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STADIUS-18-04

Abstract:

This thesis addresses the fundamental question in room acoustic signal processing concerning the appropriateness of different parametric models for room acoustics. In order to improve the perceived sound quality, room acoustic signal processing algorithms require the acoustic response of the room to be represented by means of parametric models and to be identified from the input and output signals of the room acoustic system. In particular, a good model should be both accurate, thus capturing those features of room acoustics that are physically and perceptually most relevant, and efficient, so that it can be implemented as a digital filter and used in practical tasks. The main goal of this thesis is then to develop accurate yet efficient parametric models for room acoustics, based on recent advances in system identification and numerical optimization, and on the physical understanding of room acoustics. Orthonormal basis functions (OBF) models are investigated, whose properties, such as orthogonality and scalability, are exploited in the development of iterative flexible algorithms, providing a significant reduction in the number of parameters compared to conventional parametric models. Improvements were obtained also in the room acoustic system identification framework using OBF adaptive filters, which present interesting properties in terms of error performance and convergence of the filter coefficients. A scalable iterative approach is adopted for the pole estimation, providing a reduction in the filter order, thus helping in addressing some of the issues encountered in RASE applications, such as echo path undermodeling in acoustic echo cancellation, or frequency allocation in inverse filtering for digital equalization. Particular attention is addressed to the low-frequency region of modal resonances, where the acoustics of small rooms is typically more problematic. In this regard, the issues of measuring RIRs at low frequencies, mostly related to high ambient noise and to the nonlinear distortions produced by the subwoofer, are addressed and a novel procedure for estimating the frequency-dependent reverberation time is suggested. Finally, an effective automatic procedure for the design of a low-complexity parametric equalizer for loudspeaker/room response equalization is proposed, and guidelines for the implementation of an existing solution for nonminimum-phase multi-channel equalization of car cabin acoustics are given.