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On sound absorbing characteristics and suitable measurement methods
Scania CV AB, Sweden.ORCID iD: 0000-0003-1604-8263
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, MWL Flow acoustics.ORCID iD: 0000-0003-4103-0129
Scania CV AB, Sweden.ORCID iD: 0000-0002-3826-3055
2012 (English)In: SAE Technical Paper 2012-01-1534, 2012, Society of Automotive Engineers, 2012Conference paper, Published paper (Refereed)
Abstract [en]

Noise encapsulations are widely used in automotive industry to enclose noise sources, such as e.g. the engine or the gearbox, to reduce externally radiated noise. The sound absorption factor of the material on the inside of the noise encapsulation is obviously vital for the sound attenuation. This parameter is in most cases determined experimentally for which there are several methods. The results received from the various methods may vary as different acoustic states are examined and thus influence the choice of method. The absorption factor is crucial since it is used in specifications to material manufacturers as well as being an input parameter in modeling the performance of the noise shield e.g. during a pass-by noise test.

In this paper, two standardized measurement methods along with a third, non-standardized method, are applied to determine the properties of an absorbing material used in a commercial noise encapsulation. The methods are based on normal-, random- and oblique incident sound waves. The first and the last methods are based on measuring the acoustic impedance from which the absorption can be calculated while the random incidence method measures the absorption directly. The results retrieved from the three methods are compared and discussed in the light of the differences between them. This paper clarifies the differences and gives a practical guidance for the choice of measurement method and the use of the different absorption factors in modeling.

Place, publisher, year, edition, pages
Society of Automotive Engineers, 2012.
Keyword [en]
Sound absorption coefficient, surface impedance, local reaction
National Category
Fluid Mechanics and Acoustics
Identifiers
URN: urn:nbn:se:kth:diva-128552DOI: 10.4271/2012-01-1534Scopus ID: 2-s2.0-84877167175OAI: oai:DiVA.org:kth-128552DiVA: diva2:648015
Conference
th International Styrian Noise, Vibration and Harshness Congress: The European Automotive Noise Conference, SNVH 2012; Graz; Austria; 13 June 2012 through 15 June 2012
Note

QC 20130913

Available from: 2013-09-13 Created: 2013-09-13 Last updated: 2016-03-11Bibliographically approved
In thesis
1. Analysis of Acoustic Absorption with Extended Liner Reaction and Grazing Flow
Open this publication in new window or tab >>Analysis of Acoustic Absorption with Extended Liner Reaction and Grazing Flow
2013 (English)Licentiate thesis, comprehensive summary (Other academic)
Abstract [en]

Acoustic absorbing liners are efficient and commonly used measures to reduce sound levels in many fields of application. The sound reducingperformance of the liners is dependent on the acoustic state, defined by e.g. the flow and sound field interacting with the liner. To enable liner optimization the impact of these factors on the liner performance must be predictable. Studies of the impact of these factors were performed with existing experimental, analytical and numerical methods at low Mach number flows and material used in truck engine compartments. The study showed significant impact of both flow and sound field onthe liner performance. The size of the impact of the flow depends on which of the existing methods and models that was used, implying theneed of complementary methods. A new numerical method to model the boundary layer effect was for this reason developed in this work. The method was shown to predict the impact of flow correctly compared to the Pridmore-Brown solution and the method was computationally efficient. The sound reducing performance of a liner exposed to complex sound field and grazing flow can be predicted using existing methods together with the new proposed method. Extra care has to be taken when bulk reacting liners are considered since additional complications compared to locally reacting surfaces occur in presence the of grazing flow.

Place, publisher, year, edition, pages
Stockholm: KTH Royal Institute of Technology, 2013. x, 46 p.
Series
Trita-AVE, ISSN 1651-7660 ; 2013:42
Keyword
Sound absorption, acoustic lining, non-locally reacting, boundary layer, grazing flow, sound field, transfer matrix method
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-128711 (URN)
Presentation
2013-09-27, K2, Teknikringen 28, KTH, Stockholm, 13:00 (English)
Opponent
Supervisors
Note

QC 20130916

Available from: 2013-09-16 Created: 2013-09-16 Last updated: 2013-09-16Bibliographically approved
2. 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, AnnaBoij, SusannGlav, Regnar

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