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Evaluation of the acoustic comfort of air-treatment systems with a multi-task procedure ; ; et al in E.A.A. (Ed.) Proceedings of the 2012 Euronoise Congress (2012, June) Detailed reference viewed: 6 (1 ULg)Timbre description of the sound of air-treatment systems for predicting acoustic confort ; Billon, Alexis ; et al Poster (2012, April) Detailed reference viewed: 25 (5 ULg)Acoustic comfort of air-treatment systems: influence of factors related to room acoustics ; ; et al Conference (2011, June 30) Detailed reference viewed: 30 (4 ULg)Modélisation des champs sonores en acoustique architecturale par un processus de diffusion ; Billon, Alexis ; et al Conference (2010, April) La modélisation des champs sonores en acoustique architecturale est d'autant plus complexe que la géométrie d'étude est elle-même complexe. En particulier, le domaine de propagation peut présenter de ... [more ▼] La modélisation des champs sonores en acoustique architecturale est d'autant plus complexe que la géométrie d'étude est elle-même complexe. En particulier, le domaine de propagation peut présenter de nombreux volumes couplés de tailles très différentes, connectés par de faibles surfaces de couplage. Dans ces conditions, les formulations analytiques classiques, basées par exemple sur la théorie de la réverbération, ou les méthodes numériques, basées par exemple sur des méthodes de tracé de rayons ou de faisceaux sonores, sont difficilement applicables, voire impossibles. Ces dernières années, une solution alternative, fondée sur l'utilisation d'une équation de diffusion pour la densité d'énergie sonore, a été développée et validée. Dans ce présent travail, nous présentons les développements permettant ainsi de modéliser les champs sonores en acoustique des salles, en prenant en compte les difféerents phénomènes propagatifs mis en jeu, tel que l'atténuation atmosphérique, l'absorption par les parois, la transmission acoustique à travers les cloisons et la réflexion mixte au niveau des surfaces. Dans un second temps, nous présentons la méthode retenue pour la résolution numérique des équations de diffusion mises en jeu. A cet effet, une interface logicielle a été spécialement développée, permettant de manipuler de manière fonctionnelle les données géométriques et acoustiques du problème, avant l'exécution d'un logiciel de calcul multi-physique, en s'affranchissant ainsi du formalisme spécifique associé au modèle de diffusion. Cette interface permet ensuite le post-traitement de l'ensemble des résultats, notamment afin de déterminer et de représenter différents paramètres classiques d'acoustique des salles (niveaux sonores, temps de réverbération. . . ). [less ▲] Detailed reference viewed: 69 (0 ULg)Sound Field Modeling in Architectural Acoustics using a Diffusion Equation Based Model ; ; Billon, Alexis et al Conference (2009, October) In this paper, an implementation of a model for room-acoustic predictions in COMSOL Multiphysics is presented. The model (called diffusion model) is based on the solving of diffusion equations instead of ... [more ▼] In this paper, an implementation of a model for room-acoustic predictions in COMSOL Multiphysics is presented. The model (called diffusion model) is based on the solving of diffusion equations instead of classical wave equations and allows simulating the sound propagation in complex geometries at high frequency. Instead of using COMSOL Multiplysics to solve directly the problem, a specific tool has been developed. It is composed of a user-friendly interface (I-Simpa) which manipulates all the physical data of the problem (geometries, acoustic properties and elements…) and generates an interoperability script to execute COMSOL Multiplysics via the generation of .m file scripts. Then, numerical results are processed by I-Simpa for the acoustic post-treatment and results display, like sound levels, reverberation times… [less ▲] Detailed reference viewed: 239 (2 ULg)Numerical simulations of the sound propagation in non rectilinear streets Billon, Alexis ; ; et al Conference (2009, October) At high frequencies, the sound propagation can be approximated by the propagation of sound particles which are reflected and absorbed at the boundaries. This propagation follows then a transport process ... [more ▼] At high frequencies, the sound propagation can be approximated by the propagation of sound particles which are reflected and absorbed at the boundaries. This propagation follows then a transport process. In urban canyons, this transport process can be reduced to a diffusion process. The resultant model allows one to obtain the spatial distribution of the acoustical energy in a street in a few seconds using a classical finite elements program. This diffusion model has been previously validated in rectilinear streets for different geometrical and acoustical parameters. In this paper, more complex geometries such as varying cross-sections, bents and streets crossings are dealt with. As the diffusion model only models the reverberated sound field, diffraction effects on the direct sound field at streets corners are added. The obtained results are then compared to scale model experiments. [less ▲] Detailed reference viewed: 53 (3 ULg)An empirical diffusion model for acoustic prediction in rooms with mixted specular and diffuse refections ; ; Billon, Alexis et al in Acta Acustica United with Acustica (2009), 95(1), 97-105 In this paper, a modification to the room-acoustic diffusion model is proposed to take different amounts of wall scattering into account. An extensive set of numerical simulations using a cone-tracing ... [more ▼] In this paper, a modification to the room-acoustic diffusion model is proposed to take different amounts of wall scattering into account. An extensive set of numerical simulations using a cone-tracing software has first been carried out, in order to highlight the impact of the scattering coefficient on the diffusion process in rooms, in terms of sound pressure levels. An iterative method is then proposed to identify, for a given value of the wall’s scattering coefficient, the diffusion constant that allows the stationary sound field to be governed by a diffusion process, regardless of the room’s geometry. Using this method, an empirical law can be proposed between the diffusion constant and the scattering coefficient. The empirical diffusion model is then compared to scale model experiments, as well as to other models from the literature, with a satisfactory agreement for the sound pressure level. However, the empirical diffusion model fails to predict the sound decay for rooms with perfectly specularly reflecting surfaces, due to the inherent concept of a diffusion process. [less ▲] Detailed reference viewed: 51 (2 ULg)Introducing atmospheric attenuation within a diffusion model room-acoustic predictions Billon, Alexis ; ; et al in Journal of the Acoustical Society of America (2008), 123(6), 4040-4043 This paper presents an extension of a diffusion model for room acoustics to handle the atmospheric attenuation. This phenomenon is critical at high frequencies and in large rooms to obtain correct ... [more ▼] This paper presents an extension of a diffusion model for room acoustics to handle the atmospheric attenuation. This phenomenon is critical at high frequencies and in large rooms to obtain correct acoustic predictions. An additional term is introduced in the diffusion equation as well as in the diffusion constant, in order to take the atmospheric attenuation into account. The modified diffusion model is then compared with the statistical theory and a cone-tracing software. Three typical room-acoustic configurations are investigated: a proportionate room, a long room and a flat room. The modified diffusion model agrees well with the statistical theory (when applicable, as in proportionate rooms) and with the cone-tracing software, both in terms of sound pressure levels and reverberation times. [less ▲] Detailed reference viewed: 42 (2 ULg)On the use of diffusion equations to model the acoustics of coupled rooms Billon, Alexis ; ; et al in Journal of the Acoustical Society of America (2008), 123(5), 3910-3910 he acoustics of coupled rooms are characterized by energy exchanges through apertures and/or partition walls. The use of systems of diffusion equations allows to predict the temporal and spatial energy ... [more ▼] he acoustics of coupled rooms are characterized by energy exchanges through apertures and/or partition walls. The use of systems of diffusion equations allows to predict the temporal and spatial energy distributions in these configurations quite accurately. In this presentation, the diffusion formalism for room acoustics-prediction is summarized. The systems of equations to be solved in the case of coupling through an aperture and through a partition wall are presented. For two rooms coupled through an aperture (two classrooms connected through an open door), the results obtained with the diffusion model are compared to experimental data, in terms of sound pressure levels and sound decays. On the other hand, for the case of two classrooms connected through a partition wall, the diffusion model is compared to experimental data in terms of sound pressure level difference only. Finally, an engineering application is presented in the configuration involving a workroom including multiple sound sources (e.g., machines) connected to offices through open and closed doors. [less ▲] Detailed reference viewed: 31 (1 ULg)Predictions of the reverberation time in high absorbent room using a modified-diffusion model Billon, Alexis ; ; in Applied Acoustics (2008), 69(1), 68-74 A modification of the diffusion model’s boundary condition, based on the Eyring absorption coefficient, to account for high walls absorption is proposed. Numerical comparisons are carried out for three ... [more ▼] A modification of the diffusion model’s boundary condition, based on the Eyring absorption coefficient, to account for high walls absorption is proposed. Numerical comparisons are carried out for three geometrical configurations (a proportionate room, a corridor and a flat enclosure). Comparisons with the statistical theory and a ray-tracing software show that the modified boundary condition increases the accuracy of the diffusion model in term of reverberation time in all the simulated configurations. An experimental comparison in the case of a non-uniformly absorbent room (a reverberation chamber covered with patches of glass wool) is also carried out. The modified-diffusion model results match well with the ray-tracing ones. Both models are in agreement with the experimental data for most of third octave bands (discrepancy close to or below 10%). However, some discrepancies up to 40% can also be observed in a few octave bands, probably due to experimental considerations and to the modal behaviour of the room at low frequencies. [less ▲] Detailed reference viewed: 31 (3 ULg)Modeling the sound transmission coupled through partition walls using a diffusion model Billon, Alexis ; ; et al in Journal of the Acoustical Society of America (2008), 123(6), 4261-4271 In this paper, a modification of the diffusion model for room acoustics is proposed to account for sound transmission between two rooms, a source room and an adjacent room, which are coupled through a ... [more ▼] In this paper, a modification of the diffusion model for room acoustics is proposed to account for sound transmission between two rooms, a source room and an adjacent room, which are coupled through a partition wall. A system of two diffusion equations, one for each room, together with a set of two boundary conditions, one for the partition wall and one for the other walls of a room, is obtained and numerically solved. The modified diffusion model is validated by numerical comparisons with the statistical theory for several coupled-room configurations by varying the coupling area surface, the absorption coefficient of each room, and the volume of the adjacent room. An experimental comparison is also carried out for two coupled classrooms. The modified diffusion model results agree very well with both the statistical theory and the experimental data. The diffusion model can then be used as an alternative to the statistical theory, especially when the statistical theory is not applicable, that is, when the reverberant sound field is not diffuse. Moreover, the diffusion model allows the prediction of the spatial distribution of sound energy within each coupled room, while the statistical theory gives only one sound level for each room. [less ▲] Detailed reference viewed: 25 (1 ULg)Diffusion-based models for predicting sound fields in rooms with mixed specular and diffuse reflection ; Billon, Alexis ; et al Conference (2007, September) A diffusion model, based on the numerical solution of a diffusion equation, was developed in the past to give accurate predictions of the sound-pressure level and of the sound decay at any location in a ... [more ▼] A diffusion model, based on the numerical solution of a diffusion equation, was developed in the past to give accurate predictions of the sound-pressure level and of the sound decay at any location in a single enclosure or in coupled rooms. However, this model is limited to rooms with diffusely reflecting walls. In this study, two methods are presented to extend this model to rooms with mixed specular and diffuse reflections, defined by the classical room-acoustics scattering coefficient. The first method models the reverberant field by using a modified diffusion process, with an empirical diffusion constant departing from the theoretical one based on the mean free path. The set of these coefficients, which depend on the scattering coefficient, has been found empirically to fit the results given by a ray-tracing program. The second method is a hybrid approach. The reverberant sound field due to the diffuse reflections is first obtained with the original diffusion model. Then the sound field due to the specular reflections is modelled with an image-source model. The results given by the two approaches are compared to experimental data obtained for several types of rooms (a classroom, an office and some long rooms). [less ▲] Detailed reference viewed: 41 (3 ULg)Aeroacoustics coupling of a misaligned jet-slot-oscillator with non-planar modes of a resonator ; Billon, Alexis ; et al Conference (2007, September) The impingement of a plane jet on a slotted plate produces a “jet-slot oscillator” emitting acoustic tones. The coupling of such tones with high-order acoustic resonances of the flow-supply duct (i.e ... [more ▼] The impingement of a plane jet on a slotted plate produces a “jet-slot oscillator” emitting acoustic tones. The coupling of such tones with high-order acoustic resonances of the flow-supply duct (i.e., above the first transverse mode cut-off frequency) is studied experimentally , in the particular case where the jet-exit and the obstacle are misaligned in the spanwise direction. An experimental set-up based on microphones and hot-wire probes is used to investigate the effect of the slot angle on the sound production mechanism. The aligned jet-slot oscillator is first studied, showing that the parallelism between the jet exit and the slot imposes the excitation of planar modes of the resonator only. Conversely, the misaligned jet-slot oscillator appears to be coupled with a combination of planar and transverse modes. The induced spanwise variation of the acoustic field phase has two consequences. First, it compensates the difference of vortices convection time in the spanwise direction, due to the obstacle misalignment. Second, it enhances the spanwise coherence of the aeroacoustic source produced by the jet-slot interaction: the vortex-tubes are shed in an oblique direction which tends to be parallel to the obstacle. [less ▲] Detailed reference viewed: 70 (1 ULg)Modelling the sound transmission through separation walls using a diffusion model Billon, Alexis ; ; et al Conference (2007, September) The diffusion model has been used successfully to evaluate the acoustic behaviour of a system of coupled rooms connected through a coupling aperture. In this paper, an extension of this model is proposed ... [more ▼] The diffusion model has been used successfully to evaluate the acoustic behaviour of a system of coupled rooms connected through a coupling aperture. In this paper, an extension of this model is proposed to deal with the propagation of sound energy through a partition wall. The diffusion model can be considered as an extension of the statistical theory to none diffuse sound fields. Numerical comparisons with the statistical theory are then carried out. The following parameters are varied: its transmission loss of the separation wall, its surface, the coupled room’s absorption coefficient and the coupled room’s volume. The agreement between the statistical theory and the diffusion model is very good. [less ▲] Detailed reference viewed: 47 (1 ULg)Sound field modeling in architectural acoustics using a diffusion equation ; ; Billon, Alexis et al Conference (2006, December) A numerical approach is proposed to model the reverberated sound field in rooms. The model is based on the numerical implementation of a diffusion model enabling spatial variations of the sound energy ... [more ▼] A numerical approach is proposed to model the reverberated sound field in rooms. The model is based on the numerical implementation of a diffusion model enabling spatial variations of the sound energy within a room, unlike the statistical theory. The proposed method allows to take into account most of complex phenomena encountered in room acoustics, like mixed reflections on walls (diffuse and specular), low and high absorption on walls, atmospheric attenuation, fitted zones. Moreover, the model can be applied to complex geometries, like multiple coupled rooms of different sizes. In this paper, the model and its numerical implementation are first detailed. Then, an application is proposed for a complex geometry defined by multiple coupled rooms with fitting objects, including low and high absorption on walls, in terms of sound level and reverberation times. The main interest of the model is that such approach requires less computational time in comparison with common approaches like ray-tracing simulations. [less ▲] Detailed reference viewed: 56 (1 ULg)Sifflements produits par un jet plan accordé heurtant une plaque fendue Billon, Alexis ; ; et al Scientific conference (2006, August) Sous certaines conditions défavorables, les bouches de soufflage des circuits de ventilation peuvent émettre des sifflements désagréables. Ce phénomène, modélisé par l’impact d’un jet plan sur une plaque ... [more ▼] Sous certaines conditions défavorables, les bouches de soufflage des circuits de ventilation peuvent émettre des sifflements désagréables. Ce phénomène, modélisé par l’impact d’un jet plan sur une plaque fendue, est étudié expérimentalement. Les fréquences des deux instabilités principales, l’instabilité de la couche de cisaillement et le mode de battement du jet, de l’écoulement sans obstacle permettent de définir le domaine d’existence de la fréquence la plus énergétique des sons auto-entretenus produits. Le cas particulier d’un jet accordé est étudié. Cette donnée permet d’anticiper la stratégie de réduction la plus adaptée. [less ▲] Detailed reference viewed: 15 (0 ULg)Frequency halving due to vortex pairing for the jet-slot oscillator Billon, Alexis ; ; et al Conference (2006, July) At the outlet of the HVCA systems, whistling can occur, due to self-sustained oscillations. In this study, the ventilation outlet is modelled by a free plane subsonic jet impinging on a slotted plate ... [more ▼] At the outlet of the HVCA systems, whistling can occur, due to self-sustained oscillations. In this study, the ventilation outlet is modelled by a free plane subsonic jet impinging on a slotted plate, leading to self-sustained tones production; this configuration is known as the jet-slot oscillator. The tone's frequency can be predicted through the vortex dynamics within the flow. For jet velocities higher than 16m/s, the tones couple with the flow-supply-duct’s resonances. These resonances control the vortex dynamics and reinforce the sound production, of about 20dB. Moreover, when the distance from the jet exit to the plate is increased and reaches 4.5 times the jet height, the fundamental frequency of the tones is suddenly halved due to some vortex pairing occurring at the end of the potential core of the jet. In this paper, the vortex pairing is observed with three different experimental techniques. Firstly, comparison between the radiated and the in-duct acoustic fields is conducted. Then the energy transfer from the fundamental to the sub-harmonics of the shear layer's velocity fluctuations is observed with anemometric measurements. Finally high speed flow visualizations are performed and allow to link the vortex impingement on the plate to the sound production. [less ▲] Detailed reference viewed: 32 (0 ULg)Prediction of the reverberation time in rooms with non uniform absorptions using a diffusion model Billon, Alexis ; Conference (2006, July) The acoustical comfort is now a comfort’s criterion which is taken into from the conception. The designers need therefore accurate tools to predict the acoustic quality of enclosures. Most of the ... [more ▼] The acoustical comfort is now a comfort’s criterion which is taken into from the conception. The designers need therefore accurate tools to predict the acoustic quality of enclosures. Most of the acoustical comfort criteria are based on the reverberation time. When the room has proportionate dimensions and an uniform absorption, the statistical theory through Sabine or Eyring formulas allows good predictions of the reverberation time. Moreover, extensions of these relations by Millington or Cremer and Müller among others give quite satisfactory when the room is composed of materials with different sound absorption. In this study, the reverberation time in an enclosure is calculated via the numerical resolution of unstationary diffusion equation, model validated in coupled and industrial rooms. Firstly, an improvement of the boundary condition is proposed for highly absorbent surfaces. The diffusion model is then compared to several formulations of the statistical theory and a ray-tracing software for a cubic room with homogeneous walls’ absorption and with non homogeneous walls’ absorptions. Finally, an experimental validation is conducted for an enclosure with non uniformly distributed absorption. [less ▲] Detailed reference viewed: 69 (4 ULg)Prediction of the acoustics of coupled rooms with the acoustic-diffusion model ; Billon, Alexis ; et al Conference (2006, June) A general model is proposed to simulate the acoustics of coupled rooms. It is based on a diffusion equation, solved numerically to perform acoustic predictions. The presence of scattering objects –or the ... [more ▼] A general model is proposed to simulate the acoustics of coupled rooms. It is based on a diffusion equation, solved numerically to perform acoustic predictions. The presence of scattering objects –or the “fittings”– is also taken into account. Distinct sub-volumes can be defined, representing either coupled volumes or zones with different fitting characteristics. Some sample results are presented, and compared with ray-tracing results and experimental data. Two situations are assessed: two coupled classrooms, and a room divided into two zones, one empty, one fitted. The diffusion-model predictions match the other data satisfactorily, both in terms of sound attenuation and sound decay. Diffusion-based results are obtained with the advantage of low computational time compared to ray-tracing results. [less ▲] Detailed reference viewed: 31 (0 ULg)Non-conﬁned jet-slot-cavity oscillator: ﬂow-acoustic interaction modeling ; Billon, Alexis ; et al Conference (2006, May) Disturbing whistling due to self-sustained oscillations can be produced in a wide variety of geometries where a sheared subsonic flow impinges on a downstream obstacle. When those self-sustained ... [more ▼] Disturbing whistling due to self-sustained oscillations can be produced in a wide variety of geometries where a sheared subsonic flow impinges on a downstream obstacle. When those self-sustained oscillations are coupled with an acoustic resonator, the acoustic production can reach dramatically high levels. The configuration known as jet-slot oscillator is studied in the particular case where the resonator is the flow-supply duct and the source region is unconfined. This sound production is due to flow-acoustic interactions and can be modeled by using the vortex-sound theory. It allows one to express the acoustic power generated by the interaction between the acoustic, velocity, and vorticity fields. They are obtained separately by using rather simple models together with experimental input data. The vorticity field is modeled with the assumption that the shear layer is rectilinear and the vorticity is only concentrated on discrete vortex-points convected at constant velocity. The vorticity being non null only in the shear layer, the velocity field is calculated only in this region where it is equal to the vortices convection speed. The source region dimensions being small compared to the acoustic wavelength, this field is considered, in harmonic regime, as a sinusoidal temporal fluctuation of a potential field. This field can be taken as resulting from the flow produced by the radiation of the flow supply duct's acoustic resonance. It is obtained by solving the Poisson equation in a simplified geometry obtained by Schwarz-Christoffel mapping. The configuration studied here presents some specific problems which must be addressed to correctly model its sound production. Experimentations are used to specify the missing parameters. The major one is the synchronisation between the vorticity field and the acoustic field which governs the way that the vortices interact with the resonant acoustic field. At last, comparison between experimental data and modeling results permits to validate the study, in terms of emitted frequency. Further developments of the model will allow to predict the emitted sound level. [less ▲] Detailed reference viewed: 78 (0 ULg) |
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