Contributions to discrete-time methods for room acoustic simulation

The sound field distribution in a room is the consequence of the acoustic properties of radiating sources and the position, geometry and absorbing characteristics of the surrounding boundaries in an enclosure (boundary conditions). Despite there existing a consolidated acoustic wave theory, it is very difficult, nearly impossible, to find an analytical expression of the sound variables distribution in a real room, as a function of time and position. This scenario represents as an inhomogeneous boundary value problem, where the complexity of source properties and boundary conditions make that problem extremely hard to solve. Room acoustic simulation, as treated in this thesis, comprises the algebraical approach to solve the wave equation, and the way to define the boundary conditions and source modeling of the scenario under analysis. Numerical methods provide accurate algorithms for this purpose and among the different possibilities, the use of discrete-time methods arises as a suitable solution for solving those partial differential equations, particularized by some specific constrains. Together with the constant growth of computer power, those methods are increasing their suitability for room acoustic simulation. However, there exists an important lack of accuracy in the definition of some of these conditions so far: current frequency-dependent boundary conditions do not comply with any physical model, and directive sources in discrete-time methods have been hardly treated. This thesis discusses about the current state-of-the-art of the boundary conditions and source modeling in discrete-time methods for room acoustic simulation, and it contributes some algorithms to enhance boundary condition formulation, in a locally reacting impedance sense, and source modelling in terms of directive sources under a defined radiation pattern. These algorithms have been particularized to some discrete-time methods such as the Finite Difference Time Domain and the Digital Waveguide Mesh.Escolano Carrasco, J. (2008). Contributions to discrete-time methods for room acoustic simulation [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/8309Palanci

Source excitation strategies for obtaining impulse responses in finite difference time domain room acoustics simulation

This paper considers source excitation strategies in finite difference time domain room acoustics simulations for auralization purposes. We demonstrate that FDTD simulations can be conducted to obtain impulse responses based on unit impulse excitation, this being the shortest, simplest and most efficiently implemented signal that might be applied. Single, rather than double, precision accuracy simulations might be implemented where memory use is critical but the consequence is a remarkably increased noise floor. Hard source excitation introduces a discontinuity in the simulated acoustic field resulting in a shift of resonant modes from expected values. Additive sources do not introduce such discontinuities, but instead result in a broadband offset across the frequency spectrum. Transparent sources address both of these issues and with unit impulse excitation the calculation of the compensation filters required to implement transparency is also simplified. However, both transparent and additive source excitation demonstrate solution growth problems for a bounded space. Any of these approaches might be used if the consequences are understood and compensated for, however, for room acoustics simulation the hard source is the least favourable due to the fundamental changes it imparts on the underlying geometry. These methods are further tested through the implementation of a directional sound source based on multiple omnidirectional point sources

Modeling continuous source distributions in wave-based virtual acoustics

All acoustic sources are of finite spatial extent. In volumetric wave-based simulation approaches (including, e.g., the finite difference time domain method among many others), a direct approach is to represent such continuous source distributions in terms of a collection of point-like sources at grid locations. Such a representation requires interpolation over the grid and leads to common staircasing effects, particularly under rotation or translation of the distribution. In this article, a different representation is shown, based on a spherical harmonic representation of a given distribution. The source itself is decoupled from any particular arrangement of grid points, and is compactly represented as a series of filter responses used to drive a canonical set of source terms, each activating a given spherical harmonic directivity pattern. Such filter responses are derived for a variety of commonly encountered distributions. Simulation results are presented, illustrating various features of such a representation, including convergence, behaviour under rotation, the extension to the time varying case, and differences in computational cost relative to standard grid-based source representations

Source excitation strategies for obtaining impulse responses in finite difference time domain room acoustics simulation,”

a b s t r a c t This paper considers source excitation strategies in finite difference time domain room acoustics simulations for auralization purposes. We demonstrate that FDTD simulations can be conducted to obtain impulse responses based on unit impulse excitation, this being the shortest, simplest and most efficiently implemented signal that might be applied. Single, rather than double, precision accuracy simulations might be implemented where memory use is critical but the consequence is a remarkably increased noise floor. Hard source excitation introduces a discontinuity in the simulated acoustic field resulting in a shift of resonant modes from expected values. Additive sources do not introduce such discontinuities, but instead result in a broadband offset across the frequency spectrum. Transparent sources address both of these issues and with unit impulse excitation the calculation of the compensation filters required to implement transparency is also simplified. However, both transparent and additive source excitation demonstrate solution growth problems for a bounded space. Any of these approaches might be used if the consequences are understood and compensated for, however, for room acoustics simulation the hard source is the least favorable due to the fundamental changes it imparts on the underlying geometry. These methods are further tested through the implementation of a directional sound source based on multiple omnidirectional point sources

Incorporating source directivity in wave-based virtual acoustics:Time-domain models and fitting to measured data

The modeling of source directivity is a problem of longstanding interest in virtual acoustics and auralisation. This remains the case for newer time domain volumetric wave-based approaches to simulation such as the finite difference time domain method. In this article, a spatio-temporal model of acoustic wave propagation, including a source term is presented. The source is modeled as a spatial Dirac delta function under the action of a series of differential operators associated with the spherical harmonic functions. Each term in the series gives rise to the directivity pattern of a given spherical harmonic, and is separately driven through a time domain filtering operation of an underlying source signal. Such a model is suitable for calibration against measured frequency-dependent directivity patterns and a procedure for arriving at time domain filters for each spherical harmonic channel is illustrated. It also yields a convenient framework for discretisation, and a simple strategy is presented, yielding a locally-defined operation over the spatial grid. Numerical results, illustrating various features of source directivity, including the comparison of measured and synthetic directivity patterns, are presented

Discrete-time modelling of diffusion processes for room acoustics simulation and analysis

Esta tesis está centrada en el modelado de la acústica de salas en espacios cerrados mediante el uso de una ecuación de transferencia radiativa y una ecuación de difusión En este trabajo se investiga cómo a través de estos modelos teóricos se pueden simular el campo sonoro en espacios complejos. Recientemente, el modelo de la ecuación de fusión ha sido prppuesto para ser utilizado en el modelado de la acústica de salas con superficies que reflejan el sonido de forma totalmente difusa. Este enfoque del uso de la ecuación de la disusión de sido intensamente investigado en los últimos años, ya que proporciona una alta eficiencia y flexibilidad para simular las distribuciones del campo sonoro en diferentes tipos de salas; sin embargo, sólo se han realizado unas pocas investigaciones con el objetivo de indagar sobre la precisión y las limitaciones de este método alternativo. Por lo tanto, en primer lugar se presenta un modelo basado en la ecuación de transferencia por radiación siendo meta principal el unificar una amplia gama de métodos geométricos de modelado de acústica de salas. Además, esta tesis está especialmente dedicada a establecer las bases y suposiciones que permitan obtener un modelo de difusión acústica como particularización del modelo de transferencia radiativa con el objetivo de conseguir una descripción clara y adecuada de sus ventajas y limitaciones desde el punto de vista teórico. Este trabajo permite enlazar directamente al modelo de la ecuación de difusión con el grupo de métodos de la acústica geométrica reforzando sus características y permitiendo una adecuada comparación con estos métodos ampliamente reconocidos. Una vez realizado este análisis teórico, esta tesis también se dedica a cuestiones relativas a la implementación numérica del modelo acústico de la ecuación de difusión . En este trabajo, se modela el campo sonoro a través de esquemas en diferencias finitas. Los resultados de este estudio proporcionan soluciones simples y practicas que muestran unos requerimientos computacionales bajos tanto de consumo de memoria como de tiempo.Navarro Ruiz, JM. (2012). Discrete-time modelling of diffusion processes for room acoustics simulation and analysis [Tesis doctoral no publicada]. Universitat Politècnica de València. https://doi.org/10.4995/Thesis/10251/1486

Local directional source modeling in wave-based acoustic simulation

Time-domain wave-based simulation approaches such as the finite difference time domain (FDTD) method allow for a complete solution to the problem of virtual acoustics over the entire frequency range, in contrast with the methods of geometric acoustics which are valid in the limit of high frequencies. They also allow for flexible modelling of sources and receivers, due to the inherently local nature of the computation, and complete access to the computed acoustic field over an enclosure. In this paper, a method for the emulation of sources of arbitrary directivity is presented, framed directly as an inhomogeneous wave equation. The additional terms in the wave equation take the form of Dirac delta functions and their distributional derivatives, and collections of such terms may be associated directly with an expansion of source directivity in terms of spherical harmonics. The local nature of the model implies a locally-defined efficient computational approach for wave-based methods defined over a spatial grid. Numerical results are presented

Physics and modelling of generation and propagation of noise by complex sources in realistic basins

The need to develop more accurate numerical tools for the propagation of noise in underwater environments is driven by the continuous increase of human activity in the sea and coastal areas. Noise has been shown to be dangerous to marine wildlife, and steps should be taken soon to mitigate it. Knowing that the primary sources of noise pollution at sea are marine propellers, one of the problems is assessing how the noise generated interacts with the environment, since up to now, the main focus was the characterization of the acoustic signature in the near field or, alternatively, the propagation of simplified acoustic sources in sea-like domains. The work conducted in this thesis assesses the modelling of complex acoustic sources and the propagation of acoustic pressure in realistic domains. A propagation model based on the solution of the acoustic wave equation in the time and space domain is implemented and used in conjunction with the Ffowcs Williams and Hawkings (FW-H) to analyze the possible patterns occurring in the underwater environment. Specifically, we analyzed the noise radiated by a marine propeller in a canal, focusing on the effects of the boundaries on the acoustic field and, secondly, the consequence of a rotating body placed underneath a free surface. We defined a new methodology called Full Acoustic Analogy (FAA) to achieve these results. This methodology aims to overcome some intrinsic limitations of the known Acoustic Analogies. The study presented here attempts to bridge the gap between noise characterization and its propagation by introducing a new methodology for evaluating flow-induced noise in a realistic environment. The propagation model developed, which used the finite-difference-time-domain method, has been compared against benchmark cases (monopole source propagating in classical waveguides) for which an analytical solution is available, and it provides accurate results of the acoustic field. Furthermore, a second analysis is conducted on two classical waveguides: the Ideal one and the Pekeris one. The solution of the wave equation in time and physical space enables the implementation of different sources, such as dipole and quadrupole; therefore, we analyzed the acoustic response of the Pekeris waveguide. The results show that the propagation of the acoustic pressure is strongly affected by the directivity pattern of the source. This was the first step in evaluating the capabilities of the solution of the acoustic equation in the presence of sources characterized by complex directivity since our ultimate goal is to evaluate the noise emitted by a propeller. In the second part, the FAA analogy is introduced, and we describe how the acoustic pressure obtained with the FW-H equation is used as a source term in the propagation model. After the validation of the new proposed methodology in an unbounded homogeneous domain, we investigate the propagation of the linear part of the noise generated by a naval propeller within a canal. Local maxima and minima of the acoustic fields arise from the interaction between the noise source and the environment; in particular, they derive from the superposition of direct and reflected waves. Moreover, a rotating body placed underneath a free surface generates a peculiar asymmetry of the acoustic field associated with the interaction between the acoustic waves and the free surface.The need to develop more accurate numerical tools for the propagation of noise in underwater environments is driven by the continuous increase of human activity in the sea and coastal areas. Noise has been shown to be dangerous to marine wildlife, and steps should be taken soon to mitigate it. Knowing that the primary sources of noise pollution at sea are marine propellers, one of the problems is assessing how the noise generated interacts with the environment, since up to now, the main focus was the characterization of the acoustic signature in the near field or, alternatively, the propagation of simplified acoustic sources in sea-like domains. The work conducted in this thesis assesses the modelling of complex acoustic sources and the propagation of acoustic pressure in realistic domains. A propagation model based on the solution of the acoustic wave equation in the time and space domain is implemented and used in conjunction with the Ffowcs Williams and Hawkings (FW-H) to analyze the possible patterns occurring in the underwater environment. Specifically, we analyzed the noise radiated by a marine propeller in a canal, focusing on the effects of the boundaries on the acoustic field and, secondly, the consequence of a rotating body placed underneath a free surface. We defined a new methodology called Full Acoustic Analogy (FAA) to achieve these results. This methodology aims to overcome some intrinsic limitations of the known Acoustic Analogies. The study presented here attempts to bridge the gap between noise characterization and its propagation by introducing a new methodology for evaluating flow-induced noise in a realistic environment. The propagation model developed, which used the finite-difference-time-domain method, has been compared against benchmark cases (monopole source propagating in classical waveguides) for which an analytical solution is available, and it provides accurate results of the acoustic field. Furthermore, a second analysis is conducted on two classical waveguides: the Ideal one and the Pekeris one. The solution of the wave equation in time and physical space enables the implementation of different sources, such as dipole and quadrupole; therefore, we analyzed the acoustic response of the Pekeris waveguide. The results show that the propagation of the acoustic pressure is strongly affected by the directivity pattern of the source. This was the first step in evaluating the capabilities of the solution of the acoustic equation in the presence of sources characterized by complex directivity since our ultimate goal is to evaluate the noise emitted by a propeller. In the second part, the FAA analogy is introduced, and we describe how the acoustic pressure obtained with the FW-H equation is used as a source term in the propagation model. After the validation of the new proposed methodology in an unbounded homogeneous domain, we investigate the propagation of the linear part of the noise generated by a naval propeller within a canal. Local maxima and minima of the acoustic fields arise from the interaction between the noise source and the environment; in particular, they derive from the superposition of direct and reflected waves. Moreover, a rotating body placed underneath a free surface generates a peculiar asymmetry of the acoustic field associated with the interaction between the acoustic waves and the free surface