RWTHedition. RWTH Aachen

Transcription

1 RWTHedition RWTH Aachen

2 Michael Vorländer Auralization Fundamentals of Acoustics, Modelling, Simulation, Algorithms and Acoustic Virtual Reality First edition 123

3 Prof. Dr. Michael Vorländer RWTH Aachen Institut für Technische Akustik Neustraße Aachen Germany ISBN e-isbn DOI / Library of Congress Control Number: ISSN Springer-Verlag Berlin Heidelberg This work is subject to copyright. All rights are reserved, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of this publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer. Violations are liable to prosecution under the German Copyright Law. The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Cover Design: deblik, Berlin, Germany Printed on acid-free paper springer.com

4 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. Reestablished 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 Publishing 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 meaningful 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 cooperation. May this series arouse the interest of the scientific community and contribute to an outstanding and solid education of our students and graduates. Sincerely, Prof. Dr. rer. nat. Burkhard Rauhut

5 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 reality 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 meaningful 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 generation, 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 available. 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 reproduction. 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 considered 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 developed by Krokstad et al. (1968) was applied in room acoustics. By the

6 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 workstations. 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 considered 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 reproduction. The methodology of simulation and auralization developed from discussion 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 (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 architectural acoustics other than room acoustics: airborne and impact sound insulation and structure-borne sound sources in buildings. Another field with application of auralization is vehicle acoustics. The global model of binaural 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 fundamentals, of methodology and strategies of acoustic simulation and auralization. With the mathematical background of third-year university students, 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.

7 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 linguistic, 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 community. 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 contributions of many helping hands. The contributions of the doctoral students Oliver Schmitz, Tobias Lentz and Dirk Schröder in our activities in auralization and virtual reality systems created a strong focus of research in the institute. Also Ingo Assenmacher, who is collaborating with us to implement acoustics in the RWTH Aachen Virtual Reality centre, must be mentioned. Inspired by requests from numerous graduate students and colleagues in Germany and abroad, the idea of extending the lecture notes into a book was born quickly. Finally, while teaching Acoustic Simulation 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, discussion 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, motivation, beauty and fun of acoustics. Jonathan Rathsam from the University of Nebraska checked the linguistic 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.

8 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

9 Content Introduction Fundamentals of acoustics Sound field equations and the wave equation Sound field quantities Derivation of the wave equation Plane waves in fluid media Plane harmonic waves Wideband waves and signals Energy and level Sound intensity Level arithmetic Frequency bands Sound sources Spherical waves Harmonic monopole source and sound power Pulsating sphere and radiation impedance Multipoles and extended sources Spherical harmonics Sound propagation Reflection of plane waves at an impedance plane Examples of wall impedances Spherical wave above impedance plane Scattering Object scattering Surface scattering Diffraction Refraction Attenuation Doppler effect... 51

10 XII Content 4 Sound fields in cavities and in rooms Cavities Modes Boundary conditions Geometrical acoustics Statistical reverberation theory Reverberation Steady-state energy density and level Structure-borne sound Waves in solid media Waves on plates and their radiation Finite-size plates Internal losses and structural reverberation time Vibrational transmission over junctions Psychoacoustics Anatomy of the peripheral hearing system Psychoacoustic characterization Loudness Temporal masking Time-varying loudness Sharpness Fluctuation strength Roughness Tonality, pitch, pitch strength Binaural hearing Head-related transfer functions Artificial heads Hearing in rooms Reverberance Strength Speech intelligibility and transparence Spatial impression Spatial variations in a room Estimation of the monaural subjective parameters Signal processing for auralization The concept of auralization Fundamentals of signal processing Signals and systems Impulse response and transfer function

11 Content XIII 7.3 Fourier transformation Analogue-to-digital conversion Discrete Fourier transformation Fast Fourier transformation Sources of errors, leakage and time windows Digital filters Characterization of sources Airborne sound sources Multipole synthesis Musical instruments Singing voice Speaking voice Anechoic recordings Structure-borne sound sources General approach D force sources Convolution and sound synthesis Discrete convolution FFT convolution Segmented convolution Binaural synthesis Binaural mixing console Spatial resolution of HRTF Simulation models Simulation methods for sound and vibrational fields Reciprocity Frequency domain models Time domain models Two-port models Transfer path models Other models Simulation of sound in rooms General CAD room model Absorption coefficients Scattering coefficients Stochastic ray tracing Point-in-polygon test Detectors

12 XIV Content Presentation of results Curved surfaces Reproducibility in stochastic ray tracing Computation times versus uncertainties case studies Image source model Classical model Audibility test Limitations Diffraction Reduction of computational load by preprocessing Hybrid image source models (deterministic ray tracing) Systematic uncertainties of geometrical acoustics Hybrid models in room acoustics Hybrid deterministic-stochastic models Construction of binaural room impulse responses Simulation and auralization of airborne sound insulation Definitions of airborne sound transmission Sound insulation of building elements Sound insulation of buildings Flanking transmission Sound transmission prediction models Auralization of airborne sound insulation Simulation and auralization of structure-borne sound Definitions of impact sound transmission Impact sound model Impact sound auralization Structure-borne interaction model Binaural transfer path synthesis Source identification and characterization Airborne sound sources Structure-borne sound sources Transfer path characterization Auralization in BTPS Aspects of real-time processing Real-time binaural synthesis HRTF in multiple degrees of freedom

13 Content XV 15.2 Room acoustical real-time auralization Source and receiver Real-time processing of image sources Real-time modelling of reverberation Hybrid real-time room auralization D sound reproduction and virtual reality systems Headphone systems Headphone equalization for binaural signals Individual filters Loudspeaker systems VBAP surround sound Ambisonics Wave field synthesis Binaural loudspeaker technology VR technology and integrated VR systems Annex Material data Tables of random-incidence absorption coefficients, α Tables of random-incidence scattering coefficients, s Tables of sound reduction indices, R References Index