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Loudspeaker Array Processing for Personal Sound Zone Reproduction

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Text of Loudspeaker Array Processing for Personal Sound Zone Reproduction

Philip Coleman
Submitted for the Degree of Doctor of Philosophy
Centre for Vision, Speech and Signal Processing Faculty of Engineering and Physical Sciences
University of Surrey Guildford, Surrey GU2 7XH, U.K.
2014
Summary
Sound zone reproduction facilitates listeners wishing to consume personal audio content within the same acoustic enclosure by filtering loudspeaker signals to create constructive and destruc- tive interference in different spatial regions. Published solutions to the sound zone problem are derived from areas such as sound field synthesis and beamforming. The first contribution of this thesis is a comparative study of multi-point approaches. A new metric of planarity is adopted to analyse the spatial distribution of energy in the target zone, and the well-established metrics of acoustic contrast and control effort are also used. Simulations and experimental results demon- strate the advantages and disadvantages of the approaches. Energy cancellation produces good acoustic contrast but allows very little control over the target sound field; synthesis-derived approaches precisely control the target sound field but produce less contrast.
Motivated by the limitations of the existing optimization methods, the central contribution of this thesis is a proposed optimization cost function ‘planarity control’, which maximizes the acoustic contrast between the zones while controlling sound field planarity by projecting the target zone energy into a spatial domain. Planarity control is shown to achieve good contrast and high target zone planarity over a large frequency range. The method also has potential for reproducing stereophonic material in the context of sound zones.
The remaining contributions consider two further practical concerns. First, judicious choice of the regularization parameter is shown to have a significant effect on the contrast, effort and robustness. Second, attention is given to the problem of optimally positioning the loudspeakers via a numerical framework and objective function.
The simulation and experimental results presented in this thesis represent a significant addition to the literature and will influence the future choices of control methods, regularization and loudspeaker placement for personal audio. Future systems may incorporate 3D rendering and listener tracking.
Email: [email protected]
Declaration of originality
This thesis and the work to which it refers are the results of my own efforts. Any ideas, data, images or text resulting from the work of others (whether published or unpublished) are fully identified as such within the work and attributed to their originator in the text, bibliography or in footnotes. This thesis has not been submitted in whole or in part for any other academic degree or professional qualification. I agree that the University has the right to submit my work to the plagiarism detection service TurnitinUK for originality checks. Whether or not drafts have been so-assessed, the University reserves the right to require an electronic version of the final document (as submitted) for assessment as above.
Acknowledgements
I would like to thank Philip Jackson for his supervision of my doctoral research. His construc- tive criticism and encouragement have brought out the best in all aspects of my research, and I am very grateful for the time he has invested, leading to both the development of this thesis and my personal development as a researcher.
I am grateful to Bang & Olufsen for conceiving and funding the POSZ project and the EPRSC for funding a portion of my doctoral research.
I have benefitted greatly from the collaboration with other postgraduate researchers as part of the POSZ project. Marek Olik has contributed greatly to the development of the Matlab toolbox underpinning the simulated and measured results presented in the thesis, assisted with development of the sound zone prototypes, and always provided useful feedback on plans and presentations. Similarly, Jon Francombe and Khan Baykaner have provided feedback and stimulating debate to help shape the direction of the research.
The POSZ project group have provided valuable feedback at the quarterly review meetings. Thanks to Russell Mason, Martin Dewhirst and Chris Hummersone from the University of Surrey, and also to Francis Rumsey. I am grateful for the engagement of many people at Bang & Olufsen, in particular Martin Møller and Martin Olsen for their assistance with experimental work, collaboration on measurement scripts and sound field synthesis implementations, and review of presentations and draft publications. The participation of Søren Bech, Jan Abildgaard Pedersen, Adrian Celestinos, Patrick Hegarty and Mørten Lydolf was also appreciated.
I would also like to acknowledge the help of Alice Duque in setting up the experimental system used for the measured results in this thesis and assisting with the public demonstrations, and for the administrative support provided by Liz James and Amy Pimperton in the CVSSP office.
I am grateful for the occasional review and feedback from Mark Barnard, Wenwu Wang, and the Machine Audition Group in CVSSP, and for comments at IoSR seminar days and informal discussions in the office with Tim Brookes, Daisuke Koya, Tommy Ashby, Toby Stokes, Cleo Pike, Andy Pearce and Kirsten Hermes.
Finally, thanks to my family and friends. Felicity has been my rock and support throughout the project, sharing the highs and lows with me along my doctoral journey. Thanks, as ever, to Ruth, Andy, Peter, Aretia, Jen, Matt, and the countless others who have loved and encouraged me throughout.
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Contents
1.3.2 Planarity control optimization . . . . . . . . . . . . . . . . . . . . . . 8
1.3.3 Robustness and regularization of sound zone systems . . . . . . . . . . 9
1.3.4 Optimal selection of loudspeaker positions . . . . . . . . . . . . . . . 10
1.4 Organization of thesis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
2 Literature review and theory 13
2.1 Sound zone problem definition . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.1.1 Acoustical description . . . . . . . . . . . . . . . . . . . . . . . . . . 14
2.1.2 Geometrical description . . . . . . . . . . . . . . . . . . . . . . . . . 17
2.2.1 Delay and sum beamforming . . . . . . . . . . . . . . . . . . . . . . . 19
2.2.2 Brightness control . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.3.2 Acoustic energy difference maximization . . . . . . . . . . . . . . . . 32
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2.4.1 Analytical approaches . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2.4.2 Least-squares solutions . . . . . . . . . . . . . . . . . . . . . . . . . . 46
2.5 Alternative approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
2.5.2 Crosstalk cancellation . . . . . . . . . . . . . . . . . . . . . . . . . . 54
3.1 Comparative studies in the literature . . . . . . . . . . . . . . . . . . . . . . . 61
3.2 Sound zone performance evaluation . . . . . . . . . . . . . . . . . . . . . . . 63
3.2.1 Acoustic contrast . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 63
3.2.2 Control effort . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 64
3.4 Measured performance in a reflective environment . . . . . . . . . . . . . . . . 82
3.4.1 System realization and geometry . . . . . . . . . . . . . . . . . . . . . 83
3.4.2 Practical performance . . . . . . . . . . . . . . . . . . . . . . . . . . 86
4.1 Approaches to single-zone plane wave reproduction . . . . . . . . . . . . . . . 94
4.1.1 Intensity-based approaches . . . . . . . . . . . . . . . . . . . . . . . . 95
4.2 Cost function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
4.3 Anechoic simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
4.4 Practical performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
4.4.2 Practical extension to stereophonic reproduction . . . . . . . . . . . . 113
4.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
5.1 Robustness and regularization in the literature . . . . . . . . . . . . . . . . . . 123
5.2 Anechoic simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
5.2.2 Robustness to mismatched setup and playback conditions . . . . . . . . 130
5.2.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
6.2 Selection procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
6.2.1 Objective function . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
6.2.2 Search algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
6.3 Optimal positioning of a fixed number of loudspeakers . . . . . . . . . . . . . 152
6.4 Positioning to achieve desired performance characteristics . . . . . . . . . . . 157
6.5 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 160
6.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 163
7.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167
7.1.3 Robustness and regularization . . . . . . . . . . . . . . . . . . . . . . 169
7.1.4 Loudspeaker selection . . . . . . . . . . . . . . . . . . . . . . . . . . 170
7.2 Further work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
7.2.2 Programme-aware control . . . . . . . . . . . . . . . . . . . . . . . . 172
7.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
7.3.2 Perception of aspects discussed in technical chapters . . . . . . . . . . 175
7.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176
C Sound field visualizations 187
D Planarity control simulation results 193
E Regularization effect on sound field 195
F Loudspeaker subsets 197
1.1 Sound zone problem overview diagram . . . . . . . . . . . . . . . . . . . . . 4
2.1 Acoustically defined two zone system . . . . . . . . . . . . . . . . . . . . . . 15
2.2 Geometrically defined two zone system . . . . . . . . . . . . . . . . . . . . . 18
2.3 Concept of the sound focusing and energy cancellation approaches . . . . . . . 20
2.4 Concept of applying energy cancellation techniques for super-directive beam- forming . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 23
2.5 Concept of the formulations of acoustic contrast control . . . . . . . . . . . . . 23
2.6 Concept of the interior reproduction problem . . . . . . . . . . . . . . . . . . 36
2.7 Comparison of WFS and HOA solutions with respect to the Kirchhoff-Helmholtz integral . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 38
2.8 Geometry relating to multi-zone coordinate translation . . . . . . . . . . . . . 43
3.1 Microphone array steering geometry . . . . . . . . . . . . . . . . . . . . . . . 68
3.2 Simulation geometry . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 72
3.3 Performance of BC, ACC and PM over frequency . . . . . . . . . . . . . . . . 75
3.4 SPL and phase distributions for BC, ACC and PM at 1 kHz . . . . . . . . . . . 76
3.5 Energy distribution for BC, ACC and PM . . . . . . . . . . . . . . . . . . . . 78
3.6 Upper frequency of ACC and PM contrast . . . . . . . . . . . . . . . . . . . . 79
3.7 Photographs of the implemented sound zone system . . . . . . . . . . . . . . . 84
3.8 Impulse response, filter calculation and performance measurement process . . . 86
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3.9 Measured BC, ACC and PM performance over frequency . . . . . . . . . . . . 88
3.10 Measured BC, ACC and PM energy distribution . . . . . . . . . . . . . . . . . 89
3.11 Measured upper frequency of ACC and PM contrast . . . . . . . . . . . . . . . 90
4.1 Performance of PC, ACC and PM over frequency . . . . . . . . . . . . . . . . 101
4.2 SPL and phase distributions for PC, ACC and PM at 1 kHz . . . . . . . . . . . 102
4.3 Energy distribution for PC, ACC and PM . . . . . . . . . . . . . . . . . . . . 104
4.4 Energy distribution at 1 kHz for PC and PM plane wave reproduction . . . . . 106
4.5 Energy distribution for PC and PM plane wave reproduction in frequency bands 107
4.6 Measured performance of PC, ACC and PM . . . . . . . . . . . . . . . . . . . 109
4.7 Measured energy distribution of PC, ACC and PM . . . . . . . . . . . . . . . . 112
4.8 Concept of synthesized stereophonic sound zones . . . . . . . . . . . . . . . . 114
4.9 Energy distribution for PC and PM stereo reproduction . . . . . . . . . . . . . 115
4.10 Combined contrast for PC and PM stereo reproduction . . . . . . . . . . . . . 116
5.1 Regularization effect under ideal conditions . . . . . . . . . . . . . . . . . . . 127
5.2 Regularization effect at 100 Hz with systematic errors . . . . . . . . . . . . . . 131
5.3 Regularization effect at 1000 Hz with systematic errors . . . . . . . . . . . . . 132
5.4 Measured regularization effect . . . . . . . . . . . . . . . . . . . . . . . . . . 138
6.1 Reference circle and arc arrays for the 10 loudspeaker case . . . . . . . . . . . 152
6.2 Measured mean contrast performance for increasing numbers of loudspeakers . 153
6.3 Measured acoustic contrast performance across frequency for 10 loudspeakers . 155
6.4 Sound pressure level distribution at 2650 Hz for ACC applied to 10 element loudspeaker arrays…