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See detailA Geometrical Acoustics Approach Linking Surface Scattering and Reverberation in Room Acoustics
Embrechts, Jean-Jacques ULg

in Acta Acustica United with Acustica (in press)

A general model of the influence of surface scattering on the reverberation time would have several applications in room acoustics. Such a model is not yet available, and it is the purpose of this paper ... [more ▼]

A general model of the influence of surface scattering on the reverberation time would have several applications in room acoustics. Such a model is not yet available, and it is the purpose of this paper to investigate a geometrical acoustics approach. Starting from the radiative transfer equation, an exponential solution is first developed for the reverberation energy decay in rooms with diffusely reflecting boundaries. Differences with the diffuse sound field (Sabine) theory are highlighted, leading to a modified formula for the reverberation time which is shown to be more in accordance with ray tracing simulations. A general model is then proposed for rooms in which specular and diffuse reflections coexist. This general model is applied in rooms where the specular contribution can be assumed to be quasi-isotropic and uniform. Under this assumption, the reverberation decay is represented by the sum of two exponential functions, depending on the scattering coefficients. However, it is shown that the influence of surfaces’ scattering on reverberation is rather limited in this case. On the contrary, rooms with a pair of parallel surfaces are prone to create significant anisotropy in the cloud of image sources. An analytical formulation is proposed in this case for the specular and diffuse contributions, provided that some assumptions are again made on the specularly reflected sound field. The final expression is not really intuitive concerning the relation between scattering coefficients and reverberation, but it contains all the variables influencing this relation. This allows fast evaluations of the effect of surface scattering in particular situations. Finally, the application of this model to room acoustics computer simulations is illustrated by an example. [less ▲]

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See detailA Diffraction Model for Acoustical Ray-Tracing Based on the Energy Flow Lines Concept
Billon, Alexis; Embrechts, Jean-Jacques ULg

in Acta Acustica United with Acustica (2013), 99

Geometrical acoustics models are currently the most popular prediction tools in room-acoustics due to their low computing load. However, they seldom take into account the diffraction occurring at free ... [more ▼]

Geometrical acoustics models are currently the most popular prediction tools in room-acoustics due to their low computing load. However, they seldom take into account the diffraction occurring at free edges. Moreover, the existing diffraction models implemented in geometrical acoustics algorithms are either limited to specularly reflected paths or either present excessive computation load. Recently, a diffraction model based on an approximation of the far-field direction of the Poynting’s vector around half-planes has been introduced for computing the scattering due to faceted dielectric objects. In this article, this model is developed to handle obliquely incident waves and is implemented within an acoustic ray-tracing software using the analogy between the Poynting’s and the acoustic intensity vectors. The proposed model can handle both diffuse and specular reflections and sets no limit in terms of reflection or diffraction order. The first results obtained by ray-tracing agree well with the uniform theory of diffraction and the boundary elements method for single and double diffraction problems even if the interference effects are neglected and as long as the number of emitted rays is sufficient. Moreover, the additional computation load to handle diffraction is shown to be low compared to the one from a classical ray-tracing algorithm. [less ▲]

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See detailTheoretical Determination of the Random-Incidence Scattering Coefficients of Infinite Rigid Surfaces with a Periodic Rectangular Roughness Profile
Embrechts, Jean-Jacques ULg; Billon, Alexis ULg

in Acta Acustica United with Acustica (2011), 97

An exact solution is developed for the sound field reflected by an infinite rigid rough surface with a periodic rectangular profile. The random-incidence scattering coefficient of this surface is derived ... [more ▼]

An exact solution is developed for the sound field reflected by an infinite rigid rough surface with a periodic rectangular profile. The random-incidence scattering coefficient of this surface is derived from the numerical calculation of this solution for several directions of the incident plane wave. Scattering coefficients’ values are given in this paper for a great number of configurations by varying the geometrical parameters of the periodic profile, leading to a very detailed description of the scattering properties of this kind of profile. In particular, it is shown that, when the ratio H/L of the depth to the spatial period of the profile is small, the key parameter is H/, the ratio of the depth of the profile to the wavelength. The random-incidence scattering coefficient tends to increase with this ratio, as long as H/ is less than 0.3. This evolution is similar to gaussian and sine-shaped profiles analysed in previous studies. For H/ >0.3, the coefficient’s value oscillates while converging to an asymptotic value which depends on the width of the wells. Resonance effects have also been highlighted for periodic rectangular profiles, in particular by comparing the computed scattering coefficients with the few measured values published in the scientific literature. The scattering coefficients’ values published in this paper can be introduced in room acoustics’ models to characterize periodic rectangular surfaces, provided that their dimensions are much greater than the wavelength. Additional measurements and BEM computations are required to deeply analyse the influence of the finite size and the acoustical absorption of real surfaces having this profile. [less ▲]

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See detailExperimental investigation of the aeroacoustic coupling between the non-planar modes of a resonator and the misaligned jet-slot oscillator
Glesser, Martin; Valeau, Vincent; Billon, Alexis ULg et al

in Acta Acustica United with Acustica (2009), 95(3), 512-518

The jet-slot oscillator is a source of aeroacoustic self-sustained tones resulting from the impingement of the vortical structures of a plane jet onto a slotted plate. The coupling of this type of ... [more ▼]

The jet-slot oscillator is a source of aeroacoustic self-sustained tones resulting from the impingement of the vortical structures of a plane jet onto a slotted plate. The coupling of this type of aeroacoustic source with high-order acoustic resonances of the flow-supply duct is experimentally studied, in the particular case where the jet-exit and the obstacle are misaligned in the spanwise direction. An experimental setup based on microphones and hot-wire probes is used to investigate this coupling. It is shown that: (i) the misaligned jet-slot oscillator can be coupled to non-planar (transverse) modes of the flow-supply duct, (ii) the vortex tubes spanwise morphology is governed by the phase pattern of the excited modes, and (iii) the impingement of the vortices on the inclined obstacle tends to occur in phase in the spanwise direction, in order to enhance the coherence of the interaction along the slot. An investigation of the excited modes as a function of the plate inclination angle and of the streamwise slot position was then conducted. For small inclination angles, the coupling involves a planar mode or a combination of planar and transverse modes. For higher misalignment angles the coupling involves transverse modes or “opposite-transverse” modes, generating vortices with an inclination angle opposite to the plate inclination angle. The influence of the coupling nature on tone production (emission frequency and SPL) was thus evaluated. [less ▲]

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See detailAn empirical diffusion model for acoustic prediction in rooms with mixted specular and diffuse refections
Foy, Cédric; Valeau, Vincent; Billon, Alexis ULg 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 ▲]

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See detailCalculation of the random-incidence scattering coefficients of a sine-shaped surface
Embrechts, Jean-Jacques ULg; De Geetere, L.; Vermeir, G. et al

in Acta Acustica United with Acustica (2006), 92(4, JUL-AUG), 593-603

Random-incidence scattering coefficients obtained by measurement in reverberation chambers (ISO 17497-1) for a particular sine-shaped surface are compared with theoretical results. Holford-Urusovskii's ... [more ▼]

Random-incidence scattering coefficients obtained by measurement in reverberation chambers (ISO 17497-1) for a particular sine-shaped surface are compared with theoretical results. Holford-Urusovskii's method and a hybrid plane wave decomposition - FEM method are applied to an infinite sine-shaped surface having the same profile, while the 3D BEM gives numerical results for the finite-size surface. Perfect rigidity (Neumann boundary condition) is assumed in all computations. The validity of the computations is compared and discussed with regard to the approximations involved. Finally, the measured and calculated values of the scattering coefficients are compared. A good correspondence is found, in particular concerning the overall dependence on frequency. Small deviations have however been observed, at low as well as at high frequencies. They will be explained by uncertainties in the measurements and discretisation errors in the calculations. [less ▲]

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See detailA tutorial on scattering and diffusion coefficients for room acoustic surfaces
Cox, T. J.; Dalenback, B.-I.; D'Antonio, P. et al

in Acta Acustica United with Acustica (2006), 92(1, JAN-FEB), 1-15

There is a need to be able to quantify how sound is reflected from a surface, in terms of how it is spatially dispersed. In recent years, diffusion and scattering coefficients have been developed to ... [more ▼]

There is a need to be able to quantify how sound is reflected from a surface, in terms of how it is spatially dispersed. In recent years, diffusion and scattering coefficients have been developed to enable room acoustic prediction models to be more accurate, and designers to evaluate the worth of diffusing surfaces. Two coefficients have been, or are about to be, enshrined in standards. With two coefficients being standardised, there is great potential for confusion. This paper is intended to bring together authors from different areas to explain where the coefficients should and should not be applied. In addition, this tutorial paper will revisit the rationale behind the coefficients and contrast the coefficient definitions and performance. [less ▲]

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See detailCase studies in measurement of random incidence scattering coefficients
Vorländer, Michael; Embrechts, Jean-Jacques ULg; De Geetere, Lieven et al

in Acta Acustica United With Acustica (2004), 90(5, SEP-OCT), 858-867

In this work measurements of random-incidence scattering coefficients in three laboratories are compared. The sample geometry is sinusoidal. These surfaces are geometrically identical, but they were ... [more ▼]

In this work measurements of random-incidence scattering coefficients in three laboratories are compared. The sample geometry is sinusoidal. These surfaces are geometrically identical, but they were constructed in different scales. So far, measurements of this kind were performed only in scale models. Using turntables in real sample size was hardly considered possible. One result of general importance is that measurements are indeed possible in real-scale reverberation rooms with turntables of 3 m diameter. There are only small differences between the real-scale and model-scale results. Some variations of the standard procedure were tested and the uncertainties identified: mounting of the sample, connection or sealing between sample and base plate, way of rotating the sample, air absorption and time variances. The results presented can be regarded as guidelines for application of ISO/DIS 17497-1. [less ▲]

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See detailDetermination of the scattering coefficient of random rough diffusing surfaces for room acoustics applications
Embrechts, Jean-Jacques ULg; Archambeau, Dominique; Stan, Guy-Bart

in Acta Acustica United with Acustica (2001), 87

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