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Färm, A., Boij, S., Glav, R. & Dazel, O. (2016). Absorption of sound at a surface exposed to flow and temperature gradients. Applied Acoustics, 110, 33-42
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, E-ISSN 1872-910X, Vol. 110, p. 33-42Article 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: 2022-07-11Bibliographically approved
Färm, A. (2016). Absorption of Sound: On the effects of field interaction on absorber performance. (Doctoral dissertation). Stockholm: KTH Royal Institute of Technology
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. p. 45
Series
TRITA-AVE, ISSN 1651-7660 ; 2016:09
Keywords
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: 2022-06-23Bibliographically approved
Färm, A., Boij, S. & Glav, R. (2015). The influence of finite sample size on surface impedance determination of materials with low sound absorpsion at low frequencies. In: : . Paper presented at Euronoise 2015, the 10th European Congress and Exposition on Noise Control Engineering, Maastricht, Netherlands, 2015-06-01.
Open this publication in new window or tab >>The influence of finite sample size on surface impedance determination of materials with low sound absorpsion at low frequencies
2015 (English)Conference paper, Published paper (Refereed)
Abstract [en]

The most common noise reducing measure is to add sound absorbing material on the domain boundaries. The boundaries covered by the material may in sumilations be represented by the surface impedance of the material. The impedance can either be modeled or determined experimentally. The experimental determination can be done by the well known standing wave tube method or by a free field method. These free field methods enable impedance determination at any angle of incidence for bulk reacting materials, as opposed to the standing wave tube method that is restricted to normal incidence or locally reacting materials. The method prescribes a point source above the surface and measurements in two points close to the sample surface. From this, the surface impedance can be deduced through the known sound field formulation. Among other things, the impact on the accuracy of the method from the field formulation, signal conditioning and sensor type have been studied in previous work. One major concern is the finite size of the material sample, and its influence on the measurement accuracy. This has previously been investigated for highly absorbing materials and it was shown to be a low frequency problem. Therefore, we focus on the impact of the finite sample in frequencies below 2 kHz. In particular, we relate the magnitude of the impact to the properties of the tested material. Also, the influence of the mounting of the material is analyzed. The study is made through analyzing numerical simulations of the experiment for a variety of setups and materials. Theoretical discussion is provided for deeper understanding of the results. The impact of the finite sample is seen to depend on the material properties, not only the setup as previously shown. Materials with high absorption are shown to be more sensitive to these errors.

National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-181690 (URN)
Conference
Euronoise 2015, the 10th European Congress and Exposition on Noise Control Engineering, Maastricht, Netherlands, 2015-06-01
Note

QC 20160225

Available from: 2016-02-02 Created: 2016-02-02 Last updated: 2022-06-23Bibliographically approved
Färm, A., Boij, S. & Glav, R. (2015). The influence of finite sample size on surface impedance determination of materials with low sound absorption at low frequencies. In: Euronoise 2015: . Paper presented at 10th European Congress and Exposition on Noise Control Engineering, Euronoise 2015, 1 June 2015 through 3 June 2015 (pp. 2165-2170). DC/ConfOrg
Open this publication in new window or tab >>The influence of finite sample size on surface impedance determination of materials with low sound absorption at low frequencies
2015 (English)In: Euronoise 2015, DC/ConfOrg , 2015, p. 2165-2170Conference paper, Published paper (Refereed)
Abstract [en]

The most common noise reducing measure is to add sound absorbing material on the domain boundaries. The boundaries covered by the material may in simulations be represented by the surface impedance of the material. The impedance can be either modeled or determined experimentally. The experimental determination can be done by the well known standing wave tube method or by a free field method. These free field methods enable impedance determination at any angle of incidence for bulk reacting materials, as opposed to the standing wave tube method that is restricted to normal incidence or locally reacting materials. The method prescribes a point source above the surface and measurements in two points close to the sample surface. From this, the surface impedance can be deduced through the known sound field formulation. Among other things, the impact on the accuracy of the method from the field formulation, signal conditioning and sensor type have been studied in previous work. One major concern is the finite size of the material sample, and its influence on the measurement accuracy. This has previously been investigated for highly absorbing materials and it was shown to be a low frequency problem. Therefore, we focus on the impact of the finite sample in frequencies below 2 kHz. In particular, we relate the magnitude of the impact to the properties of the tested material. Also, the influence of the mounting of the material is analyzed. The study is made through analyzing numerical simulations of the experiment for a variety of setups and materials. Theoretical discussion is provided for deeper understanding of the results. The impact of the finite sample is seen to depend on the material properties, not only the setup as previously shown. Materials with high absorption are shown to be more sensitive to these errors. 

Place, publisher, year, edition, pages
DC/ConfOrg, 2015
Keywords
Acoustic fields, Acoustic noise, Acoustic variables control, Sampling, Sound insulating materials, Absorbing materials, Angle of Incidence, Experimental determination, Finite sample sizes, Free field methods, Measurement accuracy, Sound absorbing materials, Standing wave tubes, Acoustic wave absorption
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-274299 (URN)2-s2.0-85080931350 (Scopus ID)
Conference
10th European Congress and Exposition on Noise Control Engineering, Euronoise 2015, 1 June 2015 through 3 June 2015
Note

QC 20200710

Available from: 2020-07-10 Created: 2020-07-10 Last updated: 2022-06-26Bibliographically approved
Färm, A., Boij, S. & Dazel, O. (2014). Prediction of acoustic surface impedance of bulk reacting lining with grazing flow. In: 21st International Congress on Sound and Vibration 2014, ICSV 2014: . Paper presented at 21st International Congress on Sound and Vibration 2014, ICSV 2014, 13 July 2014 through 17 July 2014 (pp. 2332-2339). International Institute of Acoustics and Vibrations
Open this publication in new window or tab >>Prediction of acoustic surface impedance of bulk reacting lining with grazing flow
2014 (English)In: 21st International Congress on Sound and Vibration 2014, ICSV 2014, International Institute of Acoustics and Vibrations , 2014, p. 2332-2339Conference paper, Published paper (Other academic)
Abstract [en]

A transfer matrix methodology to determine the acoustic properties of multi-layered absorbers in different environments is proposed in this paper. The methodology allows inclusion of grazing flow and the boundary layer effects on the surface properties, avoiding the need of several complementary methods to obtain the surface properties of a sound reducing material in a specific environment. The predicted surface properties are given as a function of angle of sound incidence, allowing for arbitrary sound fields to be simulated. This is a useful tool in for example automotive applications such as engine bays where multi-layered bulk reacting sound absorbing materials are exposed to flow and complex sound fields. Correct prediction of the acoustic performance of absorbing material where flow is present enables optimization of the noise reducing components for which conflicting requirements such as weight and space constraints are present as well.

Place, publisher, year, edition, pages
International Institute of Acoustics and Vibrations, 2014
Keywords
Acoustic field measurement, Acoustic fields, Acoustic impedance, Acoustic properties, Acoustic wave absorption, Acoustics, Boundary layers, Transfer matrix method, Absorbing materials, Acoustic performance, Automotive applications, Boundary layer effects, Complementary methods, Sound absorbing materials, Space constraints, Surface impedances, Surface properties
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-167038 (URN)2-s2.0-84922646339 (Scopus ID)9781634392389 (ISBN)
Conference
21st International Congress on Sound and Vibration 2014, ICSV 2014, 13 July 2014 through 17 July 2014
Note

QC 20160207

Available from: 2015-05-21 Created: 2015-05-21 Last updated: 2022-06-23Bibliographically approved
Glav, R. & Färm, A. (2013). Analysis of a cylindrical micro-perforated resistive silencer. In: 42nd International Congress and Exposition on Noise Control Engineering 2013, INTER-NOISE 2013: Noise Control for Quality of Life. Paper presented at 42nd International Congress and Exposition on Noise Control Engineering 2013: Noise Control for Quality of Life, INTER-NOISE 2013, 15 September 2013 through 18 September 2013, Innsbruck, Austria (pp. 5867-5876). OAL-Osterreichischer Arbeitsring fur Larmbekampfung
Open this publication in new window or tab >>Analysis of a cylindrical micro-perforated resistive silencer
2013 (English)In: 42nd International Congress and Exposition on Noise Control Engineering 2013, INTER-NOISE 2013: Noise Control for Quality of Life, OAL-Osterreichischer Arbeitsring fur Larmbekampfung , 2013, p. 5867-5876Conference paper, Published paper (Refereed)
Abstract [en]

This paper presents an analytical wave decomposition model for predicting the transmission loss a cylindrical silencer with both annular and baffled micro-perforated screens. Numerical simulation shows the fundamental characteristics as well as the potential to achieve large attenuation using micro-perforations. The numerical model is verified by measurements using the 2-microphone technique and shown to be a useful tool in practical design. Clear from the analysis is the sensitivity of the micro-perforated silencer to changes in both porosity and overall layout.

Place, publisher, year, edition, pages
OAL-Osterreichischer Arbeitsring fur Larmbekampfung, 2013
Keywords
Numerical models, Fundamental characteristics, Micro-perforations, Transmission loss, Wave decomposition, Acoustic variables control
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-151079 (URN)2-s2.0-84904498254 (Scopus ID)978-163266267-5 (ISBN)
Conference
42nd International Congress and Exposition on Noise Control Engineering 2013: Noise Control for Quality of Life, INTER-NOISE 2013, 15 September 2013 through 18 September 2013, Innsbruck, Austria
Note

QC 20160122

Available from: 2014-09-15 Created: 2014-09-15 Last updated: 2022-06-23Bibliographically approved
Färm, A. (2013). Analysis of Acoustic Absorption with Extended Liner Reaction and Grazing Flow. (Licentiate dissertation). Stockholm: KTH Royal Institute of Technology
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. p. x, 46
Series
Trita-AVE, ISSN 1651-7660 ; 2013:42
Keywords
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: 2022-06-23Bibliographically approved
Färm, A., Boij, S., Göransson, P. & Glav, R. (2013). On internal mean flow in porous absorbers and its effect on attenuation properties. In: Proceedings of Meetings on Acoustics: Volume 19, 2013. Paper presented at 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 (pp. 1-6). Acoustical Society of America (ASA), 19
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, p. 1-6Conference 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
Keywords
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: 2022-06-23Bibliographically approved
Färm, A. & Boij, S. (2013). The Effect of Boundary Layers on Bulk Reacting Liners at Low Mach Number Flows. In: : . Paper presented at 19th AIAA/CEAS Aeroacoustics Conference, 27-29 May, 2013, Berlin, Germany. American Institute of Aeronautics and Astronautics
Open this publication in new window or tab >>The Effect of Boundary Layers on Bulk Reacting Liners at Low Mach Number Flows
2013 (English)Conference paper, Published paper (Other academic)
Abstract [en]

Sound absorbing linings are effective noise treatments in many applications in order to meet noise emission requirements. Stricter noise requirements set harder demands on the performance of the liners, why better prediction models of their performance have to be developed. As of today, several models to predict the sound reducing properties in the presence of flow exist and are shown to give diverging absorption properties for locally reacting liners exposed to high Mach number flows. The effect of flow on absorption properties is often seen as an issue that only needs to be addressed at high Mach number flows. In this paper, the existing models are applied to bulk reacting liners exposed to low Mach number flows and the resulting absorption coefficients are compared. Predictions of absorption coefficients clearly show that the effect of flow needs to be considered also at low Mach number flows and that the difference between the prediction models is indeed significant at low Mach number flows. This shows the importance in choosing the correct model for a specific application in order to avoid introducing erroneous prediction on the effect of flow. This study thus gives well-grounded evidence of the importance to include flow effects in modeling of sound absorptive linings even at low Mach number flows.

Place, publisher, year, edition, pages
American Institute of Aeronautics and Astronautics, 2013
Keywords
Sound absorption, surface impedance, grazing flow, liners
National Category
Fluid Mechanics and Acoustics
Identifiers
urn:nbn:se:kth:diva-128559 (URN)10.2514/6.2013-2075 (DOI)2-s2.0-84883681935 (Scopus ID)
Conference
19th AIAA/CEAS Aeroacoustics Conference, 27-29 May, 2013, Berlin, Germany
Note

QC 20130913

Available from: 2013-09-13 Created: 2013-09-13 Last updated: 2024-03-18Bibliographically approved
Färm, A., Boij, S. & Glav, R. (2012). On sound absorbing characteristics and suitable measurement methods. In: SAE Technical Paper 2012-01-1534, 2012: . Paper presented at th International Styrian Noise, Vibration and Harshness Congress: The European Automotive Noise Conference, SNVH 2012; Graz; Austria; 13 June 2012 through 15 June 2012. Society of Automotive Engineers
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
Keywords
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-s2.0-85072490539 (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: 2022-06-23Bibliographically approved
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