INTERNATIONAL SOUND AND VIBRATION DIGEST Volume 1, Number 3 Date: January 26, 1995 Editor-in-Chief: Malcolm J. Crocker, Auburn University, USA Assistant Editor: Yana Sokolova, Auburn University, USA Editorial Board: Duan-shi Chen, Jiao Tong University, Shanghai, CHINA Frank Fahy, ISVR, Southampton University, UK Jean L. Guyader, INSA de Lyon, FRANCE Colin H. Hansen, University of Adelaide, AUSTRALIA Hanno Heller, DLR, Braunschweig, GERMANY Nikolay Ivanov, Baltic State University, St. Petersburg, RUSSIA Finn Jacobsen, Technical University of Denmark, DENMARK G. Krishnappa, Institute for Machinery Research, NRC, CANADA Conny Larsson, Uppsala University, SWEDEN Martin V. Lowson, University of Bristol, UK Leonid M. Lyamshev, Andreev Acoustics Institute, Moscow, RUSSIA Eric Marsh, Penn State University, USA M.L. Munjal, Indian Institute of Science, Bangalore, INDIA David E. Newland, The University of Cambridge, UK Michael P. Norton, University of Western Australia, AUSTRALIA A. Selamet, The University of Michigan, Ann Arbor, USA Andrew F. Seybert, University of Kentucky, Lexington, USA Jan W. Verheij, TNO, Delft, THE NETHERLANDS. Current number of subscribers: 1198 To send a submission to the IS&V DIGEST or to subscribe or unsubscribe, send information by E-mail to yanas@eng.auburn.edu. TODAY'S DIGEST CONTENTS ITEM 1. INTRODUCTION. ITEM 2. OVERVIEW: Overview of the Boundary Element Method. ITEM 3. PROFESSIONAL SOCIETY: East-European Acoustical Association. ITEM 4. LABORATORY: Penn State Center for Acoustics and Vibration. ITEM 5. CONFERENCES: Acoustic Emission Working Group (AEWG) 38'th Regular Meeting and Primer, NASA Langley Research Center. ITEM 6. 15th International Congress on Acoustics in Norway. ITEM 7. Symposium on Turbomachinery Noise (ASME). ITEM 8. SHORT COURSES. ITEM 9. READER QUESTION: Russian Acoustics Library. ITEM 10. READER QUESTION: Analysis of Speech Signals. ITEM 11. BOOK REVISION: Second edition of "Sound Intensity" by Dr. Frank Fahy. ITEM 12. BOOK REVIEWS. ITEM 13. BOOK ANNOUNCEMENTS. ITEM 14. TECHNICAL PAPER: Overview of Active Noise Control Systems. ***************************************************************************** ITEM 1. INTRODUCTION. ***************************************************************************** We are pleased to bring you the third issue of the IS&V DIGEST. As before the DIGEST contains technical articles, information on professional societies, research programs, news about new books, conferences, readers questions, and other announcements. In this issue the OVERVIEW (Item 2) on the BOUNDARY ELEMENT METHOD (BEM) is contributed by Andy Seybert. This numerical technique has rapidly increased in importance in recent years. Along with the FINITE ELEMENT TECHNIQUE (FEM) the BEM has become one of the most powerful tools engineers and scientists have for the prediction of interior sound fields in cavities and exterior sound fields produced by structural radiation and shadowing effects caused by bodies. The professional society featured in this issue is the East-European Acoustical Association (Item 3) which is based in St. Petersburg, Russia. Our series on Acoustical Laboratories continues with the Center for Acoustics and Vibration at Penn State University (Item 4). Two subscriber's questions are included (Items 9 and 10). Such subscriber's questions are welcome in the DIGEST. Please reply directly to the authors. Several new books are reviewed in this issue, including ENVIRONMENTAL AND ARCHITECTURAL ACOUSTICS by Maekawa and Lord, and THEORY AND APPLICATION OF STATISTICAL ENERGY ANALYSIS (Second Edition) by Lyon and DeJong (Item 11). In addition a number of other announcements appear on conferences, short courses, new books and last, but not least, a continuation of a series of Technical Papers on ACTIVE NOISE CONTROL by Colin H. Hansen. (Item 14). The Editorial Board and I would be very interested to hear readers comments on the IS&V DIGEST and whether you find it useful. Please let us hear which items you find most interesting and any items you feel should be included or excluded. Items for publication should be submitted to yanas@eng.auburn.edu. Please check that line length and spacing of contributions is correct on your computer screen before transmission. Since an increasing number of conference, and short course contributions are being received, these should preferably be kept brief wherever possible. Malcolm J. Crocker Editor-in-Chief ***************************************************************************** ITEM 2. OVERVIEW OF THE BOUNDARY ELEMENT METHOD. ***************************************************************************** The boundary element method (BEM) is a numerical technique for calculating the sound radiated by a vibrating body or for predicting the sound field inside a cavity such as a vehicle interior. The BEM may also be used to determine the sound scattered by an object such as a microphone or for predicting the performance of silencers or mufflers. The Boundary Element Mesh Building the BEM mesh or model is the first step in solving a problem with the BEM. This is done by representing the surface geometry of the structure (or cavity ) with nodes and elements. Boundary elements are either triangular or quadrulateral in shape. Elements may be classified according to order as well as shape. Linear elements are elements in which the geometry and the acoustic variables (sound pressure and vibration on the surface of the body) are represented by a linear or first-order approximation. Quadratic elements provide a second-order or quadratic approximation (i.e., they have curvature). Compared to linear elements, fewer quadratic elements are needed to obtain a result of given accuracy. The size of the elements must be chosen to be small enough to obtain an acceptable solution, but not so small as to result in excessive computer time. In general, a BEM mesh must meet three requirements to obtain an acceptable solution. First, it must model the geometry of the body accurately. This means that all major surfaces, as well as edges and corners, must be accurately represented. (However, it is not necessary to model regions of the body which it can be concluded beforehand are not important acoustically, such as surface irregularities that are small compared to the acoustic wavelength.) Second, the mesh must be made fine enough to represent the distribution of vibration on the surface of the body. This can be done by using at least two quadratic (or four linear) elements per structural wavelength. Finally, the mesh must be made fine enough to represent the sound pressure distribution on the surface of the body. This can be done in most cases by selecting the element size to be no larger than one-half of the acoustic wavelength for the highest frequency of interest if quadratic elements are used, and one-quarter of the acoustic wavelength if linear elements are used. These last two requirements usually conflict, so one should select the largest element size that satisfies both requirements. Most commercially available pre- and post-processing programs developed for the finite element method (FEM) may be used as well for constructing BEM meshes. One uses shell or plate elements where the physical and material properties of the elements are immaterial, since boundary elements have no thickness or properties. These same programs may be used to visualize the results of the BEM using their standard contouring capabilities. Input to the Boundary Element Model The BEM mesh covers the entire surface of the radiating body (or cavity). One needs to know some information (the so-called boundary conditions) about the problem at every node point of the mesh. Thus, for example, for most radiation problems one knows the vibration velocity normal to the surface at every point. Both magnitude and phase of the velocity must be known. If a portion of the surface is covered by a sound absorbing material, one must know the acoustic impedance of the material at each of the nodes that lie thereon. Output of the Boundary Element Model The BEM calculates the sound pressure distribution on the surface of the body or cavity from the geometry and the distribution of vibration velocity or other boundary conditions provided by the user. Once the sound pressure and the vibration velocity are known on the surface, the sound intensity, sound power, and sound radiation efficiency may be found. Secondly, the sound pressure, particle velocity, and sound intensity can be calculated at so-called field points, i.e., points in the acoustic domain that are not on the surface of the body or cavity. These field points can even lie on a reflecting plane such as the ground. The BEM will determine the sound pressure level at all points in the acoustic domain, both in the near and the far fields. The BEM will also calculate the sound pressure level in the "shadow zone" behind a body due to radiation from the other sides. All of this is done without any additional approximations or restrictions than those discussed previously, i.e., the need for an adequate mesh and the requirement to know the boundary conditions on the surface of the body. THE BEM AS AN ENGINEERING ANALYSIS TOOL The BEM is a relatively new tool compared to other acoustical analysis techniques such as the FEM. Much progress has been made in enhancing and tailoring the BEM for acoustics. Like the FEM, however, it is computationally and memory intensive, even more so for certain applications. Despite relatively long solution times, the BEM is often the method of choice where ease of modeling is desired and where accuracy is important. Because the BEM is based on an exact analytical formulation (known as the Helmholtz integral equation), the user does not need to be concerned about when or where the method is applicable. Comparison between the BEM and the FEM The BEM and the more familiar FEM share a number of features such as common underlying assumptions, the use of element technology to effect a solution, and the use of pre- and post-processing to manage the information obtained from each method. In some sense, however, the FEM is the more general of the two methods. The FEM may be used for non-linear problems such as when the sound pressure is extremely high or when the acoustic domain is nonhomogeneous, as for example when there are temperature gradients. In its usual form, the BEM is restricted to linear, homogeneous problems. However, a modification of the BEM, referred to as the multi-domain BEM, overcomes this last limitation for many problems of practical interest. For most radiation problems, the BEM is preferred over the FEM. With the FEM, one would need to extend the surface mesh into the acoustic domain using three-dimensional volume elements. This results in a large number of elements, but more important, yields an error since no matter how far the mesh is extended, it is not possible to represent an infinite domain with a finite model using the FEM. This illustrates the fundamental difference between the BEM and FEM: with the BEM one discretizes only the surface of the radiating body while with the FEM the entire acoustic domain must be discretized. Because with the BEM all numerical approximations are confined to the surface, a coarser mesh can be used as compared to the FEM for the same accuracy. Most commercially available FEM programs are written for structural applications and, therefore, do not have many of the features that are important for acoustical modeling even if their element libraries contain acoustic elements. For example, most FEM programs do not calculate sound intensity or sound power, nor is it possible with most FEM programs to use an impedance boundary condition (for modeling absorbing surfaces). However, it is sometimes useful to perform an eigenvalue search of an acoustic domain. For this objective, the FEM is ideally suited. However, it is possible with the BEM to use a forced response calculation to determine the resonance frequencies of the acoustic domain. Apart from the broad generalizations cited above regarding solution time, accuracy, and applicability to specific problems, the advantages of the BEM over the FEM, or vice versa, must be examined for each problem. However, a primary advantage of the BEM over the FEM for many problems in acoustics, is its simplicity which allows the new user to become proficient in a much shorter time. Because the boundary element mesh is only a surface mesh, it is easy to construct and does not have as many potential variations as with the FEM. This allows the user to spend time where it is most needed - applying engineering knowhow to solve noise control problems. A. F. Seybert seybert@mach.ut.TU-Berlin.DE ******************************************************************************* ITEM 3. EAST-EUROPEAN ACOUSTICAL ASSOCIATION. ******************************************************************************* The East-European Acoustical Association (EEAA) was officially established at its first Congress in St. Petersburg, Russia during October 4-5, 1990. As a public and scientific non-governmental organization the Association sees its primary goals as promoting creative initiatives among acoustical scientists and engineers, facilitating the development of their professional skills, ensuring their social rights and spreading national achievements in acoustical ecology into the world-wide community. The Congress elected Prof., Dr. Tech. Sci. Viatcheslav T. Liapunov from the Krylov Shipbuilding Research Institute (KSRI) in St. Petersburg as the first President of the Association, and Sr. Tech. Sci. Alexei V. Ionov from KSRI was appointed to manage the section in charge of the international activities of the Association. After the death of Prof. V. Liapunov the Second Congress elected Prof., Dr. Tech. Sci. Alexei S. Nikiforov from KSRI as the President of the Association . At present there are about 530 specialists who are members of the Association. The Association includes: 1. The St. Petersburg division (Russia). Chairman: the President of EEAA, Alexei S. Nikiforov. 2. The Moscow division (Russia). Chairman: the vice-president of EEAA, Georgiy L. Osipov. 3. The Kiev division (Ukraine). Chairman: the president of EEAA, Victor T. Grinchenko. 4. The Southern division (Nikolaev, Ukraine). Chairman: the vice-president of EEAA, Georgiy P. Nerubenko. 5. The Kazakhstan division (Alma-Ata, Kazakhstan). Chairman: the vice-president of EEAA Serik S. Omarov. The Association has the following scientific-technical sections: 1. At the central management (St. Petersburg): noise and vibration control; sound-techniques and electro-acoustics; ultrasonic techniques; medical acoustics; mathematical problems of acoustics; hydroacoustics; acoustical measurements and apparatus; noise and vibration excited by flow; environmental problems of acoustics; piezo-active materials; hearing and speech for technical applications; acoustic signal processing. 2. At the Moscow division (Moscow): architectural-building acoustics; noise and vibration of above ground and underground transportation vehicles; noise of aircraft; industrial noise. 3. At the Kiev division (Kiev): acoustics of shell structures. 4. At the Southern division (Nikolaev): ferro-hydrodynamics in technical acoustics; technical vibroacoustics in industry. 5. At the Kazakhstan division (Alma-Ata): acoustical materials, industrial noise; mathematical problems of acoustics. The Association began publishing a scientific-technical journal "Technical Acoustics" in Russian in 1992. The publication of the English version of the "Technical Acoustics" journal began in 1994. The editorial board consists of A. Ionov (Editor-in-chief), E. Myshinsky (Associate Editor), S. Kovinskaya (Assistant Editor), M.J. Crocker, M. Heckl, T. Kihlman, A. Nikiforov, G. Pavic. The Association is a member of the International Institute of Noise Control Engineering (IINCE) and of the International Association Against Noise (AICB). The Association has concluded cooperative agreements with the following: the Federation of Acoustical Societies of Europe (FASE), the Acoustical Committee of the Polish Academy of Science, the Acoustical Society of Poland, the Acoustical Commission of the Hungarian Academy of Science and the Lithuanian Acoustical Society. Prof. Alexei S. Nikiforov East-European Acoustical Association St. Petersburg, Russia Tel: (812) 127-93-48 FAX: (812) 127-93-23 ***************************************************************************** ITEM 4. PENN STATE CENTER FOR ACOUSTICS AND VIBRATION. ***************************************************************************** INTRODUCTION Research in acoustics and vibration is one of Penn State's main strengths. The steady growth of research in acoustics and vibration in recent years has established the Penn State program as the largest of its kind at a major research university in the USA. The Center for Acoustics and Vibration, housed in the College of Engineering, ensures the continued commitment of Penn State to acoustics and vibration research. The creation of the Center for Acoustics and Vibration (CAV) parallels modern research trends towards broadly based, cross-disciplinary programs. This is an age of large-scale, large-budget research programs that demand teams of highly skilled scientists and engineers and expensive laboratory facilities. CAV brings together unique research teams in a way that fosters collaborative efforts to produce research initiatives. Communication, a fundamental ingredient of successful research, is often neglected in modern research efforts. CAV, through the technical affinity group concept, opens lines of communication to encourage a free flow of ideas among colleagues. This is a challenge that all large research organizations in universities, government or the corporate world must meet. MEMBERSHIP More than fifty Penn State faculty members and over one hundred graduate students are directly involved in acoustics and vibration research. The combined research funding of the faculty exceeds $7M/ year. This funding sustains over thirty laboratories. CAV RESEARCH AFFINITY GROUPS CAV has implemented the affinity group concept. As research projects have become more and more cross-disciplinary, the traditional definitions dividing engineering and applied science departments have become less important. In parallel with research trends common in other large research organizations, CAV affinity groups realize a flat organizational structure, offering participants from different departments optimum opportunity for contact and communication. The CAV affinity group concept encourages researchers with diverse backgrounds to work together in areas of common technical interest. Made up of researchers and their students, the groups focus on emerging and traditional areas of acoustics and vibration research. RESEARCH AREAS Research areas include: 1) emerging research areas of adaptive structures, active control, machinery prognostics and condition monitoring, and rotorcraft acoustics and dynamics 2) traditional areas of flow-induced noise, propagation and radiation, structural vibration and acoustics, and materials evaluation. As research is a dynamic process, other affinity groups will coalesce when heightened interest in an emerging technology brings together a critical mass of CAV researchers. SEMINARS Research affinity groups organize regular formal seminars and workshops to explain current research and discoveries. A good portion of the communication between members also occurs during informal meetings. ALLIED PENN STATE PROGRAMS The CAV actively cooperates with Penn State's other acoustics and vibration organizations. Penn State's Applied Research Laboratory, an established, national research facility funded by the US Navy, supports CAV faculty and students, and many of its researchers work within CAV affinity groups. GRADUATE PROGRAM Penn State recognized the importance of acoustics as a cross-disciplinary activity over twenty-five years ago with the establishment of a separate graduate degree program in acoustics, and research in this area is carried out in many different departments in the university. Penn State's Graduate Program in Acoustics attracts graduate students whose career objectives mesh well with the CAV's research activities. The Graduate Program in Acoustics has a strong partnership with the CAV and many of its faculty act as CAV group leaders and members. Eric Marsh Assistant Professor Penn State University 322 Reber Building University Park, PA 16802 (814) 865-5242 phone (814) 863-8682 fax E-mail: erm7@psu.edu ***************************************************************************** ITEM 5. ACOUSTIC EMISSION WORKING GROUP (AEWG) 38'th REGULAR MEETING AND PRIMER, NASA LANGLEY RESEARCH CENTER. ***************************************************************************** ACOUSTIC EMISSION WORKING GROUP (AEWG) 38'th REGULAR MEETING AND PRIMER NASA LANGLEY RESEARCH CENTER HAMPTON, VIRGINIA, MAY 1-4, 1995 ANNOUNCEMENT & CALL FOR PAPERS Topics of interest include: Materials: metals, polymers, composites, concrete, wood, rock, etc. Structures: pressure vessels, pipelines, bridges, air- and space-craft, etc. Manufacturing: welding, machining, casting, composite processing, etc. Diagnostics: leak detection, maintenance, wear, machine condition, etc. Instrumentation: sensors, signal and parameter measurement systems, calibration, analysis algorithms, etc. Standards/Codes ASTM, ASME, CARP, etc. Commercial Presentations Meeting Schedule: Monday: Primer - Introduction to Acoustic Emission and Acousto- Ultrasonics (8:00 - 4:30) - See the following information Tuesday: Technical Sessions of Regular Meeting (8:00 - 4:30) Social (4:30 - 6:00) Wednesday: Technical Sessions of Regular Meeting (8:00 - 4:30) Banquet (6:30) Thursday* Tour of NASA Langley Research Center NDE Research Facilities (8:30 - 11:00) *Another workshop, entitled "Recent Advances in Smart Materials for Aerospace Applications", will be held May 4-5, 1995 at the NASA Langley Research Center. For information, contact Robin Elder (804-864-7159 or by E-mail at r.c.elder@larc.nasa.gov) General Information: Please submit a provisional title and brief abstract of proposed presentations by March 1, 1995. Please mail or fax using the information to: Dr. William H. Prosser FAX (804) 864-4914 Program Chair, 38'th AEWG Meeting NASA Langley Research Center MS 231 Hampton, VA 23681-0001 Notification of acceptance of papers will be mailed by March 14, 1995. Preliminary programs will be mailed by April 1, 1995. The fee for the regular meeting is $45. This includes a printed copy of the abstracts, refreshments each day of the meeting, the Social on Tuesday, the Banquet dinner on Wednesday, and the tour. Rooms have been reserved at the Radisson Hotel Hampton (700 Settlers Landing Road, Hampton, VA 23669, 1(800) 333-3333; mention AEWG for the special rate). Rates are $55.00 and the block of rooms will be held until April 17, 1995. Bus transportation will be provided each day between the hotel and the conference site. The area is served by airports in Newport News and Norfolk. All sessions will be held at the NASA Langley Research Center. Note: Due to security requirements, all foreign nationals must gain advance permission to visit NASA Langley. Contact your Embassy as soon as possible for instructions. For additional information, contact Dr. William H. Prosser (804-864-4960 or by E-mail at w.h.prosser@larc.nasa.gov) Primer Information "Introduction to Acoustic Emission and Acousto-Ultrasonics", a one day course on acoustic emission and acousto-ultrasonics will be held to provide an introduction to the basics of AE and A/U. This is an opportunity to familiarize yourselves with the most up-to-date information in the technology by attending a series of classes conducted by experts in the field. The fee of $30 for the Primer will include six hours of instruction, refreshments, and course notes. A tentative list of speakers and topics to be covered are: Introduction to AE Dr. Adrian Pollock - Physical Acoustics Corporation Wave Propagation Dr. Michael Gorman - Digital Wave Corporation and Signal Analysis Metals and Alloys Dr. Steve Carpenter - University of Denver Composite Laminates Dr. Marvin Hamstad - University of Denver and Pressure Vessels Introduction to A/U Mr. Alex Vary - NASA-Lewis Research Center Paid attendees of the Primer may attend the technical sessions of the regular AEWG meeting at a reduced rate of $30. Registration Form 38'th Acoustic Emission Working Group Meeting Mail or FAX to: Dr. William H. Prosser FAX (804) 864-4914 Program Chair, 38'th AEWG Meeting Phone (804) 864-4960 NASA Langley Research Center MS 231 Hampton, VA 23681-0001 Abstract Submission: If you wish to make a presentation during the technical sessions, please provide a title and a brief (200 words or less) abstract. Please E-mail (w.h.prosser@larc.nasa.gov) an ASCII text version of the abstract or include a 3.5" diskette (Mac or IBM) with an ASCII text version for inclusion in the final program. Depending on the number of presentations scheduled, time slots will be approximately 20 minutes. ***************************************************************************** ITEM 6. 15TH INTERNATIONAL CONGRESS ON ACOUSTICS IN NORWAY. ***************************************************************************** The 15th International Congress on Acoustics will take place in Trondheim, Norway 26-30 June 1995. The program will consist of structured sessions with specially invited papers, plenary papers, contributed papers, poster and demo sessions, and round table discussions. A total of more than 850 papers have been accepted by the program committee. Authors are reminded that the deadline for manuscripts is 15 Feb 95. Deadline for reduced registration fee is 1 Feb 95. Contact ica95@tele.unit.no to get an e-mail version of the updated program and instructions about registration and hotel reservation. The ICA'95 secretariat can also be contacted via fax: +47 73 59 51 50 _______________ Truls Gjestland SINTEF DELAB truls.gjestland@delab.sintef.no tel: +47 73 59 26 45 fax: +47 73 59 43 02 ****************************************************************************** ITEM 7. SYMPOSIUM ON TURBOMACHINERY NOISE. ****************************************************************************** CALL FOR PAPERS AMERICAN SOCIETY OF MECHANICAL ENGINEERS (ASME) '95 International Mechanical Engineering Congress and Exposition (IMECE) SAN FRANCISCO HILTON / NOVEMBER 12-17, 1995 SYMPOSIUM ON TURBOMACHINERY NOISE The ASME is organizing a Symposium on Turbomachinery Noise for the 1995 IMECE to be held in San Francisco on November 12-17. The symposium is jointly sponsored by the Flow-Induced Vibration and Noise Committee and the Turbomachinery Noise and Vibration Committee of the Noise Control and Acoustics Division. Analytical and experimental papers are sought on all aspects of turbomachinery noise and vibrations. Studies of any type or size of compressor, fan, pump, and turbine noise will be considered. Topics of interest include, but are not limited to: aeroacoustic problems, flow/structure interaction, and structural response including blade, vane, or case vibration. Papers on other topics related to turbomachinery noise, including analysis techniques, flow studies, or new measurement methods will also be considered. Papers can focus on current problems, from noise measurements defining a problem to research studies focused on a particular element of a problem. Papers presenting unique experimental and/or analytical tools for machinery noise problems or on proposed solutions to specific problems are also welcome. Industrial and consumer product applications are of special interest. SELECTION OF PAPERS Authors should submit three copies of an abstract to one of the organizers. Acceptance is based on a formal peer review of the full paper. All papers must conform to ASME journal standards and will be published in a bound volume. SCHEDULE January 31, 1995 Abstracts due to organizers February 15, 1995 Preliminary acceptance notification March 15, 1995 Manuscripts due June 1, 1995 Final acceptance notification July 17, 1995 Camera ready papers due ORGANIZERS Richard C. Marboe: Applied Research Laboratory / Penn State University / PO Box 30 / State College, PA 16804 Phone: 814-863-3019 FAX: 814-865-3287 e-mail: rcm@arlvax.psu.edu William B. Swim: Tennessee Technological University / Dept. of Mech. Eng. / Cookeville, TN 38505 Phone: 615-372-3273 FAX: 615-372-6172 e-mail: wbs7801@tntech.edu Niranjan Humbad: Ford Motor Company / Climate Control Division ITC / Dearborn, MI 48120 Phone: 313-323-8243 FAX: 313-390-4278 ***************************************************************************** ITEM 8. SHORT COURSES. ***************************************************************************** 1) The Institute of Sound and Vibration Research, Southampton University, UK, will run a three day short course on 'Engineering Applications of Statistical Energy Analysis' during 3-5 April, 1995. Lectures will be presented by academics and practicing engineers who have extensive experience in using SEA. Emphasis will be placed on practical applications of SEA in the following areas: Building Acoustics, Noise in Aircraft, Noise in Road Vehicles, Engine and Machinery Noise and Vibration, and Vibration of Space Launch Vehicles and Payloads. Experimental techniques will discussed in detail. Commercial and academic software will be demonstrated. A workshop will be held to enable participants to discuss specific problems with the course lecturers. The first morning will be devoted to introductory lectures for noise and vibration engineers not familiar with SEA. The cost of the course, including notes, coffee, tea, lunches and course dinner, but excluding accommodation, will be about 700 pounds sterling. For further details, please send your name, address and tel/fax/nos, to Ann Barrett, Conference Secretary, ISVR, University of Southampton, Southampton, SO9 5NH, UK. Tel. +44 703 592310: Fax +44 703 593033 .... or e-mail me. Frank Fahy, Professor of Eng. Acoustics fjf@soton.ac.uk ****************************************************************************** 2) Sensimetrics Corporation will offer its second Course In Speech Synthesis in Mariefred, Sweden, August 7-11, 1995. The lectures during morning sessions will be presented by Dr. Kenneth N. Stevens; in the afternoons, he and other Sensimetrics staff members will assist students during laboratory sessions. The course is just prior to the International Congress of Phonetic Sciences in Stockholm. The location, Gripsholms Vaerdshus in Mariefred, about 60 km from Stockholm, is a comfortable hotel with excellent food and a beautiful waterfront setting. The cost of the course is US$2500 for attendees with an industrial or commercial affiliation, and US$1500 for university faculty or research staff. Hotel accommodations are additional. Students will be provided with computers, software and all other materials needed to synthesize and analyze speech for the course. The course is limited to 30 students. Those interested in attending should contact Jennifer Bradley for further details at jennifer@sens.com. Robert Berkovitz, President Sensimetrics Corporation bob@sens.com ****************************************************************************** ITEM 9. RUSSIAN ACOUSTICS LIBRARY. ****************************************************************************** Perhaps Digest readers will be interested to help me. I have what amounts to amongst the best Russian language acoustics libraries that covers the period up to the end of the 1970s. There are many rare books, some published as early as the 1930s. Also there are some Eastern European language books. I doubt that even some specialized libraries in Russia itself (like Akusticheskii Institute of the Akademy of Sciences) have a similar collection. Unfortunately, I cannot keep this library any longer at home and I would like to (in order of preference): 1) Donate it to a Canadian library, university department or government research institution that might be interested to maintain it (there are a lot of definitive materials in general theory, hydroacoustics, antennas and ultrasound, electroacoustics, building acoustics, physics, physiology, etc. some signed by authors) or 2) Sell it as a whole to any other collector inside or outside Canada. Note that I will not accept any proposals to sell individual books. I do not have an English language catalog of them, nor do I have the time to make one. Your help with finding a new home for them will be much appreciated. Sincerely Nahum Goldmann, ARRAY Development Inc., Ottawa, Canada Phone (613)733-0399, FAX (613)733-5691 e-mail ____ NOTE: READERS SHOULD REPLY DIRECTLY TO MR. GOLDMANN. (THANK YOU, THE EDITORS) **************************************************************************** ITEM 10. ANALYSIS OF SPEECH SIGNALS. **************************************************************************** Dear Readers: For the analysis of speech signals I need a reliable, robust formant tracker. It is certain that many formant trackers have already been build by others. So, before I start developing and programming my own formant tracker, I want to ask a couple of questions: 1. What is the best formant tracker? Probably nobody can give a definite answer to this question, and therefore the question should be reformulated: Who has experience with formant tracking, and can recommend a certain algorithm, or list the pros and cons of an algorithm? 2. Does anybody have an algorithm or a program (preferably with sources) for a good formant tracker? If there is enough interest, I will prepare a summary. Those who do NOT want their names and comments in this summary, should indicate this. Greetings, Dr. W.A.J. Strik Dept. of Language and Speech E-mail : STRIK@LET.KUN.NL P.O. Box 9103 Tel.nr.: 31-80-612908 NL-6500 HD Nijmegen Fax nr.: 31-80-615939 The Netherlands ---- NOTE: READERS SHOULD REPLY DIRECTLY TO DR. STRIK. (THANK YOU, THE EDITORS) **************************************************************************** ITEM 11. SECOND EDITION OF "SOUND INTENSITY" BY DR. FRANK FAHY. **************************************************************************** Further to Finn Jacobsen's notes on Sound Intensity, I am pleased to inform readers that a second edition of my book 'Sound Intensity' will be available in paperback in late spring 1995: cost - about 30 pounds sterling. New material includes: transient and instantaneous intensity; greater focus on the relationship between energy transport and mean intensity; in-depth discussion of the sources of error associated with spatial and spectral sampling techniques; discussion of the significance of new sound field indicators; summary of principal elements of the International Standards for instrumentation and sound power determination; and new examples of the application of sound intensity measurement to sound field characterisation and source location. Frank Fahy fjf@ soton.ac.uk ************************************************************************** ITEM 12. BOOK REVIEWS. ************************************************************************** ENVIRONMENTAL AND ARCHITECTURAL ACOUSTICS. Z. Maekawa, Kobe University P. Lord, University of Salford E & FN Spon, an imprint of Chapman & Hall, 2-6 Boundary Row, London SE1 8HN, UK, 1994. 5 by 8.5 inches, 377 pages. Overview of the Text Environmental and Architectural Acoustics, published in 1994, is based on Prof. Maekawa's 1968 book, Architectural Acoustics (printed in Japanese). For this edition, Maekawa has written an essentially brand new text with classic and modern results in the field of acoustics. Prof. Lord worked with Maekawa to translate the text and assist with its presentation into English. The authors have succeeded in developing an easily read, concise guide to introductory acoustics, with emphasis on acoustical design for human comfort and enjoyment. The book's ten chapters start with the absolute basics - a discussion of the nature of sound waves and the phase relationship between velocity and pressure. In addition to the standard treatment of wave phenomena, a discussion of speech and the human perception of sound is made. The balance of the text begins with the definition of various quantities such as sound pressure, intensity, reverberation time, and structural vibration. Two chapters on experimental measurement techniques include discussion of common level ratings and their effect on people. A description of sound absorbing materials, acoustic isolation, and structural isolation techniques follows. The book closes with a discussion of indoor and outdoor acoustical design, including the classic tradeoffs of room design (e.g., the effect of reverberation time on sonority vs. speech intelligibility). Comments on the Text The presentation is straightforward and lucid. Lengthy mathematical derivations are avoided, but the authors point the interested reader to the appropriate papers for further reading. The book can be picked up and read as an acoustical design guidebook. While the first half of the book will be familiar to the student of physics or engineering, the second half presents many important results in environmental and architectural acoustics. In preparing the text, the authors have summarized a comprehensive selection of work. While the authors only include key results, many of the book's sections present the work of multiple researchers, even when contradictions exist (the discussion outlines any discrepancies). The authors use illustrations liberally throughout the text; the reader will especially appreciate the schematics of several concert halls built in the last 150 years. These drawings, placed at the end of the book, allow readers to test their understanding of the material. In short, the book is an excellent introduction into the practical application of acoustics. Although Environmental and Architectural Acoustics lacks the number of example and homework problems typically found in an engineering textbook, it could be used in an introductory course. Table of Contents by Chapter 1. Fundamentals of Sound Waves and Hearing 2. Noise and Vibration - Measurement and Rating 3. Room Acoustics 4. Sound Absorption - Materials and Construction 5. Airborne Sound Insulation 6. Isolation of Structure-Borne Noise and Vibration 7. Noise and Vibration Control in the Environment 8. Acoustic Design of Rooms 9. Electro-Acoustic Systems 10. Addenda ---------- Eric Marsh Assistant Professor Penn State University 322 Reber Building University Park, PA 16802 (814) 865-5242 phone (814) 863-8682 fax E-mail: erm7@psu.edu **************************************************************************** NOISE, ITS MEASUREMENT, ANALYSIS, RATING AND CONTROL by J. S. Anderson and M. Ratos-Anderson, Avebury Technical, Ashgate Publishing Ltd., Aldershot, England, 494 pp. + xii, 1993, $84.95 This book contains eight chapters, four appendices, a glossary and an index. Each chapter is provided with a good number of figures and many references. Many of the references are useful and current; some seem quite dated. The authors state that the book is intended as a text for students at universities and colleges and those involved in private study. It is unfortunate that for a book primarily intended for students, that questions and answers are not provided. That will tend to reduce its usefulness as a textbook. The book contains a wealth of useful theoretical and practical information ranging from principles of sound propagation, measurement and analysis of sound, noise scales, indices and rating procedures, through sound insulation and sound absorption, room acoustics, silencers, the ear and hearing loss, to noise sources, identification and control. Although it was originally intended mainly for a British audience, it is obviously useful to a wide spectrum of readers. Malcolm J. Crocker ******************************************************************************** THEORY AND APPLICATION OF STATISTICAL ENERGY ANALYSIS Second Edition by Richard H. Lyon and Richard G. DeJong, Butterworth-Heineman, Boston, Massachusetts, USA. 277 pp.+ix, 1995, ISBN 0-7506-9111-5, $89.95 This book is a revision of the earlier 1975 book with a similar title by the first author. The first edition was published by MIT Press. This book is divided into two main parts: I. Basic Theory (containing Chapters 1-4), and II. Engineering Applications (containing Chapters 5-15). The first part of the book (for which the first author had primary responsibility for revision) remains essentially unchanged, with the exception of some minor details such as the addition of figures to chapter one, minor corrections to some equations, and small changes in notation, etc. The text and equations are virtually identical in Chapters 1-4 of the first and second editions which may not be so surprising considering that the basic theory has not changed over the last 20 years. The second part of the book (for which the second author had primary responsibility for revision) has undergone extensive change, however. Although some of the titles of chapters 5-15 in Part 2 remain the same, they are reordered, and almost all of the text and equations are different. In addition, Part 2 contains many new figures and the presentation of new and useful experimental results from the literature. Surprisingly, the number of references given (66) in the second edition of this book is smaller than in the first edition and several SEA applications and commercial softwares now available are not mentioned at all. This is a pity. However, on the whole the authors are to be congratulated. This book remains the only extended attempt to review SEA theory and applications in existence. The appearance of this revised and updated second edition will be welcome news to many of our readers. Malcolm J. Crocker ******************************************************************************* ITEM 13. BOOK ANNOUNCEMENTS. ******************************************************************************* The following seven English language books, proceedings, and journals may be ordered through INTERNATIONAL SCIENTIFIC PUBLICATIONS, P.O. Box 13, Auburn, AL 36831, USA, Telephone: 334-826-3444; FAX: 334-826-7149. Prices for the books below are postpaid in the USA and Canada. Prices of books and journals airmailed to all other countries are also shown. Checks should be made (in US dollars) to International Scientific Publications and drawn on a US bank. They should be mailed to the address above. 1) NOISE AND VIBRATION CONTROL IN VEHICLES, Edited by N. I. Ivanov and M. J. Crocker, INTERPUBLISH, St. Petersburg, Russia, 1994, 352 pp, ISBN 5-7325-0090-1. $70 USA or $80 airmail overseas. This 352 page book is written in English by 11 authors and covers the main principles of noise and vibration control in different vehicles including: automobiles, trucks, farm tractors, road making machinery and construction machinery such as movable compressors. The book discusses noise sources, noise reduction methods and noise standards. Both theoretical approaches and experimental results are included in the book's 13 chapters. 2) NOISE CONTROL IN RUSSIA, Edited by O. V. Rudenko and S. A. Rybak, NPK INFORMATICA, Tiblisi, Republic of Georgia, 1992, 263 pp., $60 USA or $70 airmail overseas. This book is believed to be the first scientific book printed privately in the former Soviet Union. It consists of an Introduction and nine survey chapters written in English by eleven Russian scientists. The topics covered range from the damping of structure-borne sound, sound insulation of structures, active noise and vibration control, vibro-dosimetry, noise and vibration control of fans, the control of intense noise, to room acoustics, environmental noise, and noise standards in Russia. 3) THIRD INTERNATIONAL CONGRESS ON AIR- AND STRUCTURE-BORNE SOUND AND VIBRATION. Montreal, Canada, June 13-15, 1994. Edited by M. J. Crocker. 2100 pp. + xiv, $135 USA or $175 airmail overseas. This three-volume book of proceedings includes the written versions of the 264 papers in English by authors from 35 countries presented at the Third International Congress in Montreal, Canada in June 1994. Topics included in the papers include: Structural Radiation and Vibration, Statistical Energy Analysis, Sound and Vibration Measurements including Sound Intensity and Structure-borne Power Flow Measurements, Sound Transmission through Structures, Passive Damping, Effects of Noise and Vibration on People, Noise and vibration Control, Automotive Sound and Vibration, Wavelet Analysis, Boundary Element Analysis, Finite Element Analysis, Aeroacoustics, Active Noise and Vibration Control, Scattering, Sound propagation in the Atmosphere, Underwater Acoustics, Machinery Diagnostics, Material Properties, and Non-destructive Evaluation. 4) TRANSPORT NOISE 94. Proceedings of the Second International Symposium, St. Petersburg, Russia, October 4-6, 1994. Edited by Svetlana Kovinskaya. 450 pp., $50 USA or $60 airmail overseas. This 450 page book contains 109 papers in English by authors from 16 countries. The papers are grouped in nine chapters including one or more on Road Transportation Noise and vibration, Railway Noise and Vibration, Aircraft Noise and Vibration, Ship Noise and Vibration, Traffic Noise, Underground Railway Noise and Vibration, Noise and Vibration Diagnostics, and Vibration and Sound Radiation of Transportation Structures. 5) NOISE-93 International Noise and Vibration Control Conference Proceedings. St. Petersburg, Russia, May 31- June 3, 1993. Edited by N.I. Ivanov and M.J. Crocker. 2000 pp. $145 airmail anywhere in the world. The eight volume book of proceedings of this conference contains 280 papers in English by authors from 40 countries. The topics included cover the following topics: New Methods in Acoustics; Statistical Energy Analysis; Finite and Boundary Element Methods; Acoustical Measurements; Intensimetry; Room Acoustics; Active Noise and Vibration Control; Acoustical Design of Mufflers; Sound Isolation and Sound Absorption; Vibration,Radiation; Propagation and Damping; Traffic Noise; Noise control in Machinery, Industrial Noise; Aeroacoustics; Noise of Aircraft, Helicopters and Rockets; Sound Radiation and Sound Propagation; Environmental Noise; Structure-borne Noise; Effects of Noise and Vibration on People; New Acoustical Materials; and Hydroacoustics. 6) NOISE ABSTRACTS AND REVIEWS. an International Journal. St. Petersburg, Russia. Editor-in-Chief N.I.Ivanov. Published in English six times each year. Annual subscription for all countries $90 for individuals and $120 for institutions and libraries (airmail included). This new journal began publication in 1994. The journal is a cooperative effort of editorial offices in Russia, the USA and Germany. Each issue contains a feature article and surveys major developments in noise and vibration control throughout the world. Each year abstracts of at least fifteen hundred to two thousand papers, reports, patents and books are included; each issue includes announcements of international meetings, recent developments, engineering studies, and a calendar of upcoming events. Each issue contains information on transportation noise, aircraft noise, industrial noise, noise in buildings, environmental noise, noise of machinery such as engines, gears and electric motors, noise standards, legislation and regulations, as well as information on acoustical materials, vibration isolators and transmission of sound through walls and structures. 7) JOURNAL OF TECHNICAL ACOUSTICS. An International Refereed Journal. St. Petersburg, Russia. Editor-in-Chief A. Ionov. Published quarterly in English by the East European Acoustical Association. Annual subscription for all countries $110 (airmail included). The English version of this new refereed technical journal began publication in 1994. It has an international editorial board and contains articles on a variety of engineering and applied topics. Both theoretical and experimental papers are being published, but even the theoretical papers are normally strongly motivated by practical problems. Each issue contains papers on topics such as: flexural wave propagation, sound insulation of structures, damping materials, vibration isolation, industrial noise and vibration, tire noise, aircraft noise, automobile noise, sound absorption and building acoustics. ***************************************************************************** ITEM 14. OVERVIEW OF ACTIVE NOISE CONTROL SYSTEMS. ***************************************************************************** OVERVIEW OF ACTIVE NOISE CONTROL SYSTEMS Colin H. Hansen Department of Mechanical Engineering University of Adelaide South Australia 5005 E-mail: CHANSEN@edison.aelmg.adelaide.edu.au One of the simplest applications of active noise control is the control of plane waves propagating in air ducts. It seems appropriate, therefore, to use the duct application to explain some of the various different approaches to active control which have been used in the past. Active control systems may generally be divided into two categories: feedforward and feedback. Each type acts to suppress the noise generated by some source, referred to here as the primary source. Feedforward controllers (which are invariably digital in nature) rely on the availability of a reference signal which is a measure of the incoming disturbance (noise or vibration). This signal must be received by the controller in sufficient time for the required control signal to be generated and output to the control source when the disturbance (from which the reference signal was generated) arrives. A reference signal is often available when the disturbance is periodic, where a measure of the disturbance at a particular time can be used to predict the disturbance at some later time. A predictive measure can also be obtained for a non-periodic disturbance if it is travelling in a confined space such as a waveguide. In this case, an upstream measurement can be used to predict the disturbance at some downstream location at a later time. Systems such as these, for which the active control system produces the control signal at the downstream location at the same time that the primary signal arrives, are referred to as "causal". Causality is a condition which all feedforward controller designs must satisfy if the noise or vibration to be controlled is not periodic. This means that for random noise, the time it takes for the primary acoustic or vibration signal to travel from the reference sensor location to the error sensor location must be greater than the processing time of the controller plus the time delay associated with the control source electro-acoustics plus the time taken for the signal to travel from the control sources to the error sensors. Periodic signals which are slowly varying need not satisfy this condition as it may be assumed that the characteristics of one period are sufficiently similar to those of the period preceding it. Causality also affects feedback controllers to a certain extent, depending on the type of control law used. For example, if velocity feedback is used, the system damping is effectively increased, and no control is possible for the first cycle of a transient disturbance. An important point to note is that if the reference signal is supplied by a microphone in the duct, the signal will be subject to contamination by the upstream travelling control disturbance, an effect which must be taken into account in system design. Tachometer based reference signals are only suitable for systems which specifically target active attenuation of discrete tones, such as would be generated by a fan in a duct (in which case the tonal frequencies would be equal to the blade passage frequency and its harmonics). In this arrangement, the tachometer output is synchronised with the rotating shaft of the fan responsible for the periodic primary noise The signal conditioning electronics convert the tachometer signal into a combination of sinusoids which provide a predictive measure of the fundamental rotational frequency of the fan and the harmonics which are to be controlled. To generate the appropriate signal to drive the control source, the reference signal is passed through a digital filter to generate the resulting control signal which is fed to a control source which introduces the control disturbance into the duct. Non-adaptive controllers are characterised by fixed digital filters, using acoustic system analysis or by using trial and error to adjust the filter characteristics to minimise the signal at the "error" microphone. Unfortunately, the physical acoustic or vibration system to be controlled rarely remains the same for very long (as even small changes in temperature or flow speed change the speed of sound significantly, resulting in large phase errors between the desired and actual control signals. At first, attempts were made to overcome this problem by updating the filter weights iteratively, based on a measurement of the r.m.s. signal at the error microphone. Current practice involves the use of an adaptive algorithm to adjust the characteristics of the adaptive filter to minimise the downstream residual disturbance, the measure of which is the instantaneous value of the squared signal detected by an "error" microphone. In this case the filter weights are updated in a time frame of the order of the digital sampling rate, and much better results are obtained. Various other aspects need to be taken into account in the filter weight update algorithm, such as the electroacoustic transfer functions of the loudspeaker and error microphone. If broadband random noise is to be controlled, a reference signal correlated with all components of the primary signal must be obtained. For the duct noise control case being considered here, this could be done by substituting a microphone in the duct for the tachometer. Unfortunately, this arrangement often leads to the reference signal being contaminated with the component of the signal from the control source which is transmitted upstream, possibly leading to system instability, which is why this arrangement is not preferred if only periodic or tonal noise is to be controlled. Feedback control systems differ from feedforward systems in the manner in which the control signal is derived. Whereas feedforward systems rely on some predictive measure of the incoming disturbance to generate an appropriate "cancelling" disturbance, feedback systems aim to attenuate the residual effects of the disturbance after it has passed. Feedback systems are thus better at reducing the transient response of systems, while feedforward systems are best at reducing the steady state response. In structures and acoustic spaces, feedback controllers effectively add modal damping and in duct systems, the feedback controller also reflects incoming waves by modifying the duct wall impedance at the control loudspeaker. Thus, unlike feedforward systems for which the physical system and controller can be optimised separately, feedback systems must be designed by considering the physical system and controller as a coupled system. A feedback controller derives a control signal by filtering an error signal, not by filtering a reference signal as is done by a feedforward controller. In active noise and vibration control systems, the characteristics of the feedback control system are chosen so as to return the system (as measured at an error sensor) to its unperturbed state as quickly as possible, subject to some system stability constraints. It is of interest to explore the physical mechanisms which are involved in active noise and vibration control. In systems characterised by propagating waves, such as a duct system, feedback systems function primarily by reflecting incoming waves (which are subsequently dissipated by internal losses in the vibro-acoustic system) as they attempt to force a pressure or vibration null at the control source location, resulting in an impedance mismatch. In structures or acoustic spaces which contain standing waves that can be described modally, feedback controllers result in a change in system resonance frequencies and damping. Feedforward control systems for periodic noise, which are directed at achieving global noise reductions in which the total sound power radiated or total system potential energy is reduced, function by affecting the source radiation impedance and in some cases the control sources can absorb sound or vibratory power. For example, loudspeakers can absorb sound power if their cone motion is appropriate. Maintaining the appropriate cone motion, however, takes considerably more power than is available for absorption. Thus, all that will be noticed is a minuscule reduction in the power required to drive the loudspeaker. However, it can be shown that this absorption phenomenon only occurs for a sub-optimally adjusted controller. For an optimally adjusted controller, no absorption occurs and the attenuation of the disturbance is achieved entirely by modification of the radiation impedance presented to the source of the primary disturbance. When the noise or vibration is completely random the control mechanisms associated with feedforward control are a little more restricted. As the disturbance is random, causality constraints prevent the control source from optimally affecting the primary source radiation impedance and for sound propagating along a duct (or vibration propagating along a structure), the principal mechanism is one of reflection and subsequent dissipation of the energy by internal system losses, although absorption of energy by the control sources can often play an important role. For free field, random sound sources, there are no known physical mechanisms that would allow global control (either feedback or feedforward) using acoustic sources, although it is possible to achieve local zones of cancellation which are generally at the expense of increased levels elsewhere. Also, global control is sometimes possible by controlling the vibration of the structure generating the noise provided that a sample of the disturbance driving the structure can be obtained sufficiently far in advance. Zones of cancellation can be achieved using either feedforward or feedback control and the reduced local sound field results from interference between the primary sound field and the field produced by the control sources. A local minimum can be achieved by placing an error sensor in the region to be controlled. Unfortunately, the region in which the noise level can be reduced by 10 dB or more is only about one tenth of a wavelength in radius, which makes this technique less than useful for anything but very low-frequency noise. It is important to remember that the physical arrangement of control sources and error sensors plays a very important role in determining the effectiveness of an active control system. Moving the locations of the control sources and sensors affects both system controllability and stability. For feedforward systems, the physical system arrangement can be optimised independently of the controller, but for feedback systems, the physical system arrangement is an important part of the controller design. Also of importance is the size of the source to be controlled compared to an acoustic (for sound control) or structural (for vibration control) wavelength at the lowest frequency to be controlled. Clearly, one small control source will be ineffective in achieving global control of a primary source which is many wavelengths in dimensions because of its inability to significantly change the primary source radiation impedance. Some of the other problems which must be addressed in the design of feedforward electronic controllers include acoustic or vibration feedback from the control source to the reference sensor and non-linear control sources (that is, when the acoustic or vibration output of the control source contains frequencies not present in the electrical input, a condition often caused by harmonic distortion in the control actuators ordriving amplifiers). Also, it is necessary to either provide on-line system identification of the electro-acoustic transfer functions of the control sources and error microphones and the acoustic delay between them (cancellation path) or else design a complex controller which does not need this information. This latter alternative could require the use of complex filter structures such as neural networks or non-linear adaptive algorithms such as genetic algorithms. One question which may be asked is, how does one decide on whether to use a feedforward or a feedback controller for a particular application? The answer is that a feedforward system should be implemented whenever it is possible to obtain a suitable reference signal, because the performance of a feedforward system is, in general, superior to a feedback system. Unfortunately, in many instances, it is not possible to obtain a suitable reference signal, such as when attempting to reduce the resonant response of an impulsively excited structure. In cases such as these, feedback control systems are especially suited and indeed are the only alternative. As has been alluded to a previously, active control can be applied effectively to both noise and vibration problems. For noise problems, feedforward control has been applied successfully to ducts and on an experimental basis to noise in aircraft cabins and motor vehicle interiors and exteriors. Feedback control has been applied successfully to ear defenders where it is not easy to sample the incoming signal in advance, making it difficult to generate an appropriate reference signal for a feedforward controller. **************************************************************************** End of the third issue of the ISV Diges