Auralization: Fundamentals of Acoustics, Modelling, Simulation, Algorithms and Acoustic Virtual Reality

341 Pages·2008·5.68 MB·English

by  Prof. Dr. Michael Vorländer (auth.)

RWTHedition
RWTHAachen
Michael Vorländer
Auralization
Fundamentals of Acoustics,
Modelling, Simulation,
Algorithms and
Acoustic Virtual Reality
First edition
123
Prof.Dr.MichaelVorländer
RWTHAachen
InstitutfürTechnischeAkustik
Neustraße50
52066Aachen
Germany
[email protected]
ISBN978-3-540-48829-3 e-ISBN978-3-540-48830-9
DOI10.1007/978-3-540-48830-9
LibraryofCongressControlNumber:2007932291
ISSN1865-0899
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Foreword
RWTH Aachen University is one of Germany’s leading universities of
technology with a long tradition of close collaboration with national and
international industry and research centres.
The University was founded as a polytechnical institute by an industry
initiative in 1870, in a then fringe area of the Prussian heavy industry. Re-
established as the Institute of Technology of North Rhine-Westphalia after
World War II, it has always shown a strong commitment to contribute to
the advance of science and engineering and to respond to the increasing
demands made by society, ultimately advancing the development of the
economy at a national and international level.
In light of this, RWTH Aachen University and Springer-Verlag Publish-
ing have recently signed a cooperative agreement on the publication of
selected works of authors from the University within a special series titled
“RWTH edition” and published in English. The present work comprises
the first volume of this series, which has evolved from the long-standing,
fruitful cooperation between the two partners with the explicit goal of
fostering the sustainable dissemination of groundbreaking research results
to the international scientific community, thereby initiating and facilitating
scientific exchange worldwide. Such a step was deemed particularly mean-
ingful considering the growing importance of interdisciplinary projects
worldwide, which are based on innovative solution-finding and sound
basic research.
As Rector of RWTH Aachen University, I would like to express my
sincere gratitude to Springer-Verlag for its commitment and ongoing co-
operation.
May this series arouse the interest of the scientific community and con-
tribute to an outstanding and solid education of our students and graduates.
Sincerely,
Prof. Dr. rer. nat. Burkhard Rauhut
Preface
Acoustic virtual reality is a rather new and growing discipline. It integrates
methods of physics and acoustic engineering with psychoacoustics and
electroacoustic sound reproduction. The keywords of acoustic virtual real-
ity are “simulation,” “auralization” and “spatial sound reproduction.” The
expression “auralization” is analogous to the well-known technique of
“visualization.” In visual illustration of scenes, data or any other meaning-
ful information, in movie animation and in computer graphics, we describe
the process of “making visible” as visualization. In acoustics, auralization
occurs when acoustic effects, primary sound signals or means of sound
reinforcement or sound transmission, are processed into an audible result.
The word auralization is used today to describe the process of signal gen-
eration, processing and reproduction as well as its result: the perceivable
sound as auralization of an acoustic problem, a room, a building, a car, or
any other industrial product.
The concept of auralization was first introduced in relation to acoustic
modelling of sound fields in rooms. Already in 1929 in Munich, Spandöck
and colleagues tried to process signals from measurements on a model
scale so that one could listen to the acoustics of a room, in spite of the fact
that the “room” was just a scale model on a laboratory table. The idea was
formulated quickly, but at that time it was extremely difficult to put it into
action. Twenty years later, in 1949, magnetic tape recorders were avail-
able. Spandöck et al. finally presented their system based on ultrasonic
signals, scale models and a tape recorder working at several speeds
(Spandöck 1934). This technique can be considered an analogue computer.
The basic elements were already present in this approach: Sound field
modelling, processing with an arbitrary sound signal and sound reproduc-
tion. The results at that time were not comparable with the expectations of
audio quality we have today. Background noise and low bandwidth limited
the listening experience, but nevertheless these experiments must be con-
sidered the first successful auralization.
With the dramatic development of computers, the concept of simulation
and auralization was reinvented by Schroeder et al. (1962) at the beginning
of the 1960s. In 1968, the first room acoustical simulation software devel-
oped by Krokstad et al. (1968) was applied in room acoustics. By the
VIII Preface
1990s, the processor speed, memory space and convolution machines were
sufficiently powerful to allow room acoustical computer simulation and
auralization on a standard personal computer or on relatively small work-
stations. In the early 1990s, the word “auralization” was introduced
(Kleiner et al. 1993). Since that time, several improvements in modelling
algorithms, in binaural processing and in reproduction technique have been
made. Today, commercial software for room acoustical simulation is con-
sidered incomplete without an option for auralization through the sound
card of the computer. Several other fields of acoustics also included the
term auralization in their vocabulary, particularly product sound design
and building acoustics.
The main theme of this book is the general technique of auralization and
its manifold applications, both offline and in real time. It covers three main
components: sound generation, sound transmission and sound reproduc-
tion. The methodology of simulation and auralization developed from dis-
cussion of methods for acoustic modelling. After starting with chapters on
a general introduction into linear acoustics in fluids and in structures and
on basic features of sound perception (psychoacoustics) in a free field and
in rooms, in Chaps. 7 to 9, an introduction is given to the aspects of signal
processing and auralization methodology in a general sense. In Chap. 10,
the reader will learn about simulation models and techniques of applying
numerical approaches for various acoustic problems.
Virtual environments must usually be created in virtual rooms or similar
enclosures. The technique of simulation is therefore explained in more detail
by placing more emphasis on acoustics in enclosures in Chap. 11, including
room acoustics auralization in Sect. 11.6. (It is presented as a first example
where the technique of auralization is applied.1) Chapters 12 and 13 deal
with strategies of simulation and auralization for applications in architec-
tural acoustics other than room acoustics: airborne and impact sound insu-
lation and structure-borne sound sources in buildings. Another field with
application of auralization is vehicle acoustics. The global model of binau-
ral transfer synthesis (BTPS), used in automotive acoustics, is discussed in
Chap. 14. The final Chaps. 15 and 16 are focused on aspects of real-time
processing and 3-D sound reproduction technology.
This book is organized as a comprehensive collection of acoustic fun-
damentals, of methodology and strategies of acoustic simulation and au-
ralization. With the mathematical background of third-year university stu-
dents, readers will easily be able to follow the main strategy of auralization
and apply their own implementations of auralization in various fields of
acoustic engineering and virtual reality. For readers interested in basic

1 Historically, it was also first established in room acoustics.
Preface IX
research, the technique of auralization may be useful for creating sound
stimuli for specific investigations in civil engineering and in architecture.
Virtual reality applications with acoustic stimuli are also crucial in linguis-
tic, medical, neurological and psychological research and in the field of
human–machine interaction.
Acknowledgments
“Auralization” has become a big field of interest in the acoustics commu-
nity. Having taught a course in “Acoustic Virtual Reality” during the past
eight years, I have recognized the need to concentrate all work done in
acoustic simulation, auralization and acoustic virtual reality systems into
a textbook. This book would not have been possible without the contribu-
tions of many helping hands. The contributions of the doctoral students
Oliver Schmitz, Tobias Lentz and Dirk Schröder in our activities in aurali-
zation and virtual reality systems created a strong focus of research in the
institute. Also Ingo Assenmacher, who is collaborating with us to imple-
ment acoustics in the RWTH Aachen Virtual Reality centre, must be men-
tioned. Inspired by requests from numerous graduate students and col-
leagues in Germany and abroad, the idea of extending the lecture notes
into a book was born quickly. Finally, while teaching “Acoustic Simula-
tion and Auralization” during a sabbatical in Florianópolis, Santa Catarina,
Brazil, the book project was started.
I am particularly grateful for the support by Pascal Dietrich who created
figures in a way I never could have done myself. The permission to use the
world’s largest database of acoustic absorption was given by Dr. Ingolf
Bork from PTB Braunschweig. Many more colleagues could be named
here who supported the book project by contributing information, discus-
sion or figures. I am grateful also for excellent feedback and support from
all members of the Institute of Technical Acoustics (ITA), of RWTH
Aachen University. First and foremost, I thank my predecessor as head of
the institute and my academic teacher, Prof. Heinrich Kuttruff, for teaching
me not only the physical and technical basics but also the inspiration, mo-
tivation, beauty and fun of acoustics.
Jonathan Rathsam from the University of Nebraska checked the linguis-
tic quality and corrected my “German English.” Due to his patience and
kind proposals for modifications, I managed to finish the book in hopefully
acceptable English, at least from the viewpoint of nonnative speakers.
Thomas Lehnert from Springer Publishers was very patient and always
helpful in clarifying formal and technical aspects in template style and
layout. The cooperation was very smooth.
X Preface
Last but not least, my family must be mentioned. For several months
I was not able to contribute acceptably to the daily life of our home, school
and other fields of family activities. I can only promise Angelika, Paul,
Tobias and Benjamin that this will be better from now on.
Aachen, June 2007 Michael Vorländer
Content
Introduction......................................................................................... 1
1 Fundamentals of acoustics......................................................... 7
1.1 Sound field equations and the wave equation.................... 8
1.1.1 Sound field quantities............................................ 9
1.1.2 Derivation of the wave equation............................ 10
1.2 Plane waves in fluid media................................................ 13
1.3 Plane harmonic waves........................................................ 15
1.4 Wideband waves and signals............................................. 15
1.5 Energy and level................................................................. 16
1.6 Sound intensity................................................................... 19
1.7 Level arithmetic................................................................. 20
1.8 Frequency bands................................................................ 21
2 Sound sources............................................................................. 23
2.1 Spherical waves.................................................................. 23
2.2 Harmonic monopole source and sound power................... 24
2.3 Pulsating sphere and radiation impedance......................... 26
2.4 Multipoles and extended sources....................................... 28
2.5 Spherical harmonics........................................................... 31
3 Sound propagation..................................................................... 35
3.1 Reflection of plane waves at an impedance plane............. 35
3.1.1 Examples of wall impedances................................ 37
3.2 Spherical wave above impedance plane............................. 41
3.3 Scattering........................................................................... 42
3.3.1 Object scattering.................................................... 42
3.3.2 Surface scattering................................................... 43
3.4 Diffraction.......................................................................... 47
3.5 Refraction........................................................................... 48
3.6 Attenuation......................................................................... 49
3.7 Doppler effect.................................................................... 51