Change search
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf
Absorption of Sound: On the effects of field interaction on absorber performance
KTH, School of Engineering Sciences (SCI), Aeronautical and Vehicle Engineering, Marcus Wallenberg Laboratory MWL. Scania CV AB, Sweden.ORCID iD: 0000-0003-1604-8263
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 [en]
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: urn:nbn:se:kth:diva-183413ISBN: 978-91-7595-883-5 (print)OAI: oai:DiVA.org:kth-183413DiVA: diva2:910860
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
List of papers
1. On sound absorbing characteristics and suitable measurement methods
Open this publication in new window or tab >>On sound absorbing characteristics and suitable measurement methods
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
Sound absorption coefficient, surface impedance, local reaction
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-128552 (URN)10.4271/2012-01-1534 (DOI)2-s2.0-84877167175 (Scopus ID)
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
2. On internal mean flow in porous absorbers and its effect on attenuation properties
Open this publication in new window or tab >>On internal mean flow in porous absorbers and its effect on attenuation properties
2013 (English)In: Proceedings of Meetings on Acoustics: Volume 19, 2013, Acoustical Society of America (ASA), 2013, Vol. 19, 1-6 p.Conference paper, Published paper (Other academic)
Abstract [en]

In vehicle applications, absorbing materials are often used to attenuate sound. In, for example, exhaust systems and on noise encapsulations, the absorber is exposed to flow. This creates a boundary layer above the absorber, which affects the impedance of the surface, and hence alters the absorption properties. In addition to this effect, the flow itself may enter the absorbent material due to high pressure and forced flow paths. An investigation of the effects that internal flow in the absorber imposes on the acoustic properties is presented. One way to describe the effect is by a change in flow resistivity. The effect is investigated for typical absorbers used in noise encapsulations for trucks. The Transfer Matrix Method is applied to calculate the resulting absorption coefficient for an absorber with changed flow resistivity due to internal flow. The possibility to model the changed properties of the absorber with internal mean flow by means of Biot theory is also explored, together with a discussion on suitable experimental methods to verify and further investigate the effects.

Place, publisher, year, edition, pages
Acoustical Society of America (ASA), 2013
Series
Proceedings of Meetings on Acoustics, ISSN 1939-800X ; 19
Keyword
Absorbent materials, Absorbing materials, Absorption co-efficient, Absorption property, Attenuation properties, Experimental methods, Flow resistivity, In-vehicle applications
National Category
Engineering and Technology
Identifiers
urn:nbn:se:kth:diva-134241 (URN)10.1121/1.4799717 (DOI)2-s2.0-84878976617 (Scopus ID)
Conference
21st International Congress on Acoustics, ICA 2013 - 165th Meeting of the Acoustical Society of America; Montreal, QC; Canada; 2 June 2013 through 7 June 2013
Note

QC 20131121. QC 20160129. QC 20160207

Available from: 2013-11-21 Created: 2013-11-20 Last updated: 2016-03-11Bibliographically approved
3. Absorption of sound at a surface exposed to flow and temperature gradients
Open this publication in new window or tab >>Absorption of sound at a surface exposed to flow and temperature gradients
2016 (English)In: Applied Acoustics, ISSN 0003-682x, Vol. 110, 33-42 p.Article in journal (Other academic) Published
Abstract [en]

In noise abatement using porous or fibrous materials, accurate determination of the surface impedance representing the absorber is decisive for simulation quality. The presence of grazing flow and non-homogeneous ambient temperature influence the reaction of the absorber and may suitably be included in a modified “effective” surface impedance. In this paper, this approach is applied to a generic case representative for the engine bay of a heavy truck, where porous shields suppress the radiated noise, e.g. during a pass-by noise test. The change in the absorption is determined numerically by solving the wave propagation through a layer of varying temperature and flow adjacent to the impedance surface for different angles of incidence. The study shows significant impact of both flow and temperature, especially for materials with low absorption. The diffuse field absorption coefficient is also derived and although the effect is less pronounced in this case, it is still important in lower frequencies and in the frequency range typical for IC engine noise. The proposed numerical method is shown to be accurate and efficient for determination of the effective impedance and moreover not limited to thin boundary layers.

Place, publisher, year, edition, pages
Elsevier, 2016
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-183440 (URN)10.1016/j.apacoust.2016.03.017 (DOI)000377232500005 ()2-s2.0-84977634166 (Scopus ID)
Note

QC 20160407

Available from: 2016-03-11 Created: 2016-03-11 Last updated: 2017-11-30Bibliographically approved
4. Edge scattering impact in free field estimation of surface impedance
Open this publication in new window or tab >>Edge scattering impact in free field estimation of surface impedance
(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
Sound absorption, Surface impedance, Edge scattering, Free field measurement
National Category
Fluid Mechanics and Acoustics
Research subject
Vehicle and Maritime Engineering
Identifiers
urn:nbn:se:kth:diva-183411 (URN)
Note

QS 2016

Available from: 2016-03-10 Created: 2016-03-10 Last updated: 2016-03-11Bibliographically approved
5. Pass-by noise simulation: - inuence of trim representation
Open this publication in new window or tab >>Pass-by noise simulation: - inuence of trim representation
(English)Manuscript (preprint) (Other academic)
Abstract [en]

The necessity of accurate pass-by noise simulations of vehicles has increased as the requirements on noise levels is becoming stricter. Also, the design of noise reducing measures is needed early in the design process when measurements are not possible to perform. The impact of the sound absorbing materials representation on simulated pass-by noise levels from a truck is analysed in this paper. The material may be fully resolved in FEM, including bulk reaction, or represented by a surface impedance, either at normal or a specic angle of incidence. The first representation requires FEM simulations and more material data. This puts higher demands on input data, and more importantly, prevents the use of BEM simulations which signicantly would improve computational efficiency. The two latter representations may be implemented in BEM. The necessary assumption of local reaction may hold for some materials, but it is not always valid. The simulations presented in this paper show that the local reaction assumption underestimates the effect of sound absorption, giving up to 5 dB higher radiated sound power levels and pass-by noise levels up to 2 dB higher than obtained using the bulk-reacting representation. The difference is shown to depend on the material properties and the position of the source in relation to the noise shields and absorbing parts. The directivity of the radiated noise is not affected, although the regions of largest sound pressure levels are more pronounced. The choice of representation of the material is shown to be important for the simulated pass-by noise levels. To choose the level of complexity in the model, it is important to be aware of the effect this may have on the accuracy of the results in order to draw correct conclusions from the results.

National Category
Fluid Mechanics and Acoustics
Research subject
Vehicle and Maritime Engineering
Identifiers
urn:nbn:se:kth:diva-183412 (URN)
Projects
Surface impedance, Bulk-reaction, Pass-by noise, Enclosure ratio
Note

QS 2016

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

Open Access in DiVA

Thesis(11720 kB)221 downloads
File information
File name FULLTEXT01.pdfFile size 11720 kBChecksum SHA-512
3947b319f03823d11ff7e5a0a281ea526efaf4b254b05030d9fe128072afbae4442983fd2ee79f6bedae7b701070de7e1d5759680f7c8e65e193cd01603b3de1
Type fulltextMimetype application/pdf

Search in DiVA

By author/editor
Färm, Anna
By organisation
Marcus Wallenberg Laboratory MWL
Fluid Mechanics and Acoustics

Search outside of DiVA

GoogleGoogle Scholar
Total: 221 downloads
The number of downloads is the sum of all downloads of full texts. It may include eg previous versions that are now no longer available

isbn
urn-nbn

Altmetric score

isbn
urn-nbn
Total: 389 hits
CiteExportLink to record
Permanent link

Direct link
Cite
Citation style
  • apa
  • ieee
  • modern-language-association-8th-edition
  • vancouver
  • Other style
More styles
Language
  • de-DE
  • en-GB
  • en-US
  • fi-FI
  • nn-NO
  • nn-NB
  • sv-SE
  • Other locale
More languages
Output format
  • html
  • text
  • asciidoc
  • rtf