U. Peter Svensson

An analytic secondary source model of edge diffraction impulse responses

A new impulse-response model for the edge diffraction from finite rigid or soft wedges is presented which is based on the exact Biot–Tolstoy solution. The new model is an extension of the work by Medwin et al. [H. Medwin et al., J. Acoust. Soc. Am. 72, 1005–1013 (1982)], in that the concept of secondary edge sources is used. It is shown that analytical directivity functions for such edge sources can be derived and that they give the correct solution for the infinite wedge. These functions support the assumption for the first-order diffraction model suggested by Medwin et al. that the contributions to the impulse response from the two sides around the apex point are exactly identical. The analytical functions also indicate that Medwin’s second-order diffraction model contains approximations which, however, might be of minor importance for most geometries. Access to analytical directivity functions makes it possible to derive explicit expressions for the first- and even second-order diffraction for certain ...

Computation of edge diffraction for more accurate room acoustics auralization

Inaccuracies in computation and auralization of room impulse responses are related in part to inadequate modeling of edge diffraction, i.e., the scattering from edges of finite surfaces. A validated time-domain model (based on analytical extensions to the Biot-Tolstoy-Medwin technique) is thus employed here to compute early room impulse responses with edge diffraction. Furthermore, the computations are extended to include combinations of specular and diffracted paths in the example problem of a stage-house. These combinations constitute a significant component of the total nonspecular scattering and also help to identify edge diffraction in measured impulse responses. The computed impulse responses are then convolved with anechoic signals with a variety of time-frequency characteristics. Initial listening tests with varying orders and combinations of diffraction suggest that (1) depending on the input signal, the diffraction contributions can be clearly audible even in nonshadow zones for this conservative open geometry and (2) second-order diffraction to nonshadowed receivers can often be neglected. Finally, a practical implementation for binaural simulation is proposed, based on the singular behavior of edge diffraction along the least-time path for a given source-edge-receiver orientation. This study thus provides a first major step toward computing edge diffraction for more accurate room acoustics auralization.

Overview of geometrical room acoustic modeling techniques

Computerized room acoustics modeling has been practiced for almost 50 years up to date. These modeling techniques play an important role in room acoustic design nowadays, often including auralization, but can also help in the construction of virtual environments for such applications as computer games, cognitive research, and training. This overview describes the main principles, landmarks in the development, and state-of-the-art for techniques that are based on geometrical acoustics principles. A focus is given to their capabilities to model the different aspects of sound propagation: specular vs diffuse reflections, and diffraction.

Robust Minimum Sidelobe Beamforming for Spherical Microphone Arrays

A robust minimum sidelobe beamforming approach based on the spherical harmonics framework for spherical microphone arrays is proposed. It minimizes the peaks of sidelobes while keeping the distortionless response in the look direction and maintaining the mainlobe width. A white noise gain constraint is also derived and employed to improve the robustness against array errors. The resulting beamformer can provide optimal tradeoff between the sidelobe level, the beamwidth and robustness, so it could be more practical than the existing spherical array Dolph-Chebyshev modal beamformer in the presence of array errors. The optimal modal beamforming problem is formulated as a tractable convex second-order cone programming program, which is more efficient than conventional element-space based approaches, since the dimension of array weight vectors can be significantly decreased by using the properties of spherical harmonics and Legendre polynomials. For the purpose of performance comparison, we also formulate current robust modal beamformers as equivalent optimization problems based on the proposed array model. Numerical results show the high flexibility and efficiency of the proposed beamforming approach.

Spatial redundancy in Higher Order Ambisonics and its use for lowdelay lossless compression

When Higher Order Ambisonics (HOA) is used to represent a sound field, the channels might contain a lot of redundancy in some cases. This redundancy can be exploited in order to provide more efficient network transmission and storage. In this work the amount of inter-channel redundancy for Higher Order Ambisonics is investigated. Furthermore, lossless compression techniques that build on this redundancy are studied, with a focus on low-delay algorithms for real-time, or two-way, applications. The presented encoding scheme results in a delay of 256 samples, but with a rather high computational complexity both for encoding and decoding. The system also preserves the desired features of the HOA format, such as the scalability and the ability to reproduce over arbitrary loudspeaker layouts.

Fast time-domain edge-diffraction calculations for interactive acoustic simulations

The inclusion of edge diffraction has long been recognized as an improvement to geometrical-acoustics (GA) modeling techniques, particularly for acoustic simulations of complex environments that are represented as collections of finite-sized planar surfaces. One particular benefit of combining edge diffraction with GA components is that the resulting total sound field is continuous when an acoustic source or receiver crosses a specular-zone or shadow-zone boundary, despite the discontinuity experienced by the associated GA component. In interactive acoustic simulations which include only GA components, such discontinuities may be heard as clicks or other undesirable audible artifacts, and thus diffraction calculations are important for high perceptual quality as well as physical realism. While exact diffraction calculations are difficult to compute at interactive rates, approximate calculations are possible and sufficient for situations in which the ultimate goal is a perceptually plausible simulation rather than a numerically exact one. In this paper, we describe an edge-subdivision strategy that allows for fast time-domain edge-diffraction calculations with relatively low error when compared with results from a more numerically accurate solution. The tradeoff between computation time and accuracy can be controlled with a number of parameters, allowing the user to choose the speed that is necessary and the error that is tolerable for a specific modeling scenario.

Robust spherical microphone array beamforming with multi-beam-multi-null steering, and sidelobe control

A spherical harmonics domain microphone array beamforming approach is proposed. It unifies 3D multi-beam forming with tractable mainlobe levels, automatic multi-null steering, sidelobe control, and robustness control into one optimization framework, using a single spherical microphone array. The optimum array weights are designed by maintaining distortionless responses in multiple mainlobe directions and guaranteeing all sidelobes below given threshold values, while minimizing the beamformer output power. A weight vector norm constraint is also employed to improve the robustness of the beamformer. A convex optimization formulation is derived, and implemented by the second order cone programming (SOCP) method. Design examples demonstrate a satisfactory performance.

Performance of some linear time-varying systems in control of acoustic feedback

Some linear time-varying (LTV) components used to control feedback in sound systems were tested experimentally in real-time simulators and rooms with and without external reverberation. Gain before instability (GBI) was measured in single channels employing frequency shifting (FS), phase modulation (PM), and delay modulation (DM) implemented on a digital signal processor. FS performed according to the established theory. For PM GBI increased almost monotonically with modulation index β, except for cases with large loop gain irregularities which displayed a reduced GBI for values of β that corresponded to low carrier suppression. Also, GBI was practically independent of the modulation frequency fm already from 0.5 Hz even when this was much lower than the correlation distance of the loop gain transfer function. Rooms with different reverberation times gave different initial (time-invariant) GBI values but these differences decreased by the use of modulation. The GBI increase was larger for cases with exter...

An integral equation formulation for the diffraction from convex plates and polyhedra

A formulation of the problem of scattering from obstacles with edges is presented. The formulation is based on decomposing the field into geometrical acoustics, first-order, and multiple-order edge diffraction components. An existing secondary-source model for edge diffraction from finite edges is extended to handle multiple diffraction of all orders. It is shown that the multiple-order diffraction component can be found via the solution to an integral equation formulated on pairs of edge points. This gives what can be called an edge source signal. In a subsequent step, this edge source signal is propagated to yield a multiple-order diffracted field, taking all diffraction orders into account. Numerical experiments demonstrate accurate response for frequencies down to 0 for thin plates and a cube. No problems with irregular frequencies, as happen with the Kirchhoff-Helmholtz integral equation, are observed for this formulation. For the axisymmetric scattering from a circular disc, a highly effective symmetric formulation results, and results agree with reference solutions across the entire frequency range.

Quality evaluation of long duration audiovisual content

In this paper a new methodology for the evaluation of perceived quality for long duration audiovisual content is presented. This method allows the investigation of unexplored dependencies between perceived quality and impairment variations over extended periods of time. Instead of providing quality scores on predefined rating scales, assessors are asked to adjust the quality level of the displayed content themselves, whenever a degradation in perceived quality occurs. This method approaches the problem of quality evaluation from a different perspective than the existing methods, which might be valuable for future research and application. Preliminary results of an experiment conducted using this methodology are presented.