ACTIVE NOISE CONTROL IN VENTILATION SYSTEMS VENTILATION IN NOISE CONTROL ACTIVE ABSTRACT In many kinds of buildings, the ventilation is hand- influence of some of these degrading physical fac- led by a mechanical ventilation system. tors. ACTIVE NOISE CONTROL IN Such ventilation systems constitute a well known The degrading factors which are of particular in- source of broadband noise. As awareness of the terest include: flow induced noise in the microp- VENTILATION SYSTEMS negative effects that subjection to low frequency hone signals, acoustic feedback between the con- noise can have on human well-being has increased, trol loudspeaker and reference microphone, and PRACTICAL IMPLEMENTATION ASPECTS so too has the requirement for quieter ventila- standing waves inside the ducts. With respect to tion installations. Traditionally, duct born noise is installation design, focus is also placed upon indu- attenuated using passive resistive silencers. These stry requests on the ANC system. Taking this into passive silencers are valued for their ability to consideration has led to a module based approach, produce a high level of attenuation over a broad in which the microphones and the loudspeaker frequency range, however they tend to become are installed in separate modules based on stan- large and bulky if designed for low frequency at- dard duct parts. tenuation. This thesis comprises four parts. The first descri- Martin Larsson The active noise control (ANC) technique is bes initial investigations of potential microphone known for its ability to produce high levels of at- installations intended to reduce flow induced tenuation in the low frequency range even with noise. The second part analyzes the influence of a relatively moderate sized ANC system. On the flow induced noise on the digital controller and other hand, ANC normally tends to be ineffective presents further investigations of microphone for higher frequencies. Accordingly, a combination modules. Further, results of measurements con- of active- and passive techniques, i.e. the construc- ducted in an acoustic laboratory according to an tion of a hybrid active/passive silencer, provides a ISO-standard are presented. The attenuation pro- duct silencer solution of manageable size which duced by the ANC system was approximately 15- also covers the low frequency range. 25 dB between 50-315 Hz even for airflow speeds The ANC systems controller normally relies on up to 20 m/s. The third part of this thesis focuses adaptive digital signal processing. Even so, adequate on the possibility of using the passive silencer with levels of attenuation are not likely to be obtained which the ANC system is combined, to reduce if the installation of the ANC system is not desig- acoustic feedback and standing waves. The fourth ned to account for the physical factors that may and final part investigates the possibility of using degrade its performance. a passive silencer to reduce standing waves in the This thesis focuses on applying ANC in ventilation duct when the ANC system is not installed near systems, with particular emphasis on analysis and the duct outlet. installation design, for the purpose of reducing the Martin Larsson Blekinge Institute of Technology Licentiate Dissertation Series No. 2008:12 2008:12 ISSN 1650-2140 School of Engineering 2008:12 ISBN 978-91-7295-152-5 Active Noise Control in Ventilation Systems Practical Implementation Aspects Martin Larsson Blekinge Institute of Technology Licentiate Dissertation Series No 2008:12 ISSN 1650-2140 ISBN 978-91-7295-152-5 Active Noise Control in Ventilation Systems Practical Implementation Aspects Martin Larsson Department of Signal Processing School of Engineering Blekinge Institute of Technology SWEDEN © 2008 Martin Larsson Department of Signal Processing School of Engineering Publisher: Blekinge Institute of Technology Printed by Printfabriken, Karlskrona, Sweden 2008 ISBN 978-91-7295-152-5 White White White White v Abstract In many kinds of buildings, the ventilation is handled by a me- chanical ventilation system. Such ventilation systems constitute a well known source of broadband noise. As awareness of the negative effects that subjection to low frequency noise can have on human well-being has increased, so too has the requirement for quieter ventilation in- stallations. Traditionally, duct born noise is attenuated using passive resistive silencers. These passive silencers are valued for their ability to produce a high level of attenuation over a broad frequency range, how- ever they tend to become large and bulky if designed for low frequency attenuation. The active noise control (ANC) technique is known for its ability to produce high levels of attenuation in the low frequency range even with a relatively moderate sized ANC system. On the other hand, ANC normally tends to be ineffective for higher frequencies. Accordingly, a combination of active- and passive techniques, i.e. the construction of a hybrid active/passive silencer, provides a duct silencer solution of manageable size which also covers the low frequency range. The ANC systems controller normally relies on adaptive digital sig- nal processing. Even so, adequate levels of attenuation are not likely to be obtained if the installation of the ANC system is not designed to account for the physical factors that may degrade its performance. This thesis focuses on applying ANC in ventilation systems, with par- ticular emphasis on analysis and installation design, for the purpose of reducing the influence of some of these degrading physical factors. The degrading factors which are of particular interest include: flow induced noise in the microphone signals, acoustic feedback between the control loudspeaker and reference microphone, and standing waves inside the ducts. With respect to installation design, focus is also placed upon industry requests on the ANC system. Taking this into consideration has led to a module based approach, in which the microphones and the loudspeaker are installed in separate modules based on standard duct parts. This thesis comprises four parts. The first describes initial investi- gations of potential microphone installations intended to reduce flow induced noise. The second part analyzes the influence of flow induced noise on the digital controller and presents further investigations of mi- crophone modules. Further, results of measurements conducted in an acoustic laboratory according to an ISO-standard are presented. The attenuation produced by the ANC system was approximately 15-25 dB between 50-315 Hz even for airflow speeds up to 20 m/s. The third part of this thesis focuses on the possibility of using the passive silencer with White White vi White White which the ANC system is combined, to reduce acoustic feedback and standing waves. The fourth and final part investigates the possibility of using a passive silencer to reduce standing waves in the duct when the ANC system is not installed near the duct outlet. White White White White vii Preface This licentiate thesis summarizes my work at the Department of Signal Processing at Blekinge Institute of Technology. The thesis is comprised of four parts: Part I Microphone Windscreens for Turbulent Noise Suppression when Apply- ing Active Noise Control to Ducts. II Performance Evaluation of a Module Configured Active Silencer for Ro- bust Active Noise Control of Low Frequency Noise in Ducts. III A Feedforward Active Noise Control System for Ducts using a Passive Silencer to Reduce Acoustic Feedback. IV A System Implementation of an Active Noise Control System Combined with Passive Silencers for Improved Noise Reduction in Ducts. White White White White ix Acknowledgments First, I would like to express my sincere gratitude to Professor Ingvar Claesson for giving me the opportunity to start as a Ph. D. student under his guidance. I am indebted to my research supervisor and friend Dr. Sven Johansson for all his help and advice during these years of study. Without his knowledge, ideas, encouragement, and outstanding ability to explain things in a comprehensible manner, this thesis would not exist. I would also like to thank Assoc. Prof. Lars H˚akansson for all his help and advice, for being a great source of knowledge, and for nice discussions ranging from loudspeaker design to fishing. Further, I wish to thank Lars-Ake˚ Mattsson and Mette Husted at Lindab AB for their help, and for arranging the measurements at the acoustic laboratory in Farum. The availability of the data from those measurements has been of great value. Thanks to all my colleagues at the Department of Signal Processing for always being friendly and helpful, and for making my time at the department more enjoyable. Further, I would like to thank my parents for teaching me good values in life, and for always supporting and being there for me. I would also like to thank Lina for her support and love, and for putting up with me being away in Ronneby. Finally, I would like to express my love for my two wonderful children, Linus and Elise. Martin Larsson Ystad, October 2008 White White White White xi Contents Publication list ...................................................... 13 Introduction ......................................................... 15 Part I Microphone Windscreens for Turbulent Noise Suppression when Apply- ingActiveNoiseControltoDucts..................................45 II Performance Evaluation of a Module Configured Active Silencer for Ro- bustActiveNoiseControlofLowFrequencyNoiseinDucts ........59 III A Feedforward