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Edge scattering impact in free field estimation of surface impedance
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.ORCID iD: 0000-0003-1604-8263
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. Scania CV AB.ORCID iD: 0000-0002-3826-3055
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL.ORCID iD: 0000-0003-4103-0129
(English)Manuscript (preprint) (Other academic)
Abstract [en]

Accurate experimental characterization of sound absorbing materials is important to ensure good quality in simulations of larger systems and to analyze materials with unknown acoustic properties. Free field methods allow characterization of material properties at arbitrary sound incidence, which is advantageous compared to standardized methods. The errors of these methods have been studied, in particular those related to the size of the test samples. These are typically seen as oscillations about the correct value, especially at lower frequencies. The errors have been related to the source position and the sample size, but the impact of the material properties has not been investigated. In this paper, the influence of these properties on the errors are investigated through measurements and numerical simulations. The studies show a dependance on the material properties. The error results both from the pressure scattered at the sample edges and the pressure reflected at the material surface. The scattered field is shown to be stronger for materials with high flow resistivity, although the impact of this field on the result is stronger on materials with low flow resistivity. In addition, a method to reduce these errors based on an analytical formulation of the scattering is proposed. The method is applied to numerical simulations and shown to signicantly reduce the impact of the scattered field on the accuracy of the surface impedance.

Keyword [en]
Sound absorption, Surface impedance, Edge scattering, Free field measurement
National Category
Fluid Mechanics and Acoustics
Research subject
Vehicle and Maritime Engineering
Identifiers
URN: urn:nbn:se:kth:diva-183411OAI: oai:DiVA.org:kth-183411DiVA: diva2:910843
Note

QS 2016

Available from: 2016-03-10 Created: 2016-03-10 Last updated: 2016-03-11Bibliographically approved
In thesis
1. Absorption of Sound: On the effects of field interaction on absorber performance
Open this publication in new window or tab >>Absorption of Sound: On the effects of field interaction on absorber performance
2016 (English)Doctoral thesis, comprehensive summary (Other academic)
Abstract [en]

Environmental noise has for decades been a well known problem, especially in urban areas. As noise requirements for vehicles are sharpened, noise reducing concepts are needed in early design stages requiring accurate simulations to support the design. Specifically for optimization of noise treatments, the absorber performance must be simulated correctly.

So called noise encapsulations are placed below the powertrain on heavy vehicles to enclose the engine and reduce noise radiation. The attenuation of the absorbers on these shields must be represented correctly in simulations, even in environments with complex sound field, cooling flow and high temperature variations which may affect the absorber performance. This thesis studies the performance variation due to different absorber representations and due to these factors and how to include this in simulations.

It is shown that the material representation significantly affects the attenuation performance in the simulations. Assuming locally reacting absorbers neglects the full interaction between the sound field and the material, which was shown to affect the noise reduction considerably. A measurement method to determine the angular dependent surface impedance was evaluated. It was shown sensitive to small samples and a method to improve accuracy was suggested. Including the angular dependence, either by full resolution or an angular dependent impedance, the field-absorber interaction is included in the simulations and more accurate results are obtained. The influence of flow and temperature fields on the absorber performance was also investigated. A method to include these effects was developed and the attenuation performance shown significant, especially for materials with bulk reaction.

In conclusion, thorough knowledge of the material behavior and the field in the applications is required to choose appropriate material representation to enable reliable simulation results.​

 

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2016. 45 p.
Series
TRITA-AVE, ISSN 1651-7660 ; 2016:09
Keyword
Sound absorption, Porous absorbers, Bulk reaction, Local reaction, Boundary layer, Grazing flow, Temperature gradients, Surface impedance, Sound field, Pass-by noise
National Category
Fluid Mechanics and Acoustics
Research subject
Vehicle and Maritime Engineering
Identifiers
urn:nbn:se:kth:diva-183413 (URN)978-91-7595-883-5 (ISBN)
Public defence
2016-04-08, F3, Lindstedtsvägen 26, KTH, Stockholm, 09:00 (English)
Opponent
Supervisors
Note

QC 20160311

Available from: 2016-03-11 Created: 2016-03-10 Last updated: 2016-03-11Bibliographically approved

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Färm, AnnaGlav, RagnarBoij, Susann
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