Virtual room modeling using hybrid digital waveguide mesh techniques

The digital waveguide (DWG) mesh is a method for simulating wave propagation in multiple dimensions. In three-dimensional form it can be used for modeling room acoustics or resonating bodies of musical instruments, for example. Until now boundary conditions in the three-dimensional DWG mesh have been implemented using methods originating from the updating functions of a one-dimensional digital waveguide. A new boundary structure, which better takes into account the 3-D mesh topology, is now introduced. With this new method, the reflection magnitude is shown to match the desired value over a much wider range of reflection coefficient values than with the earlier method.

The digital waveguide mesh is an extension of the one-dimensional digital waveguide technique. Waveguide meshes are used for simulation of two- and three-dimensional wave propagation in musical instruments and acoustic spaces. The original waveguide mesh algorithm suffers from direction-dependent dispersion. In this paper we show that this problem may be reduced by using an interpolated rectilinear mesh. In the analysis part we show the analytical solution for the wave propagation speed and numerical simulations of the magnitude response and phase speed in both the original and the interpolated two-dimensional waveguide mesh algorithms. We demonstrate by simulation that the wave propagation characteristics of the proposed interpolated waveguide mesh are independent of direction and thus the remaining errors caused by dispersion may be corrected with a postprocessor.

A novel application-specific hardware (ASH) unit was designed to form the building block of the Meshotron -- aparallelisation network for three-dimensional (3D) digital wave guide-mesh (DWM) room acoustic models. The rectangular mesh topology was elected. This ASH unit was tested using professional hardware simulation tools, assuming 32-bit integer data. Room impulse responses (RIR) were obtained for a set of small models under different test conditions, using both single-unit and multi-unit configurations. They proved exactly identical to those obtained using 3D DWM modelling software for the same models and test conditions, which validates the design.

The Digital Waveguide Mesh (DWM) is a numerical simulation technique that has been shown to be suitable for modelling the acoustics of enclosed spaces. Previous work considered an approach using an array of spatially distributed receivers based on sound intensity probe theory to capture spatial room impulse responses (RIRs) from the DWM. A suitable process to facilitate spatial encoding of the DWMinto second-order spherical harmonics has also been explored. The purpose of this paper is to introduce a new alternative processing scheme based on the Blumlein Difference Technique. Both the previous and currently presented techniques are newly formulated with the main processing in the frequency domain. In addition the ability of the newly proposed technique for capturing the 2nd order components is confirmed and further processing of the receiver array is considered to extend the usable frequency range.

Digital waveguide mesh has emerged as an efficient and straightforward way to model small room impulse response because it solves the wave equation directly in the time domain. In this paper, we investigate the performance of the 3-D rectangular digital waveguide mesh in modeling the low frequency portion of room impulse response. We find that it has similar performance compared to the popular image source method when modeling the low frequency portion of the early specular reflections of the room impulse responses.

The digital waveguide mesh can be used to simulate the propagation of sound waves in an acoustic system. The accurate simulation of the acoustic characteristics of boundaries within such a system is an important part of the model. One significant property of an acoustic boundary is its diffusivity. Previous approaches to simulating diffuse boundaries in a digital waveguide mesh are effective but exhibit limitations and have not been analyzed in detail. An improved technique is presented here that simulates diffusion at boundaries and offers a high degree of control and consistency. This technique works by rotating wavefronts as they pass through a special diffusing layer adjacent to the boundary. The waves are rotated randomly according to a chosen probability function and the model is lossless. This diffusion model is analyzed in detail, and its diffusivity is quantified in the form of frequency dependent diffusion coefficients. The approach used to measuring boundary diffusion is described here in detail for the 2-D digital waveguide mesh and can readily be extended for the 3-D case.

Digital waveguide mesh (DWM) models are numerical solvers for the wave equation in -dimensions. They are used for obtaining the traveling-wave solution in practical acoustical modeling applications. Although unstructured meshes can be used with DWMs, regular mesh topologies are traditionally used due to their implementation simplicity. This letter discusses the accuracy of first-order approximations to numerical derivatives on more general unstructured mesh topologies. The results are applied to structured, regular mesh topologies as used in DWM modeling. A comparison of 2-D and 3-D DWM topologies with respect to the accuracy of first-order approximations to numerical derivatives is presented.

RoomWeaver is a digital waveguide mesh (DWM) development environment for virtual acoustic spaces. Scripting is used to define room geometry, object, surface and material properties, together with source and receiver parameters. Different mesh types and topologies are accommodated through a user-extensible plug-in architecture with the selected DWM grown to fit the defined room at simulation run-time. Visualization options allow mesh behavior and wave propagation to be observed in 2D or 3D. Excitation signals can be selected accordingly and resultant room impulse responses can be obtained in mono, stereo, binaural or surround formats. Digitract is a speech synthesis research tool that uses 1D and 2D digital waveguide models to simulate the human vocal tract. 1D simulation is already well established and Digitract allows direct comparison with 2D DWM models that offer improved control over formant bandwidths and greater naturalness in synthesized speech. Mesh types and topology can be defined together with reflection values at lip, glottis, and, in the 2D case, inner wall boundaries. Vibrato and pitch-slide can be introduced to the input and dipthongs and tripthongs can also be synthesized. 3D graphical feedback illustrates the behavior of the acoustic pressure waves present within the tract model.

The digital waveguide mesh (DWM) has proven to an efficient and accurate method for simulating multi-dimensional wave propagation in various applications such as physical modeling of musical instruments and room acoustics. However, problems appear when fitting a DWM to an arbitrary boundary because of the geometric constraints of a given mesh element. A finer mesh grid is often used in an attempt to resolve this situation, which entails an associated computational cost increase. This paper presents a conformal method for the rectilinear DWM as an efficient alternative. The proposed conformal method aims at better approximating rigid boundaries that are normally not well suited for a rectilinear DWM structure. It is inspired by the conformal method developed for the Finite Domain Time Difference (FDTD) scheme where a cell associated with the boundary is split with respect to a particular criterion and the material constant of the cell is adjusted accordingly [1]. By means of the interleaved waveguide network (IWN) [2], the conformal method is successfully achieved in the DWM.

We describe the influence of the separation accuracy via semi-blind source separation (semi-BSS) upon the measurement accuracy of the room impulse responses. Tatekura et al. had proposed a simultaneous measurement method of the room impulse responses with ensemble music. In this method, the room impulse responses were measured by separating ensemble music into each instrument sound reproduced from each loudspeaker. However, the influence of the separation accuracy via semi-BSS upon the measurement accuracy of the room impulse responses has not been clarified. Therefore, we evaluated the measurement accuracy of the room impulse responses and the separation accuracy by using the different instrument sounds. The room impulse responses were measured between two loudspeakers and a control point with 11 kinds of musical instrument sounds. From the result of the measurement accuracy, the difference between the maximum value and the minimum value was 10 dB. However, from the result of the separation accuracy, most of value held equivalent measurement accuracy. Therefore, under these measurement conditions, this method is expected to be able to measure the room impulse responses by optional instrument sounds.

The digital waveguide mesh (DWM) was first introduced as a modeling technique for musical acoustics applications in 1993 as a natural extension to the 1D digital waveguide that is widely used in physical modeling sound synthesis. DWM related work at York has focused on a number of areas with notable results including reverberation simulation using the 2D triangular mesh, modeling enclosed spaces with the 3D tetrahedral mesh, improved anechoic absorbing boundary conditions (ABCs) and the analysis and validation of spatial properties of DWM-based room impulse response measurements. This paper presents recent results from three current areas of research. RoomWeaver is a development environment that facilitates research into the application of DWM-based models for virtual acoustic spaces, incorporating new hybrid mesh-types in 2D and 3D through the use of KW-pipes. The second parallel research area involves the development of new boundary formulations that extend the anechoic ABC to the more general frequency dependent case with variable diffuse reflection control. Finally, Digitract is a speech synthesis research tool based on a 2D DWM model of the human vocal tract that offers improved control over formant bandwidths and greater naturalness in the production of speech sounds.

Digital waveguide mesh (DWM) models offer a simple, accurate, time-domain, numerical solution of the wave equation. A specific case where such accurate and computationally simple solutions are needed is the acoustical modeling of open or closed volumes. It is possible to model 3-D propagation of waves in enclosures such as rooms using DWM models. Generally, idealized omnidirectional sources are used for obtaining the impulse response in the DWM. However, real-life sound sources are never completely isotropic, causing wavefronts with directional properties. This paper presents two methods to simulate analytical and empirical directivities in 3-D DWM models in the far-field. The first method is based on the direct excitation of the mesh with the velocity component of the directional source and is used to simulate analytical sources. The second method is based on the weighting of velocity components generated by an omnidirectional source at different octave-bands and is used to simulate sources with frequency-dependent empirical directivity functions. A simple interpolation method for obtaining a closed-form description of the directivity function from incomplete directivity data is also proposed. Simulation results are presented for two sources in an acoustical model of a rectangular room.

Digital waveguide mesh (DWM) models are time-domain numerical methods providing computationally simple solutions for wave propagation problems. They have been used in various acoustical modeling and audio synthesis applications including synthesis of musical instrument sounds and speech, and modeling of room acoustics. A successful model of room acoustics should be able to account for source and receiver directivity. Methods for the simulation of directional sources in DWM models were previously proposed. This paper presents a method for the simulation of directional microphones in DWM-based models of room acoustics. The method is based on the directional weighting of the microphone response according to the instantaneous direction of incidence at a given point. The direction of incidence is obtained from instantaneous intensity that is calculated from local pressure values in the DWM model. The calculation of instantaneous intensity in DWM meshes and the directional accuracies of different mesh topologies are discussed. An intensity-based formulation for the response of a directional microphone is given. Simulation results for an actual microphone with frequency-dependent, non-ideal directivity function are presented.

The digital waveguide mesh (DWM) is a discrete-time numerical method for modeling the propagation of traveling waves in multidimensional mechanical and acoustic systems. Despite the fact that the DWM is not as computationally efficient as a 1-D digital waveguide, it is still widely used for sound synthesis of musical instruments and for acoustical modeling of rooms because of the simplicity of the implementation. However, large-scale realization of the digital waveguide mesh is not adequate for many simulations because of its relatively high and direction-dependent dispersion error. The influence of dispersion error can be reduced by using a denser mesh structure though with extra computational costs. This paper presents a method for efficiently interconnecting rectangular DWM sub-domains of different mesh density. The method requires only small overlapped buffer regions to be added for each sub-domain. This allows the selective use of higher and lower density grids in a single simulation based on spatially-dependent dispersion error criteria.

The digital waveguide mesh models multidimensional wave propagation in discrete space and time. The finite difference wave equation used in the mesh is expressed in the space-time domain, which is not straightforward to reveal the analytical expression for the impulse response between a source and a receiver. This letter describes a procedure to derive an analytical impulse response expression between two nodes in a 2-D rectangular digital waveguide mesh based on the wave equation. The analytical expression can provide helpful insights and help verify simulation results for the digital waveguide mesh.