Optimization-based reproduction of diffuse audio objects

The creation of a diffuse sound event from a single audio signal is an important signal processing task, for instance in spatial audio reproduction or audio coding. Current algorithms based on decorrelation filters or frequency-dependent panning typically cause artifacts due to transients or time-domain aliasing. In this paper, we propose an optimization-based approach to diffusion that creates a set of filters to approximate a desired distribution of frequency-dependent propagation directions to create the perception of a diffuse sound field with a multi-channel audio system. Thus, the diffusion can be optimally adapted to a specific reproduction scenario. In addition, the transient response can be purposefully improved by imposing constraints on the time-domain filter coefficients

Individual Listening Zone with Frequency-Dependent Trim of Measured Impulse Responses

Acoustic Contrast Control (ACC) has been widely used to achieve individual audio delivery in shared environments. The effectiveness of this method is reduced when the control is performed in reverberant environments. Even if control filters are computed using measured transfer functions, the robustness of the system is affected by the presence of reverberation in the plant matrix. In this paper a new optimization method is presented to improve the ACC algorithm by applying a frequency-dependent windowing of the measured impulse response used for the filter computation, thus removing late reflections. The effects of this impulse response optimization are presented by means of sound zoning results obtained from experimental measurements performed in a car cabin

Sound field reproduction

This thesis is concerned with the problem of reproducing a desired sound field with an array of loudspeakers. A theory based on functional analysis and the theory ofintegral equations is developed for the study of this problem. An attempt is made to develop a mathematical framework that can be adopted as a generalized theory of sound field reproduction. The reproduction problem is formulated as an acoustical inverse problem, in which the target sound field is given on the boundary of a control volume located in the interior of the loudspeaker array, while the loudspeaker signals required for the reproduction of the desired field are to be determined. The loudspeaker array is initially modeled as a continuous distribution of secondary sources, mathematicallyrepresented by a single layer potential, whose density is to be determined. The singular value decomposition of the integral operator involved is proposed as a method for solving the inverse problem. Closed form expressions are derived for the singular system for thecases of secondary sources arranged on a sphere and on a circle. An attempt is also made to extend the calculation to unbounded geometries, such as an infinite line and aplane. The inverse problem under consideration is in general ill-posed, and the existence and uniqueness of its solution are studied in relation to sound fields of practical interest. It is shown that an exact and unique solution exists for a large family of sound fields.Strategies are proposed for overcoming the problem of nonexistence and nonuniqueness of the solution, arising in cases such as the reproduction of focused sources or when the operating frequency corresponds to one of the Dirichlet eigenvalues of the control region.An important analogy is also drawn between the problem of sound field reproduction and the theory of acoustic scattering. In a later part of this work, the assumptions of a continuous layer of secondary sources and of a single operating frequency are removed, and the resulting consequences are analyzed. The experimental validation of some of the theoretical results is described in the final part of the thesis. A large spherical loudspeakerarray is used in an attempt to reproduce the sound field generated by a single virtual source, located in the exterior of the array. Experimental results are in good agreement with the theoretical results over a wide range of frequencies