Loudspeaker Array Processing for Personal Sound Zone Reproduction

239 Pages·2014·6.11 MB·English

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
⃝c Philip Coleman 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.
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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 Introduction 1
1.1 Motivation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
1.2 Problem statement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 3
1.3 Contributions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
1.3.1 Comparative performance of sound zone methods . . . . . . . . . . . . 6
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 Sound focusing approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18
2.2.1 Delay and sum beamforming . . . . . . . . . . . . . . . . . . . . . . . 19
2.2.2 Brightness control . . . . . . . . . . . . . . . . . . . . . . . . . . . . 20
2.3 Energy cancellation approaches . . . . . . . . . . . . . . . . . . . . . . . . . 22
2.3.1 Acoustic contrast control . . . . . . . . . . . . . . . . . . . . . . . . . 24
2.3.2 Acoustic energy difference maximization . . . . . . . . . . . . . . . . 32
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viii Contents
2.4 Sound field synthesis approaches . . . . . . . . . . . . . . . . . . . . . . . . . 33
2.4.1 Analytical approaches . . . . . . . . . . . . . . . . . . . . . . . . . . 34
2.4.2 Least-squares solutions . . . . . . . . . . . . . . . . . . . . . . . . . . 46
2.5 Alternative approaches . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
2.5.1 Active noise control . . . . . . . . . . . . . . . . . . . . . . . . . . . 51
2.5.2 Crosstalk cancellation . . . . . . . . . . . . . . . . . . . . . . . . . . 54
2.6 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 55
3 Control method comparison 59
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
3.2.3 Planarity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 65
3.3 Anechoic simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
3.3.1 Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 69
3.3.2 Control method comparison . . . . . . . . . . . . . . . . . . . . . . . 73
3.4 Measured performance in a reflective environment . . . . . . . . . . . . . . . . 82
3.4.1 System realization and geometry . . . . . . . . . . . . . . . . . . . . . 83
3.4.2 Practical performance . . . . . . . . . . . . . . . . . . . . . . . . . . 86
3.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 91
4 Planarity control optimization 93
4.1 Approaches to single-zone plane wave reproduction . . . . . . . . . . . . . . . 94
4.1.1 Intensity-based approaches . . . . . . . . . . . . . . . . . . . . . . . . 95
4.1.2 Control of pressure in a spatial domain . . . . . . . . . . . . . . . . . 96
4.2 Cost function . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 98
4.3 Anechoic simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 100
Contents ix
4.3.1 Performance characteristics . . . . . . . . . . . . . . . . . . . . . . . 101
4.3.2 Plane wave approximation using planarity control . . . . . . . . . . . . 104
4.4 Practical performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 108
4.4.1 Measured performance characteristics . . . . . . . . . . . . . . . . . . 108
4.4.2 Practical extension to stereophonic reproduction . . . . . . . . . . . . 113
4.5 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 118
5 Robustness and Regularization 121
5.1 Robustness and regularization in the literature . . . . . . . . . . . . . . . . . . 123
5.2 Anechoic simulations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 126
5.2.1 Regularization under ideal conditions . . . . . . . . . . . . . . . . . . 126
5.2.2 Robustness to mismatched setup and playback conditions . . . . . . . . 130
5.2.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 135
5.3 Practical Performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 137
5.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 142
6 Optimal Loudspeaker Selection 143
6.1 Optimal loudspeaker placement . . . . . . . . . . . . . . . . . . . . . . . . . 144
6.2 Selection procedure . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
6.2.1 Objective function . . . . . . . . . . . . . . . . . . . . . . . . . . . . 147
6.2.2 Search algorithm . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 149
6.2.3 Method . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 150
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
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7 Conclusions and further work 165
7.1 Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 167
7.1.1 Sound zone performance characteristics . . . . . . . . . . . . . . . . . 167
7.1.2 Novel sound zone optimization . . . . . . . . . . . . . . . . . . . . . . 168
7.1.3 Robustness and regularization . . . . . . . . . . . . . . . . . . . . . . 169
7.1.4 Loudspeaker selection . . . . . . . . . . . . . . . . . . . . . . . . . . 170
7.2 Further work . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
7.2.1 3D personal audio . . . . . . . . . . . . . . . . . . . . . . . . . . . . 171
7.2.2 Programme-aware control . . . . . . . . . . . . . . . . . . . . . . . . 172
7.2.3 Dynamically located sound zones . . . . . . . . . . . . . . . . . . . . 172
7.3 Discussion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 173
7.3.1 Sound zone experience . . . . . . . . . . . . . . . . . . . . . . . . . . 174
7.3.2 Perception of aspects discussed in technical chapters . . . . . . . . . . 175
7.4 Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 176
A Planarity metric 179
B Simulated line array results 183
C Sound field visualizations 187
D Planarity control simulation results 193
E Regularization effect on sound field 195
F Loudspeaker subsets 197
Bibliography 201