Active noise control primer
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This document provides an overview of active noise control and discusses several key topics: - It introduces active noise control and describes how it uses adaptive feedforward control to reduce noise. - It discusses fundamentals of sound such as sound waves, frequency analysis, and human perception of sound. - It covers passive and active approaches for noise control in free space, enclosed spaces, and ducts. - It provides an overview of the components and requirements of an active noise controller, including the digital filter, adaptive algorithm, and cancellation path modeler.
Active noise control primer
- 1. ACTIVE NOISE CONTROL PRIMER
- 2. AlP Series in Modern Acoustics and Signal Processing ROBERT T. BEYER, Series Editor-in-Chief Physics Department, Brown University EDITORIAL BOARD YOICHI ANDO, Faculty of Engineering, Kobe University, Kobe, Japan FLoYD DUNN, Bioacoustics Research Lab, University of Illinois, Urbana , Illinois JOHN ERDREICH, Ostergaard Associates, West Orange, New Jersey CHRIS fuLLER, Department of Mechanical Engineering, Virginia Polytechnic Institute, Blacksburg, Virginia WILLIAM HARTMANN, Department of Physics, Michigan State University, East Lansing, Michigan IRA HIRSCH, Central Institute for the Deaf and the Department of Psychology, Washington University, S1. Louis, Missouri HERMAN MEDWIN, Naval Postgraduate School, Monterey, California JOANNE L. MILLER, Department of Psychology, Northeastern University, Boston, Massachusetts LARRY ROYSTER, Department of Mechanical and Aerospace Engineering, North Carolina State University, Raleigh, North Carolina JULIA DOSWELL ROYSTER, Environmental Noise Consultants, Raleigh, North Carolina WILLIAM A. VON WINKLE, New London, Connecticut BOOKS IN THE SERIES Producing Speech, Contemporary Issues for Katherine Safford Harris, edited by Fredericka Bell-Berti and Lawrence 1. Raphael Signals, Sound, and Sensation, by William M. Hartmann Computational Ocean Acoustics, by Finn B. Jensen, William A. Kuperman, Michael B. Porter, and Henrik Schmidt Pattern Recognition and Prediction with Applications to Signal Characterization, by David H. Kil and Frances B. Shin Oceanography and Acoustics: Prediction and Propagation Models, edited by Alan R. Robinson and Ding Lee Handbook of Condenser Microphones, edited by George S.K. Wong and Tony F.W. Embleton Seismic Wave Propagation and Scattering in the Heterogeneous Earth, by Haruo Sato and Michael C. Fehler Active Noise Control Primer, by Scott D. Snyder
- 3. ACTIVE NOISE CONTROL PRIMER Scott D. Snyder University of Adelaide, Australia With 75 Illustrations AlP PRESS , Springer
- 4. Scott D. Synder Department of Mechanical Engineering University of Adelaide Adelaide, South Australia 5005 Australia Series Editor: Robert T. Beyer Physics Department Brown University Providence, RI 02912 USA Library of Congress Cataloging-in-Publication Data Snyder, Scott D. Active noise control primer / Scott D. Snyder. p. cm. - (Modern acoustics and signal processing) Includes bibliographical references and index. ISBN 978-1-4612-6437-8 ISBN 978-1-4419-8560-6 (eBook) DOI 10.1007/978-1-4419-8560-6 1. Noise control. 2. Acoustical engineering. 1. Title. II. AlP series in modern acoustics and signal processing TD892 .S58 2000 620.2'3-dc21 99-040962 Printed on acid -free paper. © 2000 Springer Science+Business Media New York Originally published by Springer-Verlag New York, Inc in 2000 Softcover reprint of the hardcover Znd edition 2000 AlP Press in an imprint ofSpringer-Veriag New York, Inc. AII rights reserved. This work may not be translated or copied in whole or in patt without the written permission of the publisher (Springer Science+Business Media, LLC), except for brief excerpts in connection with reviews or scholafly analysis. Use in connection with any form of information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed is forbidden. The use of general descriptive names, trade names, trademarks, etc., in this publication, even if the former are not especially identified, is not ta be taken as a sign that such names, as understood by the Trade Marks and Merchandise Marks Act, may accordingly be used freely by anyone. Production managed by Jenny Wolkowicki; manufacturing supervised by Jeffi-ey Taub. Typeset by KP Company, Brooklyn, NY, from the author's files. 9 8 7 6 5 4 3 2 1 ISBN 978-1-4612-6437-8
- 5. To Gill, Tom and Isaac
- 6. Series Preface Soun is noght but air y-broke -Geoffrey Chaucer end of the 14th century Traditionally, acoustics has formed one of the fundamental branches of physics. In the twentieth century, the field has broadened considerably and has become increasingly interdisciplinary. At the present time, specialists in modem acoustics can be encountered not only in physics departments, but also in electrical and mechanical engineering departments, as well as in mathematics, oceanography, and even psychology departments. They work in areas spanning from musical instruments to architecture to problems related to speech perception. Today, six hundred years after Chaucer made his brilliant remark, we recognize that sound and acoustics is a discipline extremely broad in scope, literally covering waves and vibrations in all media at all frequencies and at all intensities. This series of scientific literature, entitled Modem Acoustics and Signal Processing (MASP), covers all areas of today's acoustics as an interdisciplinary field. It offers scientific monographs, graduate-level textbooks, and reference materials in such areas as architectural acoustics, structural sound and vibration, musical acoustics, noise, bioacoustics, physiological and psychological acoustics, speech, ocean acoustics, underwater sound, and acoustical signal processing. Acoustics is primarily a matter of communication. Whether it be speech or music, listening spaces or hearing, signaling in sonar or in ultrasonography, we seek to maximize our ability to convey information and, at the same time, to minimize the effects of noise. Signaling has itself given birth to the field of signal processing, the analysis of all received acoustic information or, indeed, all information in any electronic form. With the extreme importance of acoustics for both modem science and industry in mind, AlP Press, now an imprint of Springer-Verlag, initiated this series as a new and promising publishing venture. We hope that this venture will be beneficial to the entire international acoustical community, as represented by the Acoustical Society of America, a founding vii
- 7. viii Series Preface member of the American Institute of Physics, and other related societies and professional interest groups. It is our hope that scientists and graduate students will find the books in this series useful in their research, teaching, and studies. As James Russell Lowell once wrote, "In creating, the only hard thing's to begin." This is such a beginning. Robert T. Beyer Series Editor-in-Chief
- 8. Preface Active noise control has become one of the most popular research topics in the "engineering" domain, with hundreds of journal papers covering dozens of associated topics reaching the academic press each year. However, despite this research effort, the number of practical, commercial implementations reaching the marketplace has been extremely slim. Apart from active headsets, and the odd air conditioning and vehicle implementation, it is difficult to think of practical examples. There are a large number of reasons for this lag between the commercial and academic worlds. Active noise control systems are very complex, usually requiring the designer to achieve some synergy between microelectronics, transducer technology and physical acoustics; having the skills to do this requires significant experience. Noise problems which are truly amenable to active control solutions are not as widespread as many people think. I cannot, for example, quiet your neighbour's dog, or stop traffic noise from entering the house built next to a superhighway, or, in most cases, even provide a practical solution to the problem of the noisy refrigerator. Even in instances where it does work, the frequency range over which control can be achieved is usually quite limited. If I dwell too long on all of these thoughts, I will be tempted to mutter a statement along the lines "an expert in a useless field!" However, having said all of these nasty things, I will say that when active noise control works, it really works. There is almost a feeling of disbelief in the audience when, for example, you reduce the level of the fundamental tone in a commercial leaf vacuum by 30 dB, or the low frequency engine noise in a vehicle cabin by a similar amount. The trick is to know when to apply the technology, what problems are amenable. This brings us to this book. This book grew out of a set of manuals and papers a colleague of mine, George Vokalek, and I wrote to support an "active control development kit." The aim of the kit was to provide the microelectronics required for commercial designers to implement active noise control systems in their various products. The problem was to give the designers some indication of how active noise control actually (physically) worked, where it could be IX
- 9. x Preface applied, and what results could be reasonably expected without going into pages of mathematical expressions. This book was my attempt at a solution. Since that time, I have found it to be a useful introduction for new graduate students, senior-level students undertaking active noise control projects, and secondary and tertiary teachers looking for new ideas to aid the instruction of fundamental physics. For those who are interested, there is an "experimental kit" which supports this text, available from the Michigan-based company Arbor Scientific: www.arborsci.com. In keeping with the aims, this book is short and descriptive, almost totally without mathematical expressions. As the title indicates, it is meant to be a "primer," an introductory text. It assumes that the reader has essentially no knowledge of acoustics, signal processing, or noise control. Hopefully, after reading the book, this will change. Scott D. Snyder
- 10. Contents Series Preface .................................................................................... vii Preface ............................................................................................... ix 1. Introduction ............................................................................... 1 Welcome to the World of Active Noise Control! ................................ I Chapter Summary.. ..... ... ..... ...... ......... ....... ....... .... ... ... .... ... ..... ... ....... ... ... 2 Do I Have to Read the Whole Book? .... ....... ....... ....... ..... .......... ..... ..... 3 What Is Active Noise Control? ............................................................. 3 Adaptive Feedforward Active Control Noise ....................................... 4 Advanced Reading . ........ ..... ............. .... ............ .... ...... ... .... ..... .... ....... ..... 5 2. Background: Fundamentals of Sound ................................... 7 What Is Sound? ..................................................................................... 7 What Is Noise? ...................................................................................... 8 Quantifying Sound Levels ......... ... ... ....... ....... .... ..... ... ... .... ... ..... ....... ... ... 8 Sound Waves ......................................................................................... 10 Frequency Analysis ..... ............. ..... ........... ... ... ....... ... ..... .... ... .... ........ ..... 14 Sine Waves ............................................................................................ 15 Fourier Analysis ...... ... .................. ... .... ....... ..... .... ... ... ... .... ..... ... ....... ... ... 19 Harmonics . ... ............ ... ... ........ .... ........ ...... ... ......... ... ... .... ... ..... ... ....... ... ... 21 1. Anything that Rotates ......... ........ ..... ... ....... .... ... ... ........ ... ......... ..... 22 2. Many Devices Which Use Mains Electricity.............................. 24 3. Devices Which Are Driven Beyond Their Capabilities .............. 25 Human Perception of Sound .. ... .... ...... .......... ......... ... ..... ....... .... ... .... ..... 27 Acceptable Sound Levels .. ... ..... ... ........ ......... ...... ... ... ....... ..... .......... ... ... 30 3. Fundamentals of Noise Control.............................................. 36 Prerequisite Discussion: Power and Impedance ....... ..... ....... .... ... ... ... ... 36 The Magnitude of Acoustic Power . .... .... ... ..... .... ... ... ..... ... ... ..... ....... ..... 40 Decibel Units for Acoustic Power ........................................................ 40 Xl
- 11. xii Contents Power, Pressure, and Hearing Loss ...................................................... 41 Real and Imaginary Impedance ............................................................ 42 What Is Noise Control? ......................................................................... 43 What Is Passive Noise Control? ........................................................... 43 What is Active Noise Control? ............................................................. 44 4. Free Space Noise Control ........................................................ 46 Passive Noise Control Approaches ....................................................... 46 Active Control Approaches ................................................................... 51 5. Enclosed Space Noise Control................................................. 67 Where Does the Sound Come From? ................................................... 67 How Does the Sound Get Out Again? ................................................. 68 How Does the Sound Field Arrange Itself? ....... ....... ... ... ..... ... .......... ... 69 Passive Noise Control Approaches ....................................................... 74 Active Noise Control Approaches ........................................................ 76 6. Control of Sound Propagation in Ducts ................................ 81 Sound Fields in Ducts ........................................................................... 81 Modes in Ducts ... .......... ....... ..... ........... ...... ....... ......... ... ........ ....... ...... ... 82 Impedance in Ducts ....... ..... ... ... ......... ..... .... .... ... ........ ............ ... .......... ... 83 Passive Noise Control Approaches ....................................................... 84 Sidebranch Resonator ..... ... ............ ... ... .... ...... ........ ..... ............... ....... ..... 85 Expansion Chamber .... ... .................... ............. ........ ...... ............ ....... ...... 86 Helmholtz Filter ..................................................................................... 87 Dissipative Passive Control Techniques ............................................... 88 Active Noise Control Approaches ........................................................ 89 Reference and Error Signal Quality.... .... .... .......... .... ... ........ ... .... ... ....... 90 Reference Sensor/Control Source Separation Distance ........................ 91 Control Source Position in the Duct .......... .... ..... ... ............. ..... ....... ...... 92 Duct Response Characteristics .. ... ..... ......... .... ..... ..... ..... ........ ... .... ......... 92 7. Active Noise Controller Overview.......................................... 95 Some Important Facts ....... ... ..... ........ ..... .... ......... ... ... ... .... ..... ... .... .... ..... 96 Digital System Requirements .... ... ..... ... .... ..... ...... ... .......... ............ ......... 96 Controller Output (Digital Control Filter) Requirements . ... ........... ... ... 104 Adaptive Algorithm Requirements ....................................................... 107 8. Controller Fundamentals ......................................................... 113 General Control System Outlines and Definitions .... ........... .... ....... ..... 114 Physical System Limitations ......... ... ... ....... .... ..... ... ... ..... ....... ... ........ ..... 119 Interfacing a Digital System ................................................................. 121
- 12. Contents xiii Background .................................................... ........................................ 121 Required Additions for Digital Control................................................ 124 Overview of the Controller ................................................................... 127 Controller Component 1: The Digital Filter ......................................... 128 What Is a Digital Filter? ................................................................... 128 Specifying the "Appropriate" Digital Filter ..................................... 132 Specifying the Digital Filter Length ................................................. 134 Controller Component 2: The Adaptive Algorithm ............................. 135 Background: Making Use of Adaptive Signal Processing ............... 135 Gradient Descent Algorithms ............................................................ 136 Evaluating the Gradient ..................................................................... 139 The Convergence Coefficient ............................................................ 140 Quantization Errors and Leakage ...................................................... 143 Slowing Down the Algorithm to Improve Performance .................. 145 Controller Component 3: Cancellation Path Modeler .......................... 146 Selecting the Sample Rate ................................................................ 150 So What Is the Optimum Sample Rate? ........................................... 154 Index .................................................................................................. 157