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The properties of the materials used for building sound proofing systems are known to exhibit large variations. These may lead to significant differences in the acoustic responses within a given material batch, particularly when resistive screens are used as a surface component for a multi-layered absorbing panel. In such thin films, it is mostly the thickness and the flow resistivity, but in some cases also the porosity, that are difficult to control in the production process. All these potential variations influence the acoustic response of the complete panel. In the present contribution, a method to isolate and evaluate the effect of uncertainties in a film is proposed. Using a transmission line approach, it is shown to be possible to predict the modification of the response induced by the uncertainties. The proposed technique is then adapted to determine uncertainty envelopes of the absorption coefficient, for experimentally acquired responses, that are closer to measured envelopes as compared to those generated using Monte Carlo simulations or simplified approaches. The method is tested both on numerical and experimental cases and shows, in both cases, a very good agreement with the reference solutions. Unlike Monte Carlo approaches, the proposed method does not require a massive computational effort which makes it suitable for real life applications., QC 20200727

This study is on the practical application of the acoustic metamaterials to the design of a silencer with high acoustic and geometric efficiencies, and negligible pressure drop. The object is to achieve broadband and low-frequency attenuation simultaneously by combining the effects of resonance, periodicity, phase difference, and impedance mismatch. An array of multiple small resonators, which is periodically applied to the wall of a duct, invokes the dispersion of sound propagating in the duct. This phenomenon is implemented to construct a virtual Herschel-Quincke (HQ) tube system. The main duct is split into two parallel ducts of the same length: a rigid duct and a duct with a wall covered by the periodic resonators. For a modelling of sound propagation in a dispersive duct, the phase speed of sound is analytically derived in the presence of a mean flow. Also, the attenuation conditions of the virtual HQ tube are proposed to establish a guideline in selecting the proper design parameters for achieving the required transmission loss (TL) in the desired frequency range. The predicted TL spectra are compared with the test results, for a virtual HQ tube system with 9 identical quarter-wavelength tube resonators, and they generally agree well. With increasing flow speed, the amount of attenuation decreases a bit, but the general spectral characteristics are maintained for vertical bar M vertical bar <= 0.1. Based on the same principle, the acoustic metamaterials (AMM) are applied to the practical silencer design to achieve the wide-band sound reduction at low- to mid-frequencies for a given small space. A virtual HQ tube having 26 cells of AMM is tested, of which a cell is composed of 3 types of quarter-wavelength tube resonators. The experiment well validates the predicted TL. The results show that TL is at least 5 dB for a wide frequency range of 230-1000 Hz. The additional volume due to the attachment of the AMM layer is only 40%, while the TL is far larger than, QC 20200103

Amplitude modulation is assumed to be a major annoyance factor of wind turbine sound. However, studies on the generation of amplitude modulation and the impact of atmospheric conditions on amplitude modulation are limited, especially in cold climates. Long-term acoustic and meteorological measurements in the vicinity of a wind farm in northern Sweden show a dependence of the occurrence of amplitude modulation on wind direction and atmospheric stability. The occurrence of amplitude modulation is highest for crosswinds from southwest, compared with the other wind directions. Moreover, the occurrence of amplitude modulation is clearly linked to atmospheric stability and highest for very stable conditions. The impact of atmospheric stability is supported by analyses of wind shear, the wind speed gradient close to the surface and the bulk Richardson number. Amplitude modulation is more likely during winter than during summer and more likely during night and early morning than during noon and early afternoon., QC20191205

Silencers are used in vehicles to reduce the noise in the engine system. How-ever, silencers themselves may produce break-out noise due to the interaction with the exhaust gas flow and structure. In this Master thesis project, the numerical simulation of vibrational behavior of housing plates of silencers is developed.The housing plate is composed of two steel plates and a fibrous material layer. Measurement results show that the fibrous material has good damping effect to decrease the vibration and radiated sound of steel plates. Steel plates are connected by spot welding. Modeling of spot welds can improve the modal assurance criterion between simulation and measurements. Interface damping is introduced into the simulation models to simulate the contact effect between two steel plates so that the simulated amplitude can have a good agreement with measurement result.Several numerical models of fibrous material are investigated. The Miki model is not chosen for the final result due to the limit of range of frequencies. The rigid frame model can simulate the sound absorption but is unfeasible for the simulation of vibration. The limp frame model can simulate the vibration of light glass wool but cannot simulate the vibration of heavy glass wool. Finally, the Biot-Allard model which is a poro-elastic model is investigated for the final result. The simulation results show good agreement with the measurement result., Ljuddämpare används i fordon för att används för att minska buller i motorsystemet. Ljuddämpare kan emellertid själva producera utbrottsbuller på grund av samspelet mellan avgassflödet och ljuddämpare. I detta examensarbete utvecklas den numeriska simuleringen av vibrationsbeteende hos husplåter för ljuddämpare. Husplåten består av två Plåtar av stål och ett fibröst materiallager. Resultaten av mätningen visar att det fibrösa materialet har god dämpningseffekt för att minska det utstrålade ljudet och vibrationen hos plåterna. Plåterna ansluts med punktsvetsning. Modellering av punktsvetsar kan förbättra kriteriet för modal försäkring mellan simulering och mätning. Dämpning av gränssnitt införs i simuleringsmodellerna för att simulera kontakteffekten mellan två plåtar så att simuleringsamplituden kan ha ett bra överensstämmelse med mätresultatet. Flera numeriska modeller av fibröst material undersöks i detta projekt. Miki-modellen används inte för slutresultatet på grund av gränsen för frekvensområden. Stiv modell kan simulera ljudabsorptionen men är omöjlig för simulering av vibrationer. Limpmodell kan simulera vibrationer av lätt glasull men kan inte simulera vibrationer av tung glasull. Slutligen undersöks Biot-Allard-modellen som är en poroelastisk modell för slutresultatet. Simuleringsresultaten visar bra överensstämmelse med mätresultatet.

The present erratum reports an error impacting the figures of a contribution published in 2018 about a simplified model for thin acoustic screens in a transfer matrix context [Gaborit, Dazel, and Göransson (2018). J. Acoust. Soc. Am. 144(1), EL76–EL81]. A mistake in the implementation of the rigid termination condition for the systems under study is identified and a correct version is proposed along with the corrected figures. It is shown that this error does not impact the conclusions of the original contribution and that the model proposed therein keeps its advantages as the approximation error remains very similar to the previously reported values., QC 20191106

The global trend towards quieter environments has been one of the key topics of acoustics research for years. The recent tightening of the regulations on noise exposure as well as the many reports on the impact of noise on human health confirm this situation and stress ever more the need for innovative mitigation strategies. Numerous efforts from many teams allowed to refine existing solutions and explore new approaches towards a lower noise level ultimately leading to a number of promising concepts. Central to this field, the use of poroelastic media and the development of realistic meta-materials are paving the way to tackle the problem. In the meantime, a great part of the most widely adopted systems to mitigate noise, such as acoustics panels for instance, resort to thin resistive screens placed on the surface to protect the bulk and control the properties. Despite often being one of the thinnest components of the systems, they have a non-negligible impact on the overall response and are subject to a number of uncertainties.The approach chosen in this thesis differs from the global trend of designing new solutions and conversely relies on investigating the effect of uncertainties inherent to all these sound proofing systems. More precisely, the work performed focuses on modelling the impact of uncertain interfaces and uncertain parameters in the thin layers used as protective, tuning or aesthetic elements. These acoustic films, and to a certain extent the thin interface zones resulting from the assembly process, are notably challenging to characterise with precision. The main goal of this thesis is then to propose strategies to account for uncertainties on the parameters of the films and interfaces and predict their impact on the overall response of the systems.Three different scientific contributions are presented in this thesis. Together they discuss modelling aspects related to the films, propose possible simplifications and demonstrate the effect of paramete, Den globala trenden mot allt tystare miljöer, har i flera år varit i fokus för forskningen inom teknisk akustik. Den senaste tidens skärpning av de reglementen som berör gränser för tillåten ljud- och bullerexponering, tillsammans med studier av bullers påverkan på hälsa, välbefinnande, inlärningsförmåga etc., understryker behovet av nya innovativa strategier. Dessa inbegriper som exempel förfining av redan existerande lösningar, utforskande av nya vägar till effektiva åtgärder för att uppnå en lägre ljudnivå, med flera. Stommen i de flesta åtgärder för att reducera ljudnivåer har sedan mitten av 1900-talet varit olika former av poroelastiska material och under de senaste decennierna även en utveckling av så kallade metamaterial, d.v.s. material vars egenskaper ej går att finna i naturen förekommande form, vilket även bereder väg för att finna nya lösningar på problemet. I dagsläget används, i de mest allmänt använda systemen för att reducera buller, till största delen tunna resistenta skikt som placeras på ytan för att skydda mot mekanisk påverkan och men även för att kunna kontrollera de akustiska egenskaperna. Trots att dessa ytskikt oftast är en av de tunnaste komponenterna i olika system, så är deras påverkan på det totala resultatet avsevärd och kan inte förbises. På grund av deras design, är tillverkningen ofta behäftad med bristande precision och de bidrar därför ofta till att de totala akustiska egenskaperna hos sammansatta system där de ingår, ofta uppvisar en spridning på grund av denna variation.I denna avhandling behandlas metoder för kunna undersöka effekten av dessa oundvikliga osäkerheter som finns i alla dessa ljudreducerande system. Mer precist, så har arbetet fokuserat på att modellera den påverkan som osäkerheten i gränsytorna och parametrarna i de tunna ytlagren innebär. Bland annat har arbetet varit inriktat på att hantera de utmaningar som ligger i att med precision karaktärisera de akustiska ytskikten och i viss utsträckning de tunna gränsyto, La tendance globale poussant à développer des environnements plus calmes est, depuis des années, un des aspects clés de la recherche en acoustique. Le récent durcissement des régulations sur l'exposition au bruit ainsi que les nombreux rapports concernant son impact sur la santé humaine rappellent le besoin criant de stratégies innovante pour palier le problème. Le travail de nombreuses équipes a permis d'améliorer les solutions existantes et d'explorer de nouvelles approches, aboutissant à de nouveaux concepts prometteurs. L'utilisation de matériaux poroélastiques et le développement de matériaux méta-poroélastiques réalistes sont un élément central de ces recherches et préfigurent des pistes viables pour résoudre le problème du bruit. En parallèle, une grande partie des systèmes courants pour le traitement des nuisances sonores, comme les panneaux acoustiques, utilisent de fins films résistifs placés en surface pour protéger les matériaux et contrôler le comportement acoustique du système. Malgré que ces films soient souvent les plus petits composants des panneaux, ils ont un impact non-négligeable sur la réponse globale et sont sujet à un certain nombre d'incertitudes.L'approche choisie dans cette thèse diffère de la tendance globale poussant au développement de nouveaux système. À l'inverse, ce travail s'emploie à modéliser l'impact des incertitudes concernant les films acoustiques et zones d'interface sur le comportement des absorbeurs et traitement acoustiques. Ces films et zones sont notoirement difficiles à caractériser avec précision à cause de leurs propriétés ou de de leur inaccessibilité. Le principal objectif de cette thèse est de proposer des stratégies pour prendre en compte les incertitudes et interfaces afin de prédire leur impact sur la réponse globale.Trois contributions scientifiques sont présentées. Ensemble, elle discutent différents aspects de modélisation se rapportant aux films et aux absorbeurs, proposent de possibles simplifications et me, QC 20191119

The spatio-temporal sparsity of room impulse responses (RIRs) degrades in the late part due to the stronger wave interference compared with the early part. Meanwhile, such an interference decays in amplitude as time progresses due to absorption in the room, resulting in a decreasing dynamic range of the measurement. Together these aspects pose challenging conditions for compressive sensing applications, such as the interpolation of RIRs measured at sparse microphone positions. In the search of sparse transformation spaces, this paper examines the application of a multi-scale directional dictionary (known as shearlets) to interpolate RIR measurements. These redundant dictionaries consist of multiple curved elementary functions, which offer a decomposition of the acoustic wavefronts into various wavelengths, propagation directions, and times of arrival. Results reported in this paper demonstrate the potential these dictionaries have to interpolate RIRs in both convex and nonconvex rooms, motivating further examination under experimental conditions and in broader frequency ranges., QC 20210914

The Cremer impedance concept based on mode merging is one method that can substantially improve the axial damping in a waveguide. Previous works on the Cremer impedance including a uniform grazing flow have exhibited unexpected phenomenon such as negative resistance in the low frequency range. The current paper is a continuation of earlier works by the authors to extend the investigation of the Cremer impedance with a focus on the low frequency range. Two independent investigations from the perspective of boundary layer effects and mode merging patterns are conducted to better understand the low frequency behavior of the Cremer impedance., QC 20190902

This paper introduces a method to recover unmeasured room impulse responses (RIRs) in acoustical spaces with unknown properties and contents, by means of a compressed sensing methodology. Methods published in the existing literature have been validated in empty, convex rooms; a limited subset of the many, diverse acoustical spaces one can encounter. It results a challenge to represent such diverse wave phenomena with a sparse set of plane waves or equivalent sources, given the coupling between the sparsity of such representations and hypotheses regarding the properties of the acoustical space and its contents, far-field measurement distances, and other parameters. In contrast to this philosophy, the method introduced in this paper exploits the sparsity inherent to the mathematical structure of the wavefronts present in the RIRs, which without further hypotheses carry themselves all the information about the wave propagation in the room. In essence, the measured RIRs are instead represented with a sparse set of curved elementary functions of various sizes, propagation directions and times of arrival, which are linked with the various shapes and locations of the unknown scatterers and boundaries in the room. The main contribution of this work is thus to enable the measurement of RIRs in more complex acoustical spaces, while keeping the number of microphones to a minimum with the use of compressed sensing. The method is formulated as a sparse optimization problem, and the solution is obtained with an iterative thresholding algorithm whose threshold value is determined from the measurements. An analysis of sensing coherence is included, and the performance of the method is experimentally evaluated with 1D microphone array measurements in two lecture rooms and one meet-ing room. For the sake of comparison, the RIRs are also linearly interpolated using a low-pass filter in the wavenumber-frequency domain. The experimental results demonstrate that the proposed method is su, QC 20190815

Experimental results on the aero-acoustic interaction at an area expansion are scarce. In this study, new experimental data is acquired of the aero-acoustic interaction at an area expansion and compared with recent modeling efforts. The aero-acoustic interaction at an area expansion with expansion ratio eta = 0.309 is investigated by measuring the scattering matrix for plane waves at 5 different flow speeds in the incompressible regime. The experimental results are complimented with a comprehensive uncertainty analysis to determine the precision of the data. The scattering coefficients together with derived quantities such as absorption coefficients and dimensionless end corrections are compared against recent analytical results. It is shown that there is consistent deviation between the models and measurement results for certain scattering coefficients. The measured end correction on Strouhal number close to the critical Strouhal number shows a relatively abrupt change. This in comparison with the models, where the change is predicted to be more gradual. Similar observations are made for the absorption coefficient for waves incident upstream of the area expansion, although a qualitative comparison is difficult to make in this case because of the strong influence of the flow downstream of the area-expansion on the results., QC 20190730

One way to obtain a complete characterization of a sound source in a duct is to measure two-port data, comprising of the source scattering matrix and the source cross-spectrum. The present paper discusses in detail the design and construction of a two-port rig to characterize high speed small fans. A new post-processing method is suggested for the source cross-spectrum data and is compared to an earlier published method. The new method is shown to behave better in highly reflective cases, but is more sensitive to flow noise. In some cases, the duct diameter is not equal to the diameter of the test object. An adaptor can then be used to match the diameters, but it will modify the two-port results. In the present paper a method to remove the adaptor influence on the measured scattering matrix and source vector is presented, and validated by measurements., QC 20190328

We consider the problem of separating and reconstructing the sound field radiated by a planar source, e.g. a vibrating plate, that faces a parallel reflector. For this purpose we propose a Fourier-based technique to process near-field microphone array measurements that are performed on a single plane between the plate and the reflector. A closed-form expression is derived in the wavenumber domain to recover the free-space sound field radiated by the plate. The fundamental assumption is that the plate scatters sound in a locally-reacting fashion. Overall, provided the admittances of the plate and the reflecting surface are known, the results demonstrate that it is possible to recover the free-field sound radiated by the plate with a reasonable accuracy., QC 20190306

The goal of the work is to find a way to improve the sound insulation properties of different types of panels in order to meet different requirements. Inspired by the nontrivial behavior of the locally resonant acoustic metamaterials, this concept is introduced into the design of structures in order to explore the potential ways to improve the sound insulation behavior in the relevant specific frequency regions. At relatively low frequency region when the bending wavelength is much longer than the distance between isolated resonators, which is also the interesting frequency range in the most part of the work, it may be assumed that the effects of the resonators are uniformly distributed over the entire surface. An impedance approach is hence proposed to estimate the sound transmission loss of the metamaterial panels in order to get more insights from physics. This is realized, in general, by integrating the equivalent impedance of the resonators together with the corresponding impedance of the host panel. Valuable theories are derived based on that, laying a solid foundation for effective/efficient design of metamaterial panels. This approach also provides a fast and reliable tool for the designs prior to a time-consuming and computationally expensive numerical simulation. Based on that, a new design for locally resonant metamaterial sandwich plates is proposed to improve the sound transmission loss performance in the coincidence frequency region. A systematic method to tune the resonance frequency of local resonators is developed. This approach also supplies a method to remove the possible side-dips associated with the resonance of the resonators. The influence of the sound radiation from the resonators is further investigated with the Finite Element models. It is proposed to embed the resonators inside the core material in order to eliminate the possible influence, and also to make a smooth surface. The metamaterial sandwich panel designed in this way combines imp, QC 20190308

This paper investigates the suppression of vibration and sound radiation of a sandwich plate through the use of periodic design. A periodic sandwich plate is constructed and its dispersion relation is calculated. The vibration and sound radiation properties of the periodic sandwich plate are studied. Via the comparison of the periodic and bare sandwich plate, the effects of the periodic design on the vibration and sound radiation are analysed. Further, to know the sound radiation properties better, sound radiation efficiency of the periodic and bare sandwich plates is compared. In addition, the effects of the boundary conditions on the properties of the periodic sandwich plate are analysed. The numerical results demonstrate that the vibration and sound radiation are greatly suppressed over the stop band of the periodic sandwich plate. The suppression can also be obtained in part of pass bands. It is also shown that the periodic design can be an effective method for the reduction of the sound radiation efficiency. The suppression for the vibration and sound is greater than that caused by only increasing the mass of the plate in the designing frequency range., QC 20190305

Perforates are used for noise control in automotive mufflers and aircraft engine liners as well as for other vehicles and machines. Their acoustic properties and noise reduction are known to depend on the mean flow field and other parameters such as temperature and acoustic excitation level. It is therefore of interest to understand how the properties of perforates varies with the level of acoustic excitation. This paper gives an overview of high level nonlinear effects on the acoustic properties of perforates. It includes semi-empirical models as well as experimental studies. Methods for studying nonlinear effects and harmonic interaction effects, for perforates, using single tone excitation and Poly-harmonic distortion models or nonlinear scattering matrices are discussed. These techniques typically require measurements with a number of different acoustic loads. It would be more attractive to directly be able to extract the nonlinear acoustic properties from a more limited set of experiments using either random or periodic excitation. Multi input - single output techniques for nonlinear system identification using broadband random excitation has been tried with limited success. One reason is the mixing of the sound pressure signal incident from the acoustic source with the sound pressure transferred to higher frequencies by nonlinear effects at the perforate sample. The present paper includes an attempt to combine band-limited broadband excitation with Poly-harmonic distortion models or nonlinear scattering matrices describing the nonlinear transfer of energy to higher frequencies., QC 20190125

This paper focusses on the modelling and prediction of two of the main mechanisms of acoustic attenuation, the dynamic drag and thermal heat transfer, of a bundle of fibres typical of lightweight fibrous porous materials. The methodology uses geometrical properties derived from microscopy, and is based on the assumption that the interaction between the shear stress fields as well as the thermal fields of neighbouring fibres, may be neglected in the predicted dynamic impedances of an individual fibre. Analytical procedures are discussed which provide an estimation of the impedances acting on infinite longitudinal and transversely orientated cylinders. The frequency-dependent viscous and thermal contributions may be scaled in terms of statistical fibre diameter distributions and orientation angles. Using these analytical models, a three-dimensional poroelastic model of coupled acoustic and structural wave propagation through a transversely isotropic, fibrous, highly porous thermal insulation material is presented., QC 20181213

Advancements in 3D print technology now allows the printing of structured acoustic absorbent materials at appropriate microscopic scale and sample sizes. Optimisation of parameter sets associated with a Kelvin Cell structure have the potential to develop various metabehaviours in the associated acoustic responses. The repeatability of the fundamental cell unit also provide a route for the development of viable macro models to simulate built up structures based on detailed models of the individual cell units. This paper describes a process to model, print and test of such a sample. Manufacturing restraints will initially guide the optimised design. Micro to macro models based on a single cell structure which are cross checked against a full visco thermal acoustic finite element model of the individual cell. A macro model based on a reduced lossy helmholtz system is then used to predict the absorptive response response under normal incidence and checked against a manufactured sample under experimental test., QC 20181214

Most engineering applications involving solutions by numerical methods are dependent on several parameters, whose impact on the solution may significantly vary from one to the other. At times an evaluation of these multivariate solutions may be required at the expense of a prohibitively high computational cost. In the present work, a multivariate finite element approach is proposed, allowing for a fast evaluation of parametric responses. It is based on the construction of a reduced basis spanning a subspace able to capture rough variations of the response. The method consists in an extension of the Well-Conditioned Asymptotic Waveform Evaluation (WCAWE) to multivariate problems, by an appropriate choice of derivative sequences, and a selection of the most relevant basis components. It is validated and demonstrated for its potential on a semi-industrial sized 3D application involving coupled poroelastic and internal acoustic domains., QC 20190109

QC 20190418

This brief communication exposes an overview of various wavenumber filters to separate the rail contribution to pass-by noise via the wave signature extraction method [Zea, Manzari, Squicciarini, Feng, Thompson, and Lopez Arteaga, J. Sound Vib. 409, 24–42 (2017)]. It has been found that the originally proposed filters underesti- mate the rail noise at frequencies above 1.6 kHz due to the presence of higher-order wave families that is unaccounted for. The goal of this let- ter is thus to propose and examine different filter functions that can cap- ture such waves, and to assess whether the rail contribution estimations can be improved., QC 20181019Copyright 2018 Acoustical Society of America. This article may be downloaded for personal use only. Any other use requires prior permission of the author and the Acoustical Society of America.The following article appeared in The Journal of the Acoustical Society of America 144, EL286 (2018) and may be found at http://asa.scitation.org/doi/10.1121/1.5063453.

For quite some time, the strive for more efficient acoustic absorbers keepsincreasing, driven by a number of psycho-physiological studies on health re-lated dangers of noise exposure. As the global wealth increases and with itthe global expectation of quieter living and working environments, manifestedin both politics and research, an important market for sound absorbing andnoise control systems develops in all industrialised countries. In the acousticcommunity, the main endeavours of the two last decades have been orientedtowards a better understanding of the dissipation phenomena in absorbers(and especially in poroelastic media) as well as proposing new topologies andstructures for these elements. These efforts have resulted in an abundant lit-erature and numerous improvements of the characterisation, modelling anddesign methodologies for a wide range of media and many different systems.The chosen research direction for the present thesis slightly deviates fromthis usual path of modelling absorbing materials as bulk media. Here theaim is to investigate the interfaces between the different components of typ-ical absorbers. Indeed, these interface regions are known to be difficult tocharacterise and controlling their properties is challenging for a number ofreasons. Interfaces in sound packages for instance are inherently by-productsof the assembly process and, even if they surely have an important impact onthe acoustic performance, they remain mostly overlooked in the establishedmodelling practices. Therefore, the overall objective of the current doctoralproject is to identify strategies and methods to simulate the effect(s) of un-certainties on the interface physical or geometrical parameters.The present licentiate thesis compiles three works which together form adiscussion about techniques and tools designed in an attempt to efficientlymodel thin layers and small details in rather large systems. As part of thework a section of physical model simplifications is di, Sedan en tid tillbaka kan en ökande efterfrågan av material och konstruk-tioner med effektivare akustiska absorptionsegenskaper skönjas. Detta drivsav ett antal psykofysiologiska studier kring hälsorisker relaterade till långva-rig bullerexponering. Till detta kommer den växande globala välfärden somger upphov till en förväntad höjning livskvalitet i form av till exempel tystareboende- och arbetsmiljöer, manifesterad i både politiska beslut och forskning.Ur detta utvecklas en viktig marknad för ljudabsorberande material, kon-struktioner och bullerreducerande system i allt fler länder. Inom forskningensom rör akustiska material och bulleråtgärder, har forskningen under de tvåsenaste decennierna framförallt varit inriktad på en bättre förståelse för dis-sipationsfenomen (omvandling av akustisk energi till andra energiformer, tillexempel värme) i absorberande material (och särskilt då i poroelastiska me-dier) samt att utveckla nya topologier och sammansatta strukturer för dennatyp av akustiska element. Dessa ansträngningar har resulterat i en omfattan-de vetenskaplig litteratur och framsteg inom karaktäriserings-, modellerings-och designmetoder, och innovativa lösningar, för olika tillämpningar .Forskningen i denna avhandling avviker något från den traditionella mo-delleringen av absorberande material, genom att undersöka gränsytorna mel-lan olika skikt och komponenter hos typiska absorbenter. Faktum är att des-sa gränssnittsregioner är kända för att vara både svåra att karaktärisera ochatt styra deras egenskaper i produktion är en utmaning av ett antal olikaskäl. Angränsande ytor i ljudabsorbenter är till exempel ofta biprodukterfrån tillverknings- och monteringsprocessen och, även om de utan tvekan haren viktig inverkan på akustiska prestanda, bortses det ofta från dessa i deflesta etablerade simuleringsmodeller. Det övergripande målet med forskning-en inom detta projektet är därför att identifiera strategier och metoder föratt modellera och simulera effekterna av os, Depuis quelques temps, l’effort pour améliorer l’efficacité des absorbeursacoustique n’a cessé d’augmenter, sous-tendu par nombre d’études psycho-physiques sur les dangers de l’exposition au bruit pour la santé. Alors quecroit la richesse globale et avec elle l’envie d’environnements de travail et devie plus silencieux (ce qui se manifeste dans les politiques publiques comme enrecherche), un marché important se développe pour les systèmes d’absorptionet de contrôle du bruit dans tous les pays industrialisés. Dans la communautéacoustique, les principaux efforts au cours des vingt dernières années se sontorientés vers une meilleure compréhension des phénomènes de dissipation dansles absorbeurs (et en particulier dans les matériaux poroélastiques) ainsi quevers la recherche de nouvelles topologies et structures pour ces éléments. Cesefforts ont mené à une littérature abondante et de nombreuses améliorationsdes méthodologies de caractérisation, modélisation et conception pour unelarge gamme de média et de nombreux systèmes.L’axe de recherche choisi pour la présente thèse diffère quelque peu duchemin classique visant à modéliser le cœur des matériaux absorbants. Ici,l’objectif est d’étudier les interfaces entre les différents composants des absor-beurs classiques. En effet, ces régions sont notoirement difficiles à caractériseret contrôler leurs propriétés est un défi complexe pour un certain nombre deraisons. Les interfaces au sein des isolants acoustiques sont intrinsèquementdes sous-produits du processus d’assemblage et, bien qu’ils aient un impactimportant sur la performance acoustique, sont le plus souvent négligés dansles modèles classiques. L’objectif global du projet doctoral est ainsi d’identi-fier des stratégies et méthodes pour simuler le ou les effets d’incertitudes surles paramètres physiques ou géométriques des interfaces.La présente thèse de Licentiate compile trois travaux qui, pris ensemble,forment une discussion autour de techniques et d’outils conçus p, QC 20180516

A coupling approach is presented to combine a wave‐based method to the standard finite element method. This coupling methodology is presented here for the Helmholtz equation but it can be applied to a wide range of wave propagation problems. While wave‐based methods can significantly reduce the computational cost, especially at high frequencies, their efficiency is hampered by the need to use small elements to resolve complex geometric features. This can be alleviated by using a standard finite element model close to the surfaces to model geometric details and create large, simply‐shaped areas to model with a wave‐based method. This strategy is formulated and validated in this paper for the wave‐based discontinuous Galerkin method together with the standard finite element method. The coupling is formulated without using Lagrange multipliers and results demonstrate that the coupling is optimal in that the convergence rates of the individual methods are maintained., QC 20180516. QC 20191106

The present paper explores a modified version the wave signature extraction (WSE) method [1], whose aim is to separate the rail contribution to the pass-by noise of railway vehicles. The method requires a line microphone array parallel to the rail, and two accelerometers on the rail in the ver- tical and lateral direction. The motivation for this work is the need to separate the rail contribution to the pass-by noise of railway vehicles. The WSE method [1] is based on the wavenumber domain filtering of pass-by data measured with a microphone array located in the near-field of the rail. The filter design does not require a priori information of the structural properties of the rail, since the required information is obtained from array pressure data and rail vibration data before and after the train passes in front of the array. The filter is such that it extracts the dispersion plot branches of the first family of horizontal and vertical bending waves (moving band-pass filter). Although the comparison with TWINS simulation data provides very promising results, there are discrepancies at the higher frequencies, possibly due to the onset of new bending wave families. Therefore in the present paper we propose a modified filtering procedure, where a bank of band-pass and low-pass filters are used instead, and the results are compared to the original WSE implementation and the TWINS simulation data. We show that the results in the higher frequency range are improved with respect to the original WSE implementation., QC 20190930

QC 20190918

In order to seek for new ways to improve the sound transmission loss at ring frequency range for curved aeronautical structures, a semi-infinite curved panel, representing aircraft fuselage, is studied in this research. First, a minor modification on Koval's classical theory for cylindrical shells is made to incorporate effective mass theory for acoustic metamaterials. Then, based on the finite element simulations, the possibility of improving the acoustic behaviour of such panels by using locally resonant acoustic metamaterial treatment is investigated. Both the host curved panel, and the host panel periodically mounted with local resonators, denoted as metamaterial curved panel, are compared from the transmission loss point of view. The observations show that the antiresonance dip appears first rather than the resonance peak in the sound transmission loss for the tested metamaterial curved panel, which may be explained by the effective impedance of the panel. The influences of the panel curvature on the dynamic behaviour of the local resonators are further discussed in comparison to the same resonantsystem acting on the flat panel. Both the finite element simulations and the theoretical estimations show the same trend, thus confirms the proposed explanation., QC 20190226

A combined design method of impedance of acoustic nacelle is developed based on the Cremer impedance. First, the Cremer method is further extended to provide the theoretical optimum impedance for infinite uniform annular ducts in the presence of a single azimuthal mode and plug flow. Then, with the Cremer impedance being the initial value, the searching optimization based on a finite element sound propagation model is conducted to determine the impedance for maximizing noise reduction under the conditions of nearly actual duct geometry and flow. A design case of impedance of a typical aeroengine inlet is used to evaluate the method. The TL contour against impedance indicates that the combined design can avoid the local solution and find successfully the globally optimum impedance, thereby leading to the maximum sound attenuation. Additionally, the Cremer impedance is quite close to the final optimum impedance in this case, thereby providing not only a good initial impedance but also a proper searching space for the search optimization. Therefore, the combined design method is of high efficiency and accuracy., QC 20190125

Particulate emissions from road transportation are known to have an adverse impact on human health as well the environment. As the effects become more palpable, stricter legislation have been proposed by regulating bodies. This puts forward a challenge for the automotive industry to develop after treatment technologies to fulfil the progressively stricter legislation. At present, the most common after-treatment technologies used for particulates are the diesel and gasoline particulate filters. The typical size distribution of the particles is such that the smallest particles in terms of size are in numbers the largest, although they are not influencing the total particle mass significantly. The most recent legislation have included restrictions on the particle number as well as particle mass. In this thesis numerical tools for studying the transport and interaction of particles in an exhaust flow are evaluated. The specific application is particle agglomeration as a mean to reduce the number of particles and manipulate the size distribution. As particles agglomerate the particle number distribution is shifted and larger sized agglomerates of particles are created reducing the number of ultra-fine particles. The particle agglomeration is obtained by forcing sudden acceleration and deceleration of the host gas carrying the particles by variations in the cross sectional areas of the geometry it is passing through. Initially, a simplified one dimensional model is utilized to assess the governing parameters of particle grouping. Grouping here means that the particles form and are transported in groups, thus increasing the probability for agglomeration. The lessons learned from the 1D-model are also used to design the three dimensional geometry: an axisymmetric corrugated pipe. Two different geometries are studied, they both have the same main pipe diameter but different diameter on the corrugations. The purpose is to find the potential onset of flow instabilities and the, QC 20181008

QC 20181228

Nonmodal instability analysis is carried out for a 2:1 elliptic cone with base flow conditions selected for a Ma=7 and two different ight altitudes, namely 33km and 21km with unit Reynolds number Re′ = 1.89 x 106 m-1 and Re′ = 1.015 x 107 m-1, respectively. The aim is to analyze the effects of transiently growing optimal disturbances and their possible relation to instability mechanisms that have been confirmed to exist in previous modal crossow. Local linear stability results obtained at several streamwise locations on the cone surface indicate that transient growth in the crossow region may be correlated to streamwise oriented structures having spanwise spacing of the same order of magnitude as which have long been known to exist in this flow., QC 20181031

Heat exchangers, consisting of tube arrays in a cross-flow are a vital component of power generation systems. They are of interest from an acoustic point of view, because they can reflect, transmit and absorb an incident sound wave; in other words, they have the potential to act as a sound absorber and even as a passive control device to prevent a thermoacoustic instability in the power generation system. This paper presents a fundamental study of the aeroacoustic response of a tube array with and without bias-flow (also called cross-flow). The study has a theoretical and experimental side. On the theoretical side, a new model, based on the assumption of quasi-steady flow, was developed to predict the acoustic reflection and transmission coefficient of a tube array with bias-flow. Also, the model by Huang and Heckl (Huang and Heckl, 1993, Acustica 78, 191-200) for the case without bias-flow was evaluated. On the experimental side, flow-duct experiments using a multi-microphone technique were performed to validate the predictions from both models. The agreement was found to be very good for low frequencies. The measurements revealed the limit of validity of the quasi-steady model in terms of the Strouhal number. Although this limit is quite low, our quasi-steady model can serve as a valuable tool for designers of heat exchangers., QC 20180927

The noise produced during a train pass-by originates from several different sources such as propulsion noise, noise from auxiliary equipment, aerodynamic noise and rolling noise. The rolling noise is radiated by the wheels and the track and is excited by the wheel and rail unevenness, usually referred to as roughness. The current TSI Noise certification method, which must be satisfied by all new mainline trains in Europe, relies on the use of a reference track to quantify the noise from new vehicles. The reference track is defined by an upper limit of the rail roughness and a lower limit of the track decay rate (TDR). However, since neither the rail roughness nor the track radiation can be completely neglected, the result cannot be taken as representing only the vehicle noise and the measurement does not allow separate identification of the noise radiated by wheel and track. It is even likely that further reductions in the limit values for new rolling stock cannot be achieved on current tracks. There is therefore a need for a method to separate the noise into these two components reliably and cheaply. The purpose of the current study is to assess existing and new methods for rolling noise separation. Field tests have been carried out under controlled conditions, allowing the different methods to be compared. The TWINS model is used with measured vibration data to give reference estimates of the wheel and track noise components. Six different methods are then considered that can be used to estimate the track component. It is found that most of these methods can obtain the track component of noise with acceptable accuracy. However, apart from the TWINS model, the wheel noise component could only be estimated directly using three methods and un- fortunately these did not give satisfactory results in the current tests., QC 20180525, Roll2Rail

The present paper investigates a modified implementation of the wave signature extraction (WSE) method [1], whose aim is to separate the rail contribution to the pass-by noise of railway vehicles. The method requires a line microphone array parallel to the rail, and two accelerometers on the rail in the vertical and lateral direction. The motivation for this work is the need to separate the rail contribution to the pass-by noise of railway vehicles. The WSE method [1] is based on the wavenumber domain filtering of pass-by data measured with a microphone array located in the near-field of the rail. The filter design does not require a priori information of the structural properties of the rail, since the required information is obtained from array pressure data and rail vibration data before and after the train passes in front of the array. The filter is such that it extracts the dispersion plot branches of the first family of horizontal and vertical bending waves (moving band-pass filter). Although the comparison with TWINS simulation data provides very promising results, there are discrepancies at the higher frequencies, possibly due to the onset of new bending wave families. Therefore in the present paper we assess the performance of a simplified WSE method, where a moving low-pass filter is used instead, and the results are compared to the original WSE implementation and the TWINS simulation data. We show that the results in the higher frequency range are improved with respect to the original WSE implementation. [1] Zea et al. Journal of Sound and Vibration 409, pp. 24–42, 2017

This paper introduces and investigates a novel single-layer Fourier-based near-field acoustic holography (NAH) technique suited to measurements with a linear microphone array in the presence of a reflector that is perpendicular to the array. The general idea is to model the reflected plane waves as weighted versions of the incident waves. Numerical experiments are performed in order to test the reconstructions against the conventional (free-field) NAH technique and reference results obtained from theoretical calculations. The most important outcome of this paper is that the reconstruction errors decrease as the wavelength of the plane waves is an integer multiple of the array length, and this is attributed to spectral leakage and windowing artefacts introduced by the use of the spatial Fourier transform. In the case of greatest leakage, that is, the wavenumbers of the plane waves are exactly in the middle of two adjacent bins of the wavenumber domain, the errors can exceed 100%. In the case of no leakage, the errors are no greater than 25%. Overall, this study points towards further investigation of the method.

Design of duct networks is challenging because the design should consider the required flow rate, acceptable noise levels, and minimum pressure drop to achieve optimum performance. This paper presents acoustic analysis in high frequency range using sound power two-ports applied to Heating, Ventilation, and Air Conditioning (HVAC) systems. To simulate the acoustic behaviour one need to model three mechanisms; the sound power generated from sound sources (e.g. Fans), the regenerated sound power caused by the flow in different elements in the network (e.g. junctions), and the sound power loss across different elements of the network. The general approach considered here is based on two-port theory that divides the duct network into two-port elements. Each element can be described by 2 x 2 scattering matrix where the state variables are the acoustic power flow in both up and downstream directions. Junctions and branching are described by multi-port elements depending on the number of elements connected to this multiport. This algorithm is compared to measurements of HVAC system located in an academic building that shows good agreement. An advantage of this approach is the ability to use the same formalism of the two-port network theory to analyse the acoustic behaviour in both low and high frequency ranges beside the flow distribution and the pressure drop., QC 20180516

A dominating source for the radiated acoustic noise from a train at low speeds is the traction motor. This noise originates from electromagnetic forces acting on the structure resulting in vibrations on the surface and thus radiated noise. It is often perceived as annoying due to its tonal nature. To achieve a desirable acoustic behavior, and also to meet legal requirements, it is of great importance to thoroughly understand the generation of noise of electromagnetic origin in the motor and also to be able to control it to a low level. In this work, experimental tests have been performed on a traction motor operated from pulse width modulated (PWM) converter. A PWM converter outputs a quasi-sinusoidal voltage created from switched voltage pulses of different widths. The resulting main vibrations at PWM operation and their causes have been analyzed. It is concluded that an appropriate selection of the PWM switching frequency, that is the rate at which the voltage is switched, is a powerful tool to influence the noise of electromagnetic origin. Changing the switching frequency shifts the frequencies of the exciting electromagnetic forces. Further experimental investigations show that the trend is that the resulting sound power level decreases with increasing switching frequency and eventually the sound power level reaches an almost constant level. The underlying physical phenomena for the reduced sound power level is different for different frequency ranges. It is proposed that the traction motor, similar to a thin walled cylindrical structure, shows a constant vibration over force response above a certain frequency. This is investigated using numerical simulations of simplified models. Above this certain frequency, where the area of high modal density is dominating, the noise reducing effect of further increasing the switching frequency is limited., QC 20180109

This paper discusses the effect of high level broad band and tonal acoustic excitation on the acoustic properties of perforates. It is based on an experimental study of the nonlinear properties of these types of samples without mean grazing or bias flow. In previous studies multiple tone and high acoustic excitation levels were studied. Both normal incidence impedance tube measurements and liners placed in a grazing incidence configuration were considered. It is known from previous studies that high level acoustic excitation at one frequency will change the acoustic impedance of perforates at other frequencies, thereby changing the boundary condition seen by the acoustic waves. The combination of tones as well as the relative phase between frequency components has also been shown to influence the result. There is usually also a broadband random level excitation in combination with the harmonic tonal excitation. The effect of this combination on the acoustic properties of a perforate has been studied., QC 20170816

The aerodynamic sound sources on a realistic landing gear are investigated using numerical phased array methods, based on array data extracted from compressible Detached-Eddy Simulations of the flow. Assuming monopole or monopole in a moving medium propagation, the sound sources are identified in the source region through various beamforming approaches: dual linear programming (dual-LP) deconvolution, orthogonal beamforming and CLEAN-SC. The predicted source locations are in good agreement with previous experimental results performed on the same nose landing gear configuration by industrial and academic partners within the ALLEGRA project. Additionally, the modeled sources are used to generate far-field spectra which are subsequently compared to the ones obtained with the Ffowcs Williams-Hawkings acoustic analogy. The results of the dual-LP approach show a good match between the far-field spectra up to a certain frequency threshold cor- responding to the quality of the mesh used. The results demonstrate the potential of numerical phased array methods as a legitimate modeling tool for aeroacoustic simulations in general and as a tool to gain insight into the noise generation mechanisms of landing gear components in particular., QC 20170608

QC 20180507

In order to counteract the problem of railway noise and its environmental impact, passing trains in Europe must be tested in accordance to a noise legislation that demands the quantification of the noise generated by the vehicle alone. However, for frequencies between about 500 Hz and 1600 Hz, it has been found that a significant part of the measured noise is generated by the rail, which behaves like a distributed source and radiates plane waves as a result of the contact with the train's wheels. Thus the need arises for separating the rail contribution to the pass-by noise in that particular frequency range. To this end, the present paper introduces a wavenumber–domain filtering technique, referred to as wave signature extraction, which requires a line microphone array parallel to the rail, and two accelerometers on the rail in the vertical and lateral direction. The novel contributions of this research are: (i) the introduction and application of wavenumber (or plane–wave) filters to pass-by data measured with a microphone array located in the near-field of the rail, and (ii) the design of such filters without prior information of the structural properties of the rail. The latter is achieved by recording the array pressure, as well as the rail vibrations with the accelerometers, before and after the train pass-by. The performance of the proposed method is investigated with a set of pass-by measurements performed in Germany. The results seem to be promising when compared to reference data from TWINS, and the largest discrepancies occur above 1600 Hz and are attributed to plane waves radiated by the rail that so far have not been accounted for in the design of the filters., QC 20170801, Roll2Rail

Comprehensive methods to cascade active multi-ports, e.g., for acoustic network prediction, have until now only been available for plane waves. This paper presents procedures to combine multi-ports with an arbitrary number of considered duct modes. A multi-port method is used to extract complex mode amplitudes from experimental data of single and tandem in-duct orifice plates for Helmholtz numbers up to around 4 and, hence, beyond the cut-on of several higher order modes. The theory of connecting single multi-ports to linear cascades is derived for the passive properties (the scattering of the system) and the active properties (the source cross-spectrum matrix of the system). One scope of this paper is to investigate the influence of the hydrodynamic near field on the accuracy of both the passive and the active predictions in multi-port cascades. The scattering and the source cross-spectrum matrix of tandem orifice configurations is measured for three cases, namely, with a distance between the plates of 10 duct diameter, for which the downstream orifice is outside the jet of the upstream orifice, 4 duct diameter, and 2 duct diameter (both inside the jet). The results are compared with predictions from single orifice measurements. It is shown that the scattering is only sensitive to disturbed inflow in certain frequency ranges where coupling between the flow and sound field exists, whereas the source cross-spectrum matrix is very sensitive to disturbed inflow for all frequencies. An important part of the analysis is based on an eigenvalue analysis of the scattering matrix and the source cross-spectrum matrix to evaluate the potential of sound amplification and dominant source mechanisms., QC 20170522, IdealVent

An application of phased array methods to numerical data is presented, aimed at identifying equivalent flow sound sources from aeroacoustic simulations. Based on phased array data extracted from compressible flow simulations, sound source strengths are computed on a set of points in the source region using phased array techniques assuming monopole propagation. Two phased array techniques are used to compute the source strengths: an approach using a Moore-Penrose pseudo-inverse and a beamforming approach using dual linear programming (dual-LP) deconvolution. The first approach gives a model of correlated sources for the acoustic field generated from the flow expressed in a matrix of cross- and auto-power spectral values, whereas the second approach results in a model of uncorrelated sources expressed in a vector of auto-power spectral values. The accuracy of the equivalent source model is estimated by computing the acoustic spectrum at a far-field observer. The approach is tested first on an analytical case with known point sources. It is then applied to the example of the flow around a submerged air inlet. The far-field spectra obtained from the source models for two different flow conditions are in good agreement with the spectra obtained with a Ffowcs Williams-Hawkings integral, showing the accuracy of the source model from the observer's standpoint. Various configurations for the phased array and for the sources are used. The dual-LP beamforming approach shows better robustness to changes in the number of probes and sources than the pseudo-inverse approach. The good results obtained with this simulation case demonstrate the potential of the phased array approach as a modelling tool for aeroacoustic simulations., QC 20160921

In a classical paper (Acustica 3, 1953) Cremer demonstrated that in a rectangular duct, with locally reacting walls, there exits an impedance (”the Cremer impedance”) that maximizes the propagational damping for the lowest mode. Later (JSV 28, 1973) Tester extended the analysis to include a plug flow and ducts of both circular and rectangular cross-section. One limitation in the work of Tester is that it simplified the analysis of the effect of flow only considering high frequencies or well cut-on modes. This approximation is reasonable for large duct applications, e.g., aero-engines, but not for many other cases of interest. Kabral et al. (Acta Acustica united with Acustica 102, 2016) removed this limitation and investigated the’exact’ Cremer impedance for circular ducts including flow effects. As demonstrated in that paper the exact solution exhibits some special properties at low frequencies, e.g., a negative real part of the wall impedance. In this paper the exact Cremer impedance is further analyzed and discussed. Also, the exact solution for rectangular ducts is presented., QC 20190109

An innovative configuration of an acoustic sandwich structure is proposed in this paper, which uses the locally resonant structures to generate stopbands in desired frequency regions and hence to increase the sound transmission loss of the panel. Effects of different types of resonators, including the mounting techniques, are investigated. The methods to broaden the effective stopbands are discussed. The acoustic properties of the sandwich panel with non-flat laminates are also studied. Numerical analyses show that good results can be obtained when combining the laminate modification with the locally resonantstructure, especially when the stopbands are designed to compensate the corresponding coincidence effects of the sandwich panel. The analysis is based on the Finite Element models constructed in COMSOL. Bloch wave vectors are derived at first Brillouin zone by using wave expansion method. Dispersion relation of the structure is discussed. Experimental validation is planned, and the results will be shown in the conference., QC 20180507

A hybrid reactive-dissipative splitter silencer offers the potential to attenuate turbomachinery noise over a wide frequency range, including the problematic low to medium frequencies. This article uses a theoretical model to investigate the performance of a hybrid parallel baffle silencer for different complex incident sound fields. This includes an incident sound field with equal modal energy density, as well as the excitation of individual higher order modes. It is shown that provided horizontal and vertical partitions are used in the reactive element, the sound attenuation performance of the reactive chamber under complex incident sound fields is equivalent to that obtained using plane wave excitation over the frequency range of interest. Furthermore, it is demonstrated that the reactive elements work at frequencies above the first cut-on mode in the inlet duct, and so they are capable of extending sound attenuation into the low to medium frequency range. This delivers an efficient hybrid silencer design that is suitable for use in power generation applications, such as gas turbine exhaust systems., QC 20190109

A numerical methodology is presented aimed at evaluating the sound field generated by acoustic sources from ground vehicles using monopoles moving in a simplified urban environment. The approach includes reflection and scattering on nearby building surfaces, differentiating it from standard pass-by test simulations, which are performed on an ideal track in a free-field. In the example shown in this study, the sound sources consist of a group of incoherent monopoles representing the flow sound from a NACA air inlet, which were identified using numerical beamforming in a previous study. Other sources such as the engine or the exhaust could also be included as long as they can be modelled with one or more monopoles. Propagation is computed using a method previously developed by the authors, based on a point-to-point moving source Green's function and a modified Kirchhoff integral under the Kirchhoff approximation to compute first-and second-order reflections on built surfaces. This methodology has the potential to be used by vehicle manufacturers to assess the acoustic impact of a particular vehicle design in urban operating conditions, or by urban planners to get local predictions of the sound levels caused by a single vehicle pass-by., QC 20171113

Several Padé-based computational methods have been recently combined with the finite element method for the efficient solution of complex time-harmonic acoustic problems. Among these, the component-wise approach, which focuses on the fast-frequency sweep of individual degrees of freedom in the problem, is an alternative to the projection-based approaches. While the former approach allows for piecewise analytical expressions of the solution for targeted degrees of freedom, the projection-based approaches may offer a wider range of convergence. In this work, the two approaches are compared for a range of problems varying in complexity, size and physics. This includes for instance the modeling of coupled problems with non-trivial frequency dependence such as for the modeling of sound absorbing porous materials. Conclusions will be drawn in terms of computational time, accuracy, memory allocation, implementation, and suitability of the methods for specific problems of interest., QC 20180522

A numerical methodology is presented to simulate the sound field generated by moving aerodynamic sound sources in a simplified urban environment. The framework is described step by step from source identification to pass-by simulations. The applicability of the methodology is demonstrated on the example of a NACA inlet, a type of submerged air inlet typical placed on cars or trains. The methodology is based on three steps that use previous research published by the authors. In a first step, the flow around the NACA inlet is solved using compressible detached-eddy simulation. In a second step, identification of the sources is performed using direct numerical beamforming with linear programming deconvolution, based on array pressure data extracted from the flow simulation. In a third step, the sources are propagated in a urban environment by means of a simplified boundary element method based on a moving point source Green’s function and a Doppler-corrected Kirchhoff integral using the Kirchhoff approximation. The ground is accounted for through the image source method. The results are analysed in terms of pass-by time-frequency descriptions and sound pressure level. Due to its fully numerical framework, this methodology can be used to evaluate the contribution of aerodynamic sound sources to the pass-by signal early in the design process of ground vehicles. It should also be possible to include other sound sources such as the engine or the exhaust in the propagation step to get a more complete pass-by description and evaluate the contribution of the individual sources to the total emitted noise, providing that they can be modelled as monopoles and have a statistically stationary frequency content., QC 20161208

Sound field analysis is a fundamental tool in the design, manufacturing, testing and diagnosis of machines and transportation means, as well as the legislations that regulate noise levels in order to minimize environmental pollution. Customary solutions to the problem of sound field analysis are microphone array technologies such as near-field acoustic holography (NAH) and beamforming. One of the challenges of using these technologies often lies in the difficulty for separating disturbing sounds from the target source, specially when these are correlated. For example, NAH requires that no reflecting surfaces are found in the vicinity of the array, which is in theory only possible in an echo-free chamber. On the other hand, beamforming is most suitably used to separate uncorrelated sound sources, which is not the case of, for instance, the noise generated by the contact between the wheel of a train and a railway track. The present thesis examines the research problems of separating a sound source from its reflections, and separating the rail noise from the total noise radiated by a passing train. The overall goal of the thesis is to push the limits of microphone array technologies in the context of sound field separation, to the end of minimizing the cost and complexity of measurements and analyses. The proposed separation methods are formulated in the wavenumber domain, and the measurements are done with uniform single layer microphone arrays. The problem of separating reflections is addressed in three different papers: (i) compact sources and a parallel reflector, (ii) planar source and a parallel reflector, and (iii) a perpendicular reflector with respect to the microphone array, and the common requirement is the knowledge of the reflector impedance. The problem of separating rail noise is studied in a fourth paper, and the proposed method is formulated such that it does not require prior knowledge of the rail properties. Upon the findings obtained in the papers, QC 20171117