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Journal of the Audio Engineering Society

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2023 April - Volume 71 Number 4

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Papers

Context-Based Evaluation of the Opus Audio Codec for Spatial Audio Content in Virtual Reality

Authors: Lee, Ben; Rudzki, Tomasz; Skoglund, Jan; Kearney, Gavin

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This paper discusses the evaluation of Opus-compressed Ambisonic audio content through listening tests conducted in a virtual reality environment.The aim of this studywas to investigate the effect that Opus compression has on the Basic Audio Quality (BAQ) of Ambisonic audio in different virtual reality contexts---gaming, music, soundscapes, and teleconferencing. The methods used to produce the test content, how the tests were conducted, the results obtained and their significance are discussed. Key findings were that in all cases, Ambisonic scenes compressed with Opus at 64 kbps/ch using Channel Mapping Family 3 garnered a median BAQ rating not significantly different than uncompressed audio. Channel Mapping Family 3 demonstrated the least variation in BAQ across evaluated contexts, although there were still some significant differences found between contexts at certain bitrates and Ambisonic orders.

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A Magnitude-Based Parametric Model Predicting the Audibility of HRTF Variation

Authors: Doma, Shaimaa; Ermert, Cosima A.; Fels, Janina

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This work proposes a parametric model for just noticeable differences of unilateral differences in head-related transfer functions (HRTFs). For seven generic magnitude-based distance metrics, common trends in their response to inter-individual and intra-individual HRTF differences are analyzed, identifying metric subgroups with pseudo-orthogonal behavior. On the basis of three representative metrics, a three-alternative forced-choice experiment is conducted, and the acquired discrimination probabilities are set in relation with distance metrics via different modeling approaches. A linear model, with coefficients based on principal component analysis and three distance metrics as input, yields the best performance, compared to a simple multi-linear regression approach or to principal component analysis--based models of higher complexity.

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Weighted Pressure and Mode Matching for Sound Field Reproduction: Theoretical and Experimental Comparisons

Authors: Koyama, Shoichi; Kimura, Keisuke; Ueno, Natsuki

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Two sound field reproduction methods, weighted pressure matching and weighted mode matching, are theoretically and experimentally compared.Weighted pressure and mode matching are a generalization of conventional pressure and mode matching, respectively. Both methods are derived by introducing a weighting matrix in the pressure and mode matching. The weighting matrix in the weighted pressure matching is defined on the basis of the kernel interpolation of the sound field from pressure at a discrete set of control points. In weighted mode matching, the weighting matrix is defined by a regional integration of spherical wavefunctions. It is theoretically shown that the weighted pressure matching is a special case of the weighted mode matching by infinite-dimensional harmonic analysis for estimating expansion coefficients from pressure observations. The difference between the two methods is discussed through experiments.

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Computationally-Efficient Simulation of Late Reverberation for Inhomogeneous Boundary Conditions and Coupled Rooms

Authors: Kirsch, Christoph; Wendt, Torben; Van De Par, Steven; Hu, Hongmei; Ewert, Stephan D.

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For computational efficiency, acoustic simulation of late reverberation can be simplified by generating a limited number of incoherent signals with frequency-dependent exponential decay radiated by spatially distributed virtual reverberation sources (VRS). A sufficient number of VRS and adequate spatial mapping are required to approximate spatially anisotropic late reverberation, e.g., in rooms with inhomogeneous distribution of absorption or for coupled volumes. For coupled rooms, moreover, a dual-slope decay might be required. Here, an efficient and perceptually plausible method to generate and spatially render late reverberation is suggested. Incoherent VRS signals for (sub-) volumes are generated based on room dimensions and frequencydependent absorption coefficients at the boundaries. For coupled rooms, (acoustic) portals account for effects of sound propagation and diffraction at the room connection and energy transfer during the reverberant decay process. The VRS are spatially distributed around the listener, with weighting factors representing the spatially subsampled distribution of absorption on the boundaries and the location and solid angle covered by portals. A technical evaluation and listening tests demonstrate the validity of the approach in comparison to measurements in real rooms.

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Comparison of Transaural Configurations Inside Usual Rooms

Authors: Vidal, Adrien; Herzog, Philippe; Lambourg, Christophe; Chatron, Jacques

This paper deals with the design of transaural systems in usual rooms, whose response has a strong influence on sound reproduction. The paper proposes to select configurations for the best perceptual rendering. However, realistic perceptive experiments cannot deal with the many possible room and loudspeaker configurations. Therefore, the authors propose to assess them using objective scores that are extrapolated from the results of perceptive tests assessing a suitable selection of rooms and loudspeaker configurations. This extrapolation then allows comparison of a much larger set of combinations, leading to the conclusion that close-to-ears configurations allow reduction of the room influence, leading to a good perceived fidelity---even inside usual rooms. Closer loudspeakers are, however, likely to be more sensitive to listener position, so the robustness of loudspeaker configurations to listenermisplacement were investigated. A suitable objective score, again based on a perceptive test, led to the surprising conclusion that some close-to-ears configurations are also robust to listener position.

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