1,720,991 research outputs found
Definition of analytical models of non-acoustical parameters for randomly-assembled symmetric and asymmetric radii distribution in parallel fiber structures
No full textFibrous materials for acoustical applications have been extensively studied over the past years. A consid-erable number of experimental tests have been carried out as a function of fiber geometrical character-istics (for example, overall density, fiber diameter and density, open porosity, etc.) in order to determineanalytical formulations for the prediction of physical parameters, primarily the airflow resistivity. Due tothe increase of computational capabilities micro-to-macro structure approach is the subject of currentresearch in order to determine transport and acoustic properties starting from certain arrangements offiber in space. Almost all research assumes fiber having the same radius with regular or random positionwithin a representative volume of material. To this end, the aim of the present paper is to provide a revi-sion of existing analytical formulations of non-acoustical parameters for two-dimensional structures hav-ing random positioning in space and symmetric and asymmetric distribution of radii. Proposedformulations will be compared with existing models and finite element simulation
Numerical methodologies for predicting the low frequency behavior of anechoic chambers
No full textIn this paper a description of the use of simplified numerical methodologies for determining the low cut-off frequency of anechoic and hemi-anechoic chambers will be presented. The anechoic chamber has been modeled as a cavity with proper surface impedance boundary conditions. Firstly, the surface impedance is determined by means of a finite element model of the wedges in a "virtual" impedance tube for a plane wave field. Successively, both analytical and numerical procedure are used for determining the complex reflection coefficient for spherical waves at oblique incidence. Finally, a complex image source approach is used for predicting the sound field within the chamber. The methodology will be applied to the new anechoic chamber of the University of Ferrara, having volumes of 650 m3and 800 m3in anechoic and hemi-anechoic configurations, respectively. Results will be compared in terms of sound decays along fixed directions and surfaces pressure distributions.In this paper a description of the use of simplified numerical methodologies for determining the low cut-off frequency of anechoic and hemi-anechoic chambers will be presented. The anechoic chamber has been modeled as a cavity with proper surface impedance boundary conditions. Firstly, the surface impedance is determined by means of a finite element model of the wedges in a "virtual" impedance tube for a plane wave field. Successively, both analytical and numerical procedure are used for determining the complex reflection coefficient for spherical waves at oblique incidence. Finally, a complex image source approach is used for predicting the sound field within the chamber. The methodology will be applied to the new anechoic chamber of the University of Ferrara, having volumes of 650 m3 and 800 m3 in anechoic and hemi-anechoic configurations, respectively. Results will be compared in terms of sound decays along fixed directions and surfaces pressure distributions.In this paper a description of the use of simplified numerical methodologies for determining the low cut-off frequency of anechoic and hemi-anechoic chambers will be presented. The anechoic chamber has been modeled as a cavity with proper surface impedance boundary conditions. Firstly, the surface impedance is determined by means of a finite element model of the wedges in a "virtual" impedance tube for a plane wave field. Successively, both analytical and numerical procedure are used for determining the complex reflection coefficient for spherical waves at oblique incidence. Finally, a complex image source approach is used for predicting the sound field within the chamber. The methodology will be applied to the new anechoic chamber of the University of Ferrara, having volumes of 650 m3 and 800 m3 in anechoic and hemi-anechoic configurations, respectively. Results will be compared in terms of sound decays along fixed directions and surfaces pressure distributions
On the Solved Turbulent Scales in Turbulent Plume Fires
No full textPlume fires are characterized by a turbulent nature with a large number of different
scales. LES is often used to solve the largest structures and to model the smallest ones. Grid size
and time steps become decisive to place the limit between resolved and modelled turbulence.
Significant information on this limit and its placement can be obtained with spectral analyses of
the specific turbulent kinetic energy. While frequency analysis is relatively easy, an analysis in
the wavenumber domain is more challenging. The IWC method, typically used in structures and
acoustics, is used here for this purpose. IWC method allows to obtain wavenumber spectra with
a better resolution than those obtained with a direct approach. Furthermore, in this paper the IWC
method is also used in its reverse form to obtain frequency spectra. Although rather dense grids
have been chosen, the number of nodes along the plume and their spacing is not such as to
guarantee detailed wavenumber spectra with the direct approach and consequently with the
reverse IWC. On the contrary, the IWC method provides wavenumber spectra in agreement with
those obtained directly, but of much higher quality
Using near-field acoustic measurements to characterise mechanical and acoustic properties of lightweight building structures
No full textThe evaluation of the vibrational field on a certain surface has a great importance in noise control engineering applications. It can be helpful, both for noise reduction purposes, and diagnostic purposes, in product optimisation, or in order to characterise the mechanical properties of the vibrating structure. Accelerometers are still the most used sensors to measure vibration. However, contactless transducers, such as scanning laser doppler vibrometers (LDV), have been widely used recently, presenting several advantages. Near-field acoustic holography may represent a valid alternative to LDV, in order to reconstruct the vibrational field on a surface from sound pressure measurements performed with an array of microphones. This paper presents the preliminary results, of a ongoing project regarding the use of near-field acoustic holography to characterise the elastic and acoustic properties of a lightweight building structure. The used test rig allows to scan the sound pressure over the vibrating surface on a grid of points, using an array of microphones. The panel's dynamic response was used to evaluate its elastic properties. The reliability of this experimental approach was assessed by comparing the results with the ones obtained using miniature accelerometers. These elastic properties were finally used as input data to model sound transmission through the plywood panel using the transfer matrix method, investigating the accuracy by comparing the numerical results with the experimental sound insulation
FE-based approach to compute the sound transmission loss of building partitions
No full textThis paper investigates a numerical approach to compute sound transmission loss of building partitions and other structures. This method requires that, a unit cell, a small portion of the considered partition, is modelled by using the Finite Element Method (FEM), and coupled with semi-infinite fluid domains on both sides. On the sending side, the structure is excited by an acoustic plane wave. Bloch-Floquet periodic boundary conditions are applied in order to consider the structure of infinite extent. The infinite sound transmission coefficient of the considered partition is computed from the sound field variables obtained from the FE simulation. To increase the accuracy of the results below the critical frequency, the finite-size radiation impedance can be considered. The transmission loss computed with this approach is comparable with the results obtained by means of the transfer matrix method (TMM). In this study the validation of the results, obtained with the FE approach, and an assement of their accuracy were achieved by comparing them with the sound transmission loss computed using the TMM. Moreover, the influence on the results of practical aspects of the implementation of the model, such as dimension of the unit cell, the number of elements in the perfectly matched layer domains, the extension of the fluid domains and the size of the mesh were investigated by means of a parametric analysis
Building materials: Influence of physical, mechanical and acoustic properties in sound prediction models
No full textSeveral models to compute the sound reduction index of a building partition, based on different approaches, can be found in the literature. Building construction involves a great variety of materials, and regardless of the chosen sound transmission simulation prediction approach, their elastic and physical properties, which are necessary as input data, have a strong influence on the accuracy of the predicted results. In this article, the influence of such properties is investigated, by means of a prediction model based on the method proposed by the recently updated standard EN ISO 12354. Moreover, the reliability of different prediction models and the consistency of their results have been tested, by simulating several building partitions made of various materials, both homogeneous and non-homogeneous
Determination of the structural response of lightweight structures by means of sound pressure measurements
No full textFrom the dynamic response of a vibrating structure, it is possible to determine trustworthy insights about the acoustic and mechanical properties of that specific element. The structural wavenumber, propagating along a certain direction, can be determined from the dynamic response of the element, measured along a line of equally spaced points. Moreover, the radiation efficiency of the structure, for example, can be computed from the complex vibration velocity, measured over a grid of points on the surface of the element. Measurements of the vibrational filed are commonly performed by using accelerometers attached to the surface of the vibrating object. By using miniature transducers, it is possible to investigate a wide range of structures, even though non-contact transducers are sometime preferred, in order to avoid any small influence of added masses on the dynamic response of very lightweight elements. Optical methods have been widely used to overcome such issue, although they require very expensive equipment, such as a laser Doppler vibrometer. Near-field acoustic holography represents an alternative contactless approach, which allows the reconstruction of the structural dynamic response from the sound pressure measured with an array of microphones. In this study the possibility to accurately determine the structural response of a lightweight structure by measuring acoustic pressure in the vicinity of the vibrating surface has been investigated. The reliability of the results has been determined by comparing the structural response reconstructed from sound pressure measurement to the response directly measured by using accelerometers. Furthermore, the elastic properties of the structures, derived from their dynamic response, have also been compared
On the comparison of different techniques for measuring complex acoustical properties of fibrous and porous materials
No full textThe knowledge of the complex properties of fibrous and porous materials has become of paramount importance in the design of quieter environments, especially for enclosed spaces. Several techniques were developed for measuring the transmission coefficient, the characteristic impedance and the propagation constant of these materials. The present work focuses on the transfer matrix technique, that is, the sound field is firstly decomposed into an incident, a reflected and a transmitted wave. Then the complex pressure and particle velocity at each side of the sample material are calculated. From these quantities the transfer matrix is constructed and finally the complex acoustical properties are obtained. In this work the experimental results obtained for porous and fibrous sample materials are compared and discussed together with the two cavity technique in an impedance tube and the predictions obtained by means of the Delany-Bazley model.The knowledge of the complex properties of fibrous and porous materials has become of paramount importance in the design of quieter environments, especially for enclosed spaces. Several techniques were developed for measuring the transmission coefficient, the characteristic impedance and the propagation constant of these materials. The present work focuses on the transfer matrix technique, that is, the sound field is firstly decomposed into an incident, a reflected and a transmitted wave. Then the complex pressure and particle velocity at each side of the sample material are calculated. From these quantities the transfer matrix is constructed and finally the complex acoustical properties are obtained. In this work the experimental results obtained for porous and fibrous sample materials are compared and discussed together with the two cavity technique in an impedance tube and the predictions obtained by means of the Delany-Bazley model
A newly developed low-cost 3D acoustic positioning system: Description and application in a reverberation room
Full text linkA newly developed low-cost 3D acoustic positioning system is used to measure the sound field inside a reverberation room. The measurement system requires a personal computer, a standard multi-channel sound card and very few external components such as 4 small amplified loudspeakers (used only for the 3D positioning system) placed at known fixed positions and a microphone, used as probe for both the positioning and the sound field measurements. A dodecahedron is used to excite the reverberation room with an MLS signal. The measurement is performed moving the microphone inside the reverberation room and measuring, point by point, the local impulse response, in order to analyze the room modes and possibly compute a spatially localized reverberation time. The results of the measurements are compared with a FEM model of the sound field inside the reverberation room
- …
