The Experts below are selected from a list of 1488 Experts worldwide ranked by ideXlab platform
David G. Blair - One of the best experts on this subject based on the ideXlab platform.
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Underwater acoustic imaging: image due to a specular reflector in the Geometrical-Acoustics limit
Journal of Marine Science and Technology, 2006Co-Authors: David G. BlairAbstract:In underwater acoustic imaging, which is used to produce high-quality images in turbid waters, a specular reflector can produce a “pseudoimage” of the receiving array at the reflecting surface. Based on the “Geometrical approximation” (which is similar to Geometrical Acoustics), formulae are derived for the size and shape of the pseudoimage for both flat and curved reflectors. For curved reflectors, described by two principal radii of curvature, the formulae also assume the “large-range approximation.” The formulae allow radii of curvature to be determined from an image. Also discussed briefly are some possible extensions and the role of nonGeometrical effects.
Desmet Wim - One of the best experts on this subject based on the ideXlab platform.
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Reverberation time and audibility in phased Geometrical Acoustics using plane or spherical wave reflection coefficients
'Acoustical Society of America (ASA)', 2019Co-Authors: Boucher, Matthew A, Rychtarikova Monika, Zelem Lukas, Pluymers Bert, Desmet WimAbstract:Room Acoustics parameters are typically predicted using some form of Geometrical Acoustics for large rooms. For smaller rooms, phased Geometrical Acoustics improves results for lower frequencies. The use of a spherical wave reflection coefficient improves the results further, yet the exact impact on room Acoustics parameters is not fully known. This work predicts the reverberation time in medium-sized rooms (27 m3
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Convergence study of phased Geometrical Acoustics using a low frequency reflection coefficient
Heverlee (Belgium), 2016Co-Authors: Boucher, Matthew Alban, Pluymers Bert, Desmet WimAbstract:This work investigates phased Geometrical Acoustics using a low frequency reflection coefficient. This reflection coefficient, which is derived from an exact solution to a half-space problem, consists of an integral of image sources located at complex coordinates and is valid for any position of the source and receiver above the reflecting plane and for an arbitrary surface impedance. The method is demonstrated by calculating the sound pressure level in a rectangular room using the image source and beam tracing methods. The improvement over using the plane wave reflection coefficient is highlighted, using the finite element method as a reference.status: publishe
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Interference effects in phased beam tracing using exact half-space solutions
'Acoustical Society of America (ASA)', 2016Co-Authors: Boucher, Matthew Alban, Pluymers Bert, Desmet WimAbstract:Geometrical Acoustics provides a correct solution to the wave equation for rectangular rooms with rigid boundaries and is an accurate approximation at high frequencies with nearly hard walls. When interference effects are important, phased Geometrical Acoustics is employed in order to account for phase shifts due to propagation and reflection. Error increases, however, with more absorption, complex impedance values, grazing incidence, smaller volumes and lower frequencies. Replacing the plane wave reflection coefficient with a spherical one reduces the error but results in slower convergence. Frequency-dependent stopping criteria are then applied to avoid calculating higher order reflections for frequencies that have already converged. Exact half-space solutions are used to derive two additional spherical wave reflection coefficients: (i) the Sommerfeld integral, consisting of a plane wave decomposition of a point source and (ii) a line of image sources located at complex coordinates. Phased beam tracing using exact half-space solutions agrees well with the finite element method for rectangular rooms with absorbing boundaries, at low frequencies and for rooms with different aspect ratios. Results are accurate even for long source-to-receiver distances. Finally, the crossover frequency between the plane and spherical wave reflection coefficients is discussed.status: publishe
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Coupled (vibro-)acoustic problems using Geometrical Acoustics and patch transfer functions
2015Co-Authors: Boucher, Matthew Alban, Pluymers Bert, Desmet WimAbstract:Geometrical Acoustics (GA) is well-suited for interior Acoustics problems at high frequencies when the boundaries are described by absorption coefficients. If the boundaries are described by an impedance, mid-frequency problems can also be solved if a phase factor, which is dependent on the total distance a ray has traveled, and a complex-valued plane wave reflection coefficient are included. However, including velocity boundary conditions in the context of GA is not as straightforward. One existing method approximates a vibrating panel as a distribution of acoustic monopoles. Another, the Green Ray Integral Method (GRIM) solves a boundary integral in which the pressure on the surface is determined by a ray tracing procedure. However, in both cases, it is assumed that the acoustic fluid does not influence the velocity field of the vibrating structure. These problems are best solved using other methods, such as the finite and boundary element methods for low frequencies and statistical energy analysis (SEA) for high frequencies. An alternative approach is presented here for coupled (vibro-)acoustic problems at mid- to high frequencies using Geometrical Acoustics and the patch transfer function (PTF) method. The PTF method is a general sub-structuring procedure in which a common surface between two domains is divided into patches and the spatially averaged pressure and normal velocity over each patch are conserved. The calculation of patch transfer functions of an acoustic domain using Geometrical Acoustics is detailed, and the approach is applied to a strong coupling case by dividing an acoustic volume into two separate volumes. In this way, convergence criteria and accuracy of the approach are quantified.status: publishe
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Coupled (vibro-)acoustic problems using Geometrical Acoustics and patch transfer functions
INT INST ACOUSTICS & VIBRATION, 2015Co-Authors: Boucher, Matthew Alban, Pluymers Bert, Desmet WimAbstract:Geometrical Acoustics (GA) is well-suited for interior Acoustics problems at high frequencies when the boundaries are described by absorption coefficients. If the boundaries are described by an impedance, mid-frequency problems can also be solved if a phase factor, which is dependent on the total distance a ray has traveled, and a complex-valued plane wave reflection coefficient are included. However, including velocity boundary conditions in the context of GA is not as straightforward. One existing method approximates a vibrating panel as a distribution of acoustic monopoles. Another, the Green Ray Integral Method (GRIM) solves a boundary integral in which the pressure on the surface is determined by a ray tracing procedure. However, in both cases, it is assumed that the acoustic fluid does not influence the velocity field of the vibrating structure. These problems are best solved using other methods, such as the finite and boundary element methods for low frequencies and statistical energy analysis (SEA) for high frequencies. An alternative approach is presented here for coupled (vibro-)acoustic problems at mid- to high frequencies using Geometrical Acoustics and the patch transfer function (PTF) method. The PTF method is a general sub-structuring procedure in which a common surface between two domains is divided into patches and the spatially averaged pressure and normal velocity over each patch are conserved. The calculation of patch transfer functions of an acoustic domain using Geometrical Acoustics is detailed, and the approach is applied to a strong coupling case by dividing an acoustic volume into two separate volumes. In this way, convergence criteria and accuracy of the approach are quantified.Book subtitle: MAJOR CHALLENGES IN Acoustics, NOISE AND VIBRATION RESEARCH, 2015status: publishe
Boucher, Matthew Alban - One of the best experts on this subject based on the ideXlab platform.
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Gefaseerde geometrische akoestiek met behulp van lage / hoge frequentie reflectiecoëfficiënten met toepassingen op absorptiemetingen ,,
2017Co-Authors: Boucher, Matthew AlbanAbstract:The prediction of sound and noise in everyday environments is of utmost importance for comfort as well as for personal health. Although measurements give invaluable information about the behavior of existing environments, designers and engineers are also in need of predictive models that are both accurate and efficient. These simulations provide detailed analysis before construction or prior to renovations and thus yield a huge return on investment. Geometrical Acoustics (GA) simulations are used throughout industry for the design of acoustic comfort in offices, classrooms, concert halls, music studios and more. Despite its prevalence, limitations of Geometrical Acoustics methods are still present. Most deficiencies stem from the high frequency assumptions of the method, meaning that GA traditionally only calculates the sound energy and is only an approximation to the wave equation. Expanding Geometrical Acoustics to lower frequencies by adding wave behavior such as scattering, diffraction and interference -- phenomena that are common in everyday experience -- is a need by practitioners and an important aim for researchers. A main focus of this work is on interference effects, which requires phase information so that sound pressures can be calculated instead of energy. An important source of information for phased Geometrical Acoustics comes from reflections from acoustical boundaries. While a plane wave assumption is valid at high frequencies, sound propagates more as a spherical wave when obstacles/walls/observers are close to the source, making spherical wave reflection coefficients important for low frequency sound prediction in Geometrical Acoustics. Including this behavior is more accurate and results in changes to acoustic parameters that are clearly noticed by most listeners. Previous work has shown that the uncertainty in sound absorption data has a strong influence on the accuracy of Geometrical Acoustics simulations. It is even known that the uncertainty in measured absorption coefficients alone is enough for simulations to give audible differences. This puts great importance on improving measurement techniques of acoustic absorption coefficients or even coming up with new methods. This motivates the second part of this thesis, which works to combine phased Geometrical Acoustics models with absorption measurement techniques.nrpages: 166status: publishe
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Phased Geometrical Acoustics using Low / High Frequency Reflection Coefficients with Applications to Absorption Measurements
2017Co-Authors: Boucher, Matthew AlbanAbstract:The prediction of sound and noise in everyday environments is of utmost importance for comfort as well as for personal health. Although measurements give invaluable information about the behavior of existing environments, designers and engineers are also in need of predictive models that are both accurate and efficient. These simulations provide detailed analysis before construction or prior to renovations and thus yield a huge return on investment. Geometrical Acoustics (GA) simulations are used throughout industry for the design of acoustic comfort in offices, classrooms, concert halls, music studios and more. Despite its prevalence, limitations of Geometrical Acoustics methods are still present. Most deficiencies stem from the high frequency assumptions of the method, meaning that GA traditionally only calculates the sound energy and is only an approximation to the wave equation. Expanding Geometrical Acoustics to lower frequencies by adding wave behavior such as scattering, diffraction and interference -- phenomena that are common in everyday experience -- is a need by practitioners and an important aim for researchers. A main focus of this work is on interference effects, which requires phase information so that sound pressures can be calculated instead of energy. An important source of information for phased Geometrical Acoustics comes from reflections from acoustical boundaries. While a plane wave assumption is valid at high frequencies, sound propagates more as a spherical wave when obstacles/walls/observers are close to the source, making spherical wave reflection coefficients important for low frequency sound prediction in Geometrical Acoustics. Including this behavior is more accurate and results in changes to acoustic parameters that are clearly noticed by most listeners. Previous work has shown that the uncertainty in sound absorption data has a strong influence on the accuracy of Geometrical Acoustics simulations. It is even known that the uncertainty in measured absorption coefficients alone is enough for simulations to give audible differences. This puts great importance on improving measurement techniques of acoustic absorption coefficients or even coming up with new methods. This motivates the second part of this thesis, which works to combine phased Geometrical Acoustics models with absorption measurement techniques.nrpages: 166status: publishe
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Convergence study of phased Geometrical Acoustics using a low frequency reflection coefficient
Heverlee (Belgium), 2016Co-Authors: Boucher, Matthew Alban, Pluymers Bert, Desmet WimAbstract:This work investigates phased Geometrical Acoustics using a low frequency reflection coefficient. This reflection coefficient, which is derived from an exact solution to a half-space problem, consists of an integral of image sources located at complex coordinates and is valid for any position of the source and receiver above the reflecting plane and for an arbitrary surface impedance. The method is demonstrated by calculating the sound pressure level in a rectangular room using the image source and beam tracing methods. The improvement over using the plane wave reflection coefficient is highlighted, using the finite element method as a reference.status: publishe
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Interference effects in phased beam tracing using exact half-space solutions
'Acoustical Society of America (ASA)', 2016Co-Authors: Boucher, Matthew Alban, Pluymers Bert, Desmet WimAbstract:Geometrical Acoustics provides a correct solution to the wave equation for rectangular rooms with rigid boundaries and is an accurate approximation at high frequencies with nearly hard walls. When interference effects are important, phased Geometrical Acoustics is employed in order to account for phase shifts due to propagation and reflection. Error increases, however, with more absorption, complex impedance values, grazing incidence, smaller volumes and lower frequencies. Replacing the plane wave reflection coefficient with a spherical one reduces the error but results in slower convergence. Frequency-dependent stopping criteria are then applied to avoid calculating higher order reflections for frequencies that have already converged. Exact half-space solutions are used to derive two additional spherical wave reflection coefficients: (i) the Sommerfeld integral, consisting of a plane wave decomposition of a point source and (ii) a line of image sources located at complex coordinates. Phased beam tracing using exact half-space solutions agrees well with the finite element method for rectangular rooms with absorbing boundaries, at low frequencies and for rooms with different aspect ratios. Results are accurate even for long source-to-receiver distances. Finally, the crossover frequency between the plane and spherical wave reflection coefficients is discussed.status: publishe
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Coupled (vibro-)acoustic problems using Geometrical Acoustics and patch transfer functions
2015Co-Authors: Boucher, Matthew Alban, Pluymers Bert, Desmet WimAbstract:Geometrical Acoustics (GA) is well-suited for interior Acoustics problems at high frequencies when the boundaries are described by absorption coefficients. If the boundaries are described by an impedance, mid-frequency problems can also be solved if a phase factor, which is dependent on the total distance a ray has traveled, and a complex-valued plane wave reflection coefficient are included. However, including velocity boundary conditions in the context of GA is not as straightforward. One existing method approximates a vibrating panel as a distribution of acoustic monopoles. Another, the Green Ray Integral Method (GRIM) solves a boundary integral in which the pressure on the surface is determined by a ray tracing procedure. However, in both cases, it is assumed that the acoustic fluid does not influence the velocity field of the vibrating structure. These problems are best solved using other methods, such as the finite and boundary element methods for low frequencies and statistical energy analysis (SEA) for high frequencies. An alternative approach is presented here for coupled (vibro-)acoustic problems at mid- to high frequencies using Geometrical Acoustics and the patch transfer function (PTF) method. The PTF method is a general sub-structuring procedure in which a common surface between two domains is divided into patches and the spatially averaged pressure and normal velocity over each patch are conserved. The calculation of patch transfer functions of an acoustic domain using Geometrical Acoustics is detailed, and the approach is applied to a strong coupling case by dividing an acoustic volume into two separate volumes. In this way, convergence criteria and accuracy of the approach are quantified.status: publishe
Victor V Krylov - One of the best experts on this subject based on the ideXlab platform.
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overview of localised flexural waves in wedges of power law profile and comments on their relationship with the acoustic black hole effect
Journal of Sound and Vibration, 2020Co-Authors: Victor V KrylovAbstract:Abstract In the present paper, the relationship between localised flexural waves in wedges of power-law profile and flexural wave reflection from acoustic black holes is examined. The Geometrical Acoustics theory of localised flexural waves in wedges of power-law profile is briefly discussed. It is noted that, for wedge profiles with power-law exponents equal or larger than two, the velocities of all localised modes take zero values, unless there is a wedge truncation. It is demonstrated that this effect of zero velocities of localised flexural waves in ideal wedges is closely related to the phenomenon of zero reflection of flexural waves from ideally sharp one-dimensional acoustic black holes. A possible influence of localised wedge modes on flexural wave reflection from one-dimensional acoustic black holes having rough edges is discussed. With regard to two-dimensional acoustic black holes, the role of localised flexural waves propagating along wedge edges that are curved in their middle plane is considered. Such waves can propagate along edges of inner holes in two-dimensional acoustic black holes formed by circular indentations in plates of constant thickness. A possible impact of such localised waves on the processes of scattering of flexural waves by edge imperfections of inner holes in two-dimensional acoustic black holes is discussed, including their influence on the efficiency of two-dimensional acoustic black holes as vibration dampers.
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Geometrical Acoustics of Lamb waves
2019Co-Authors: Victor V KrylovAbstract:In the present work, an overview of the developments of the Geometrical Acoustics (GA) theory of Lamb waves in plates of variable thickness is given, based mainly on the original results of the present author. The main attention is paid to the lowest order Lamb modes in plates of variable thickness, i.e. flexural and quasi-longitudinal plate waves. It is shown that the GA approach is an ideal tool to describe propagation of ultrasonic Lamb waves in complex plate-like and wedge-like structures. In particular, it is demonstrated that the developed GA theory involving both lowest order Lamb modes can be used for theoretical description of the classical problem of Rayleigh surface wave reflection from the tip of an elastic wedge of arbitrary angle, both at normal and at oblique incidence. The GA approach operating with flexural waves alone can be used for the development of the theory of localised waves propagating along sharp edges of different wedge-like structures. Another important application of GA is the development of the theory of ‘acoustic black holes’ for flexural waves that can absorb almost all of the incident wave energy. The obtained theoretical results are illustrated by recent experiments
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On the theory of smooth topographic waveguides for Rayleigh waves
2019 IEEE International Ultrasonics Symposium (IUS), 2019Co-Authors: Victor V KrylovAbstract:In the present paper, it is demonstrated that the existence of guided modes of Rayleigh waves on some types of smooth solid surfaces, often called 'smooth topographic waveguides', can take place under the condition of total internal reflection of Rayleigh waves from the 'external' areas of surfaces surrounding the 'internal' areas of wave localisation. In the framework of the Geometrical Acoustics approximation, the possibility of total internal reflection of Rayleigh waves in smooth topographic structures of complex geometry is linked to the presence of internal areas on the surfaces characterised by the geometry-modified angular-dependent local phase velocities of Rayleigh waves that are smaller in the direction of guided wave propagation than their velocities in the surrounding external areas. The above-mentioned condition of wave localisation is illustrated by theoretical calculations of the dispersion curves of guided waves for several examples of guided wave propagation. The obtained results for the dispersion curves of localised waves are compared with the known solutions, where available.
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nonlinear evolution of initially sine like wedge acoustic waves
Internaltional Ultrasonics Symposium, 1993Co-Authors: Victor V Krylov, Andreas P Mayer, David F ParkerAbstract:The nonlinear behaviour of antisymmetric wedge acoustic waves propagating along the tip of a sharp elastic wedge is investigated theoretically. The nonlinear evolution equation is derived taking into account Geometrical-Acoustics approximation for wedge waves. In contrast to the case of surface acoustic waves for which the quadratic nonlinearity dominates the lowest order of nonlinearity in this equation is cubic. For arbitrary propagation distances, the numerical solution taking into account 10 interacting wave harmonics has been carried out. The results show that an initially sine-like antisymmetric wedge wave distorts to a wave of trapezoidal form propagating with changed phase velocity
Francois Coulouvrat - One of the best experts on this subject based on the ideXlab platform.
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numerical simulation of sonic boom from hypersonic meteoroids
AIAA Journal, 2015Co-Authors: Martin Henneton, Olaf Gainville, Francois CoulouvratAbstract:Meteoroids entering the Earth atmosphere at high hypersonic velocities are sources of sonic booms that are recorded as infrasound signals at the ground level. The boom pressure field is simulated by solving Euler equations for a spherical meteoroid. The numerical challenge is to capture the acoustical regime of weak shock waves in the very far field at several hundreds or thousands times the meteoroid diameter. Computational fluid dynamics simulations are then matched to nonlinear Geometrical Acoustics for long-range atmospheric propagation down to the ground. The numerical process is validated through comparison with an analytical model, considering for perfect gases the meteoroid as a line source of strong shock in the near field, matched to a weak shock N-wave in the far field. Compared with a perfect gas, real gas effects at thermochemical equilibrium induce a reduced amplitude at the source, along with a shorter signal duration at the ground level. Simulations are illustrated for the well-documented ...
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numerical simulation of sonic boom from hypersonic meteoroids
AIAA Journal, 2015Co-Authors: Martin Henneton, Olaf Gainville, Francois CoulouvratAbstract:Meteoroids entering the Earth atmosphere at high hypersonic velocities are sources of sonic booms that are recorded as infrasound signals at the ground level. The boom pressure field is simulated by solving Euler equations for a spherical meteoroid. The numerical challenge is to capture the acoustical regime of weak shock waves in the very far field at several hundreds or thousands times the meteoroid diameter. Computational fluid dynamics simulations are then matched to nonlinear Geometrical Acoustics for long-range atmospheric propagation down to the ground. The numerical process is validated through comparison with an analytical model, considering for perfect gases the meteoroid as a line source of strong shock in the near field, matched to a weak shock N-wave in the far field. Compared with a perfect gas, real gas effects at thermochemical equilibrium induce a reduced amplitude at the source, along with a shorter signal duration at the ground level. Simulations are illustrated for the well-documented ...
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sonic boom in the shadow zone a Geometrical theory of diffraction
Journal of the Acoustical Society of America, 2002Co-Authors: Francois CoulouvratAbstract:Geometrical Acoustics predicts the amplitude of sonic booms only within the carpet. Inside the Geometrical shadow zone, a nonlinear, Geometrical theory of diffraction in the time domain is proposed. An estimation of magnitude orders shows that nonlinear effects are expected to be small for usual sonic booms. In the linear case, the matching to Geometrical Acoustics yields an analytical expression for the pressure near the cutoff. In the shadow zone, it can be written as a series of creeping waves. Numerical simulations show that the amplitude decay of the signal compares favorably with Concorde measurements, while the magnitude order of the rise time is correct. The ground impedance is shown to influence the rise time and peak amplitude of the signal mostly close to the cutoff. In the case of a weakly refractive atmosphere (low temperature gradient or downwind propagation), the transition zone about the cutoff is large, the transition is smooth, and the influence of ground absorption is increased.
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continuous field radiated by a Geometrically focused transducer numerical investigation and comparison with an approximate model
Journal of the Acoustical Society of America, 1993Co-Authors: Francois CoulouvratAbstract:Spherical caps are widely used to produce efficiently focused ultrasonic fields. Due to the curvature of the source surface, no exact analytical expression of the radiated field is known. The approximate model of Williams and O’Neil [J. Acoust. Soc. Am. 17, 219‐227 (1946); 21, 516–526 (1949)] is expected to be limited to high‐frequency, slightly focusing sources. In order to determine more precisely the limits of that model, and to dispose of an efficient tool for describing precisely the focused field, a new numerical method is presented. It is based on two expansions of the pressure field into spherical harmonics in two conveniently chosen domains. The expansions are then matched. Validity of the method is assessed by making a comparison with the exact analytical solution for the planar case. Then, the sound field radiated by a sharply focusing radiator is investigated. Comparison of the results of the numerical procedure with the approximate model indicates a much wider domain of validity of the model than generally thought. More precise conditions for the applicability of the model are derived, that follows from simple considerations of Geometrical Acoustics.