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Room acoustic synthesis can be used in virtual reality (VR), augmented reality (AR) and gaming applications to enhance listeners' sense of immersion, realism and externalisation. A common approach is to use geometrical acoustics (GA) models to compute impulse responses at interactive speed, and fast convolution methods to apply said responses in real time. Alternatively, delay-network-based models are capable of modeling certain aspects of room acoustics, but with a significantly lower computational cost. In order to bridge the gap between these classes of models, recent work introduced delay network designs that approximate Acoustic Radiance Transfer (ART), a geometrical acoustics (GA) model that simulates the transfer of acoustic energy between discrete surface patches in an environment. This paper presents two key extensions of such designs. The first extension involves a new physically-based and stability-preserving design of the feedback matrices, enabling more accurate control of scattering and, more in general, of late reverberation properties. The second extension allows an arbitrary number of early reflections to be modeled with high accuracy, meaning the network can be scaled at will between computational cost and early reverberation precision. The proposed extensions are compared to the baseline ART-approximating delay network as well as two reference GA models. The evaluation is based on objective measures of perceptually-relevant features, including frequency-dependent reverberation times, echo density build-up, and early decay time. Results show how the proposed extensions result in a significant improvement over the baseline model, especially for the case of non-convex geometries or the case of unevenly distributed wall absorption, both scenarios of broad practical interest.

Computer games and virtual reality require digital reverberation algorithms, which can simulate a broad range of acoustic spaces, including locations in the open air. Additionally, the detailed simulation of environmental sound is an area of significant interest due to the propagation of noise pollution over distances and its related impact on well-being, particularly in urban spaces. This paper introduces the waveguide web digital reverberator design for modeling the acoustics of sparsely reflecting outdoor environments; a design that is, in part, an extension of the scattering delay network reverberator. The design of the algorithm is based on a set of digital waveguides connected by scattering junctions at nodes that represent the reflection points of the environment under study. The structure of the proposed reverberator allows for accurate reproduction of reflections between discrete reflection points. Approximation errors are caused when the assumption of point-like nodes does not hold true. Three example cases are presented comparing waveguide web simulated impulse responses for a traditional shoebox room, a forest scenario, and an urban courtyard, with impulse responses created using other simulation methods or from real-world measurements. The waveguide web algorithm can better enable the acoustic simulation of outdoor spaces and so contribute toward sound design for virtual reality applications, gaming, and auralization, with a particular focus on acoustic design for the urban environment.

In room acoustics simulation and virtualization applications, accurate wall termination is a perceptually crucial feature. It is particularly important in the setting of wave-based modeling of 3D spaces, using methods such as the finite difference time domain method or finite volume time domain method. In this paper, general locally reactive impedance boundary conditions are incorporated into a 3D finite volume time domain formulation, which may be specialized to the various types of finite difference time domain method under fitted boundary termination. Energy methods are used to determine stability conditions for general room geometries, under a large family of nontrivial wall impedances, for finite volume methods over unstructured grids. Simulation results are presented, highlighting in particular the need for unstructured or fitted cells at the room boundary in the case of the accurate simulation of frequency-dependent room mode decay times.

Finite difference solutions to the wave equation are simple and flexible modeling tools for approximating physical systems in audio and room acoustics. Each model is characterized by a matrix operator and the time-stepping solution by a sequence of powers of the matrix. Spectral decomposition of representative matrices provide some practical insight into solution behavior and in some cases stability. In addition to computed eigenvalue spectra, pseudospectra provide a description of numerical amplification due to rounding errors in floating point arithmetic. The matrix analysis also shows that certain boundary implementations in non-cuboid geometries can be unstable despite satisfying conditions derived from von Neumann and normal mode analyses.

Visual rendering is the process of creating synthetic images of digital models. The modelling of sound synthesis and propagation in a virtual space is called sound rendering. In this article we review different audiovisual rendering techniques suitable for realtime rendering of three-dimensional virtual worlds. Virtual environments are useful in various application areas, for example in architectural visualisation. With audiovisual rendering, lighting and acoustics of a modelled concert hall can be experienced early in the design stage of the building. In this article we demonstrate an interactive audiovisual rendering system where an animated virtual orchestra plays in a modelled concert hall. Virtual musicians are conducted by a real conductor who wears a wired data dress suit and a baton. The conductor and the audience hear the music rendered according to the acoustics of the virtual concert hall, creating a lifelike experience.

An integrated computer graphics and audio signal processing system is developed for producing real‐time audiovisual performances, such as a virtual concert. Animated musician models play MIDI‐coded music, with hands and fingers synchronized to each note by kinematic calculations. Instrument sounds are synthesized by physical modeling techniques. Acoustic response of the concert hall is calculated based on its geometric model and material information. Ray tracing and image source methods are used for early reflections, whereas a recursive filter structure, consisting of comb and allpass filters, models the later diffuse reverberation. For three‐dimensional sensation of the environment, the direct sound is auralized with a FIR approximation of the listener’s head‐related transfer function (HRTF), and the image sources are processed with the interaural amplitude and time differences (IAD and ITD). The system implementation is distributed over a high‐speed network using workstations for the visual and interactive parts and a dedicated digital signal processor for sound synthesis and HRTF convolution. The listener may freely move in the virtual concert hall using a mouse controller. Methods for conducting the virtual orchestra with a tracker baton are under development. [Work supported by the Academy of Finland.]