not sufficient and that good correspondence was achieved between calculation results and the experimental data by taking the ground effect into consideration.
The performance of a finite impedance wedge barrier
Noise Control Engineering Journal, 2003
The problem of sound screening by a wedge shaped barrier with general boundary conditions is considered. The theoretical methods used for evaluating the effectiveness of a noise barrier on the ground often appeal to one model for considering the wave reflection from the ground, and another model accounting for the wave diffraction at the top of the wedge barrier. For a point-like sound source, there are several attractive models for evaluating the sound field diffracted by either an ideally hard or an ideally soft wedge, but few treat the case of a wedge of more general surface boundary conditions. The purpose of the present work is to carry out a numerical investigation based on an existing diffraction model where the edge diffracted field is given as a high frequency asymptotic expression. The sound excitation source is taken as linear and parallel to the edge of the wedge, hence the two-dimensional character of the problem. The expression of the field diffracted by the edge of the absorbing wedge is adapted from a model where it is developed from the solution for plane wave incidence. More specifically, the solution to the case of the line source in that model is elaborated by considering a wave spectrum decomposition of a cylindrical wave. Numerical results of calculations on an absorbing wedge in free space show that the amplitude of the edge diffracted field increases with increasing hardness of the wedge. This field amplitude is also found to increase with an increasing value of the opening angle of the wedge, which is in agreement with the results for the case of an ideally hard wedge irradiated by a spherical source. Another point of interest, which is also considered in this study, is the case of a hard wedge on a ground with mixed boundary conditions, and which is considered for both the case of a point source and the case of a line source. Further examples are considered for predicting the insertion loss of an absorbing barrier in a typical road traffic situation. The barrier is erected on ground consisting of a combination of two grounds with different impedances, namely that of asphalt on the source side, and of grass on the receiver side. The results of numerical calculations show that the noise shielding performance of the impedance barrier diminishes both for increasing hardness of the barrier and for increasing value of the barrier angle. This latter conclusion is also in agreement with previous findings for a hard wedged barrier, with a point-like sound source. The divergence of the sound source is also found to have a minor effect on evaluating the effectiveness of the noise barrier.
Sound Diffraction Over Noise Barriers with Added Devices Installed on the Top Edge
Acta Physica Polonica A, 2015
Airborne sound insulation index of the noise barrier panel and the sound absorption coefficient of the barrier surface are the acoustic parameters that are usually determined in specialized laboratories, however they can be also determined in situ. Acoustic characteristics of a barrier include also the diffraction index difference determined from comparison of barriers with plain top edges and barriers with added devices installed on the top edge. The index is determined from the impulse response values determined for the acoustic wave propagation path from the sound source to a set of properly distributed measurement points. By means of the same method, one can also determine the difference in a barrier's acoustic effectiveness between the plain top barrier structure and its version with added devices mounted on the top. The paper presents measurement results for three types of added devices mounted on the top edge of the barrier. The diffraction index differences have been determined for each added device type and the acoustic effectiveness for each device has been compared with the plain top edge acoustic barrier.
An Acoustics Intensity Based Investigation οf the Energy Flow Over the Barriers
Acta Physica Polonica A, 2010
Many of theoretical research of the acoustics fields provides useful information about pressure fields, but none currently offers a full mapping of the acoustic energy flow (vectorial effects) in front and back of any scattering systems working in 3D real environmental conditions. Interference, diffraction and scattering of waves made the real field very complex and difficult to the theoretical modelling. This is one of the reasons why the experimental investigations of acoustic fields using sound intensity (SI) technique are so effective and serviceable methods. The visualization of acoustic energy flow in real-life acoustic 3D space fields can explain many particulars energetic effects (perturbations and vortex flow, effects of scattering in direct and near field, etc.), concerning the areas in which it is difficult to make numerical modeling and analysis with the CFD-FSI-CAA simulation methods. The sound intensity image represents a more accurate and efficient information compare to the spatial sound fields modelled. The article presents the application of SI technique to graphic presentation of spatial distribution the acoustic energy flow over the barriers of various geometrical shapes structures located in a three-dimensional space. As the results of research, the graphic analysis of the sound intensity flux in 2D and 3D space is show. Visualisation of research results is shown in the form of intensity streamlines in space and as a shape of flow wave or isosurface in three-dimensional space. Numerous examples illustrate the application of the SI measurement for practical problems at the vibroacoustical diagnostic and noise abatement, as well as to the validation of results of CFD/CAA numerical modelling. The differences, if appearance, mainly result from the fact that theoretical forecasting uses far too big simplifications or that it is impossible to obtain proper data on real physical features of the tested area and structures.
Applied Acoustics
This paper describes scale modelling method to measure the acoustic performance of a nearly-enclosed barrier and corresponding predictions using an existing 2.5-D Boundary Element Method(BEM) program. Preliminary investigation results show the deterioration in performance of a nearly-enclosed barrier due to the resonance effect that led to high pressure levels radiating into the surroundings via the topped opening. Absorptive material added to the inner surface of the barrier can effectively improve this phenomenon. Measurements on one-twentieth scale model of barriers, viaducts and vehicle structures were carried out outdoors under controlled conditions. The measured results show the transmission loss of transparent panels on the top were not adequate to make the measured results as high as the predictions. A modified scale model, by coating all the surfaces with rubber, was remeasured. The results from retested tests and calculations were in good agreement each other, which indicate that the 2.5-D BEM code can provide a reliable description of the acoustic performance of a nearly-enclosed barrier. Then the program was able to be employed into the investigation of barrier performance on every area with different acoustic features in the surrounding environment. As expected, the attenuation of the nearly-enclosed barrier averaged around 15 dB in the near filed and around 10 dB in the far field. The number effect of incoherent point sources on the performance is discussed as well for the study of railway traffic noise. The increased number of incoherent point sources can result in smoother and lower attenuations for the whole sound field.
ACOUSTIC INTRINSIC PERFORMANCES OF NOISE BARRIERS: ACCURACY OF IN SITU MEASUREMENT TECHNIQUES
Laboratory and in situ methods have been used for measuring intrinsic performances of noise barriers, as prescribed by the European standard series EN 1793. The use of in situ techniques is promising, but their accuracy has to be duly verified, even in comparison with well-known standardized procedures. Sound insulation and reflection properties have been measured through a MLS-based technique in an outdoor test field. The paper analyzes the procedures that mainly influence the accuracy: correction for wave spreading and time windowing. Repeatability of the in situ method for sound insulation is satisfying and its results look consistent with simple prediction models. Nevertheless, in situ data can be overestimated at low frequencies, due to the overlapping of the transmitted and diffracted components. The method has to be carefully employed when the sample shows apertures as slits or holes, unless a different kind of sound propagation is assumed at the receiving side. A good agreement was found between in situ and laboratory sound insulation data, while in situ and laboratory absorption properties show poorer correlation.
Determination of Acoustic Properties of Noise Barriers
2016
Recently major improvements in the measurement methods for determining the in-situ values of noise barriers have been implemented. Laboratory and in-situ measurements have been compared. The results are not directly comparable, due to differences between the diffuse, omnidirectional sound field in the laboratory and the limited incidence angles that occur in the field. Additionally, new measurement methods have been developed by the University of Twente and SoundInSight, which are suitable to determine the acoustic properties of noise barriers in-situ, for any complex sound field.
The Journal of the Acoustical Society of America, 2013
The sound reflection and the airborne sound insulation of noise barriers can be measured in-situ according to CEN/TS 1793-51,2,3,4. The procedure, based on impulse response measurements close to the noise barrier and in the free field, is robust and easily applicable; it allows to get the results in real-time just after the measurements, in situ or in laboratory, applying a well defined post-processing to the raw data. Some errors may anyway occur: for sound reflection, when the signal subtraction procedure1 leaves a small residual; for airborne sound insulation, when the barrier under test is highly insulating and so the transmitted signal is very low. This kind of problems is not always easy to recognize when on site. The European standard explains the measurement procedure in details, but a criterion for validating the measurements and prevent the acquisition of possible invalid data is missing. The authors analyzed many measurements on different noise barriers, performed on outdoor test sites during an inter-laboratory test organized in the frame of the European project QUIESST5,6,7, having among its objectives the improvement of the CEN/TS 1793-5 measurement method. The analysis of this large amount of data suggested that the above mentioned measurement problems may be identified and corrected evaluating the signal-to-noise ratio (SNR) of critical parts of the impulse responses being processed. These criteria are rigorously described here for the first time and illustrative examples are presented.