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Archive-name: active-noise-control-faq
Posting-Frequency: monthly Last-modified: 1996/02/22 Version: 1996-03-14 See reader questions & answers on this topic! - Help others by sharing your knowledge ------------------------------------------------------------- Frequently Asked Questions: Active noise control ------------------------------------------------------------- SUMMARY: The FAQ you are now reading discusses active noise control, a novel way of using basic physics to control noise and/or vibration. What is an FAQ, you say? Well, the Internet supports thousands of "newsgroups" -- discussion forums covering every imaginable topic. An FAQ (Frequently Asked Questions list) is a summary written to answer specific questions that arise repeatedly in the newsgroups. This particular FAQ was written for the newsgroups news:alt.sci.physics.acoustics and news:comp.dsp, which focus on acoustics and digital signal processing, respectively. This FAQ has four purposes: * Provide concise, accurate answers to common questions about active noise control. * Dispel popular misconceptions about what active noise control can and cannot do. * Refer interested readers to web links, magazine articles, technical references, and other sources of information. * Stimulate public interest in acoustics. CONTENTS 1. Introduction 1.1. What's new in the Active Control FAQ 1.2. Finding the most recent FAQ 1.3. Contributors 1.4. Administrative trivia 1.5. Basics: what is sound? Frequency? Wavelength? 2. General discussion of active control 2.1. What is active control of noise/vibration? 2.2. Is active control new? 2.3. Are there different kinds of active control? 2.4. Is active noise control like noise masking? 2.5. How can adding sound make a system quieter? 2.6. When does active control work best? 2.7. What is adaptive active control? 2.8. What are some typical applications? 2.9. Are all 'active headphones' the same? 2.10. What are the benefits of active control? 2.11. What was that short story by Arthur C. Clarke? 2.12. How can I do a simple, inexpensive active control demo? 3. Finding more information 3.1. What is the active control newsletter? 3.2. What companies produce active control products? 3.3. What universities teach active noise control? 3.4. How can I learn more via Internet? 3.5. Are there short courses about active control? 3.6. References from the general literature 3.7. References from the technical literature ============================================= Subject: 1. Introduction - - - - - - - - - - - - - - - - - - - - - - - Subject: 1.1. What's new in the Active Control FAQ The Acoustical Society of America recently awarded its 1994 Science Writing Award for this FAQ. The Science Writing Award is intended to "recognize and stimulate distinguished writing (or producing) that improves public understanding and appreciation of acoustics." The award, one of two given each year, has never before been given for a work published only on the Internet. An article based on this FAQ appeared in the most recent issue of _Echoes_, the quarterly newsletter of the Acoustical Society of America (Spring 1996). Date: Topic added or changed: 1996/02/22 updated short course info (3.5) 1996/01/23 link to Digisonix home page (3.4) 1996/01/11 some info on anti-noise computer headset (2.9) 1995/12/12 links to universities (3.3) 1995/12/04 rearranged sections; added section on amplified earmuffs (2.9); new web links (3.4); buzzword generator (2.3); archive-name changed back to original 1995/11/27 archive-name changed 1995/11/06 Clarke story (2.11); low-cost ANC (2.12) 1995/10/23 link to acoustics FAQ (3.4); new popular references (3.6) 1995/08/24 Causal Systems home page (3.4) 1995/06/26 Digisonix short course (3.5) 1995/04/11 active control newsletter (3.1) 1995/03/03 cross-posted to *.answers 1995/02/24 expanded intro, revised format, added basics (1.5) 1995/02/23 new references (3.6); info on short courses (3.5) 1995/01/24 cross-posted to comp.dsp 1994/12/22 revised list of applications (2.8) 1994/12/12 added new references 1994/10/04 expanded description of mechanisms; corrected typos 1994/06/14 initial release - - - - - - - - - - - - - - - - - - - - - - - Subject: 1.2. Finding the most recent FAQ The Active Noise Control FAQ is updated monthly; see the version date cited above. You have several options to obtain the latest version: * Usenet: the FAQ is posted monthly to these newsgroups: news:alt.sci.physics.acoustics, news:comp.dsp, news:alt.answers, news:comp.answers, and news:news.answers * Anonymous ftp: ftp://rtfm.mit.edu/pub/usenet/news.answers/ * Email: mail-server@rtfm.mit.edu (send usenet/news.answers/active-noise-control-faq) Like most FAQs, this is a living, evolving document. Please e-mail contributions, comments, praise, and criticisms to the FAQ maintainer (ruckman@oasys.dt.navy.mil) or post to news:alt.sci.physics.acoustics. In particular, please contribute the following: * Companies/universities that teach courses on active control * Companies that sell active control products * Interesting references from the general literature * Comments from readers who do not know much about acoustics To cite this FAQ as a reference, I suggest a citation like this: Ruckman, C.E. (1995) "Frequently Asked Questions: Active Noise Control," Internet FAQ document. Available via anonymous ftp from ftp://rtfm.mit.edu/pub/usenet/news.answers/, or via Usenet in news:news.answers. - - - - - - - - - - - - - - - - - - - - - - - Subject: 1.3. Contributors The following people contributed to the discussions upon which this FAQ is based: * rtm@sabine.acs.psu.edu (Ralph T. Muehleisen) * chrisl@sparc.ncpa.olemiss.edu (Chris Lawrenson) * lajoie@eskimo.com (Stephen Lajoie) * S.E.Mercy@acoustics.salford.ac.uk (Susan Mercy) * dieh1232@w250zrz.zrz.TU-Berlin.DE (Rolf Diehl) * jsv@acpub.duke.edu (Jeffrey Stuart Vipperman) * mbronzel@vtmers1.me.vt.edu (Marcus Bronzel) * nielsen@tele.unit.no (Johan L. Nielsen) * chansen@aelmg.adelaide.edu.au (Colin Hansen) * M.A.Schonewille@CTG.TUDelft.NL (Michel Schonewille) * sl@la.dtu.dk (Soeren Laugesen) * Todd Toles (E70TET1@WPO.CSO.NIU.EDU) * stever@quiknet.com * john.gilliver@gmrc.gecm.com (John Gilliver) * nomader@eskimo.com (Lee Leggore) * and many others! - - - - - - - - - - - - - - - - - - - - - - - Subject: 1.4. Administrative trivia Copyright (c) 1994,1995,1996 by Christopher E. Ruckman All rights are reserved. Christopher E. Ruckman ("Author") hereby grants permission to use, copy and distribute this document for any NON-PROFIT purpose, provided that the article is used in its complete, UNMODIFIED form including both the above Copyright notice and this permission notice. Reproducing this article by any means, including (but not limited to) printing, copying existing prints, or publishing by electronic or other means, implies full agreement to the above non-profit-use clause. Exceptions to the above, such as including the article in a compendium to be sold for profit, are permitted only by EXPLICIT PRIOR WRITTEN CONSENT of Christopher E. Ruckman. Disclaimer: This document does not necessarily represent the opinion of the US Government, nor of anyone other than the Author. Any mentions of commercial products, company names, or universities are solely for information purposes and do not imply any endorsement by the Author or his employer. The Author provides this article "as is." The Author disclaims any express or implied warranties including, but not limited to, any implied warranties of commercial value, accuracy, or fitness for any particular purpose. If you use the information in this document in any way, you do so entirely at your own risk. - - - - - - - - - - - - - - - - - - - - - - - Subject: 1.5. Basics: what is sound? Frequency? Wavelength? If you are not familiar with how sound works, the following brief refresher course may help. Dont be put off by occasional technical jargon; most of the FAQ includes analogies and examples to illustrate ideas in plain language. (The author apologizes to acousticians everywhere for presuming to summarize their craft in a mere three paragraphs!) Sound is a pressure wave traveling in air or water. A sound wave resembles the surface wave made when you throw a stone into a calm pool of water, except that * the sound wave consists of tiny fluctuations in the air pressure rather than fluctuations in water height, * a sound wave can travel in three dimensions rather than two, and * the wave speed is much faster (340 meters per second in air). Sound is usually generated by vibration of an object or surface such as a speaker cone, a violin body, or human vocal cords. The vibrating surface "radiates" pressure waves into the adjoining air or water as sound. (Sound can also be generated by turbulent airflow, by bubbles collapsing, or by many other phenomena.) The frequency (number of wave crests per unit time that pass a fixed location) measures the tone or pitch of a sound. For example, a bass guitar plays lower frequencies than a violin. The wavelength, or distance between wave crests, is related to frequency: lower frequencies have longer wavelengths. In air, all frequencies of sound travel at the same speed. When bending waves travel through a flexible structure, however, low frequencies travel faster than high frequencies. In this context, noise is simply *unwanted* sound. There is an old trick question: "If a tree falls in the forest and nobody is there to hear it, does it make any noise?" The answer is "no" because sound cannot be *noise* unless somebody hears it and finds it offensive. However, if the question is phrased "Does it make any *sound*," then you have a deep philosophical question to ponder! ============================================= Subject: 2. General discussion of active control - - - - - - - - - - - - - - - - - - - - - - - Subject: 2.1. What is active control of noise/vibration? The question is usually posed like this: "I heard about a new noise control technology called Active Something-Or-Other ... can I use it to make my house quiet when the kid next-door plays 'Black Sabbath' on his electric guitar?" The technology in question is "active noise control," a.k.a. "active noise cancellation," a.k.a. "anti-noise," and it is one of the hot research topics in acoustics these days. Here is the bottom line: yes, active noise control works in the proper circumstances, but no, you cannot use it to soundproof an entire house. Active control is sound field modification, particularly sound field cancellation, by electro-acoustical means. In its simplest form, a control system drives a speaker to produce a sound field that is an exact mirror-image the offending sound (the "disturbance"). The speaker thus "cancels" the disturbance, and the net result is no sound at all. In practice, of course, active control is somewhat more complicated; see below. The name differentiates "active control" from traditional "passive" methods for controlling unwanted sound and vibration. Passive noise control treatments include "insulation", silencers, vibration mounts, damping treatments, absorptive treatments such as ceiling tiles, and conventional mufflers like the ones used on todays automobiles. Passive techniques work best at middle and high frequencies, and are important to nearly all products in todays increasingly noise- sensitive world. But passive treatments can be bulky and heavy when used for low frequencies. The size and mass of passive treatment usually depend on the acoustic wavelength, making them thicker and more massive for lower frequencies. The light weight and small size of active systems can be a critically important benefit; see later sections for other benefits. In control systems parlance, the four major parts of an active control system are: * The plant is the physical system to be controlled; typical examples are a headphone and the air inside it, or air traveling through an air-conditioning duct. * Sensors are the microphones, accelerometers, or other devices that sense the disturbance and monitor how well the control system is performing. * Actuators are the devices that physically do the work of altering the plant response; usually they are electromechanical devices such as speakers or vibration generators. * The controller is a signal processor (usually digital) that tells the actuators what to do; the controller bases its commands on sensor signals and, usually, on some knowledge of how the plant responds to the actuators. Analog controllers may also be used, although they are somewhat less flexible and thus more difficult to use. - - - - - - - - - - - - - - - - - - - - - - - Subject: 2.2. Is active control new? The idea of active noise control was actually conceived in the 1930s (see the Lueg patent mentioned below), and more development was done in the 1950s. However, it was not until the advent of modern digital computers that active control became truly practical. Active control became a "mainstream" research topic in the 1970s and 1980s, and in recent years research papers have been published at the rate of several hundred per year. There are now several rather large companies that specialize in active control products, and the topic is widely studied in universities and government research laboratories. - - - - - - - - - - - - - - - - - - - - - - - Subject: 2.3. Are there different kinds of active control? There are two basic approaches for active noise control: active noise cancellation (ANC) and active structural-acoustic control (ASAC). In ANC, the actuators are acoustic sources (speakers) which produce an out-of-phase signal to "cancel" the disturbance. Most people think of ANC when they think of active noise control; some examples are mentioned below. On the other hand, if the noise is caused by the vibration of a flexible structure, then ASAC may be more appropriate than ANC. In ASAC, the actuators are vibration sources (shakers, piezoceramic patches, etc.) which can modify how the structure vibrates, thereby altering the way it radiates noise. (The distinction between ANC and ASAC is somewhat arbitrary, since both cases correspond to a controller using actuators to reduce the plant response.) Active vibration control is a related technique that resembles active noise control. In either case, electromechanical actuators control the response of an elastic medium. In active noise control, the elastic medium is air or water through which sound waves are traveling. In active vibration control, the elastic medium is a flexible structure such a satellite truss or a piece of vibrating machinery. The critical difference, however, is that active vibration control seeks to reduce vibration *without* regard to acoustics. Although vibration and noise are closely related, reducing vibration does not necessarily reduce noise. Actually, you can generate your own catchy phrases with the following handy buzzword generator. Simply choose one word from each column, string them all together without commas, and paste the result or its acronym into your document or conversation! / Column A \ / Column B (optional) \ / Column C \ | ----------- | | ------------------- | | ------------ | | active | | vibration | | cancellation | < adaptive > < noise > < control > | semi-active | | sound | | damping | | | | structural-acoustic | | suppression | \ / \ vibro-acoustic / \ / - - - - - - - - - - - - - - - - - - - - - - - Subject: 2.4. Is active noise control like noise masking? Active noise control is quite different from noise masking. Acoustic masking is the practice of intentionally adding low-level background sounds to either a) make noise less distracting, or b) reduce the chance of overhearing conversations in adjoining rooms. In active noise control, the system seeks not to mask offending sound, but to eliminate it. Masking increases the overall noise level; active control decreases it, in some locations if not all. - - - - - - - - - - - - - - - - - - - - - - - Subject: 2.5. How can adding sound make a system quieter? It may seem counter-intuitive to say that adding more sound to a system can reduce noise levels, but the method can and does work. Active noise control occurs by one, or sometimes both, of two physical mechanisms: "destructive interference" and "impedance coupling". Here is how they work: On one hand, you can say that the control system creates an inverse or "anti-noise" field that "cancels" the disturbance sound field. This works by the principle of destructive interference. A sound wave is a moving series of compressions (high pressure) and rarefactions (low pressure). If the high-pressure part of one wave lines up with the low-pressure of another wave, the two waves interfere destructively and there is no more pressure fluctuation (no more sound). Note that the matching must occur in both space *and* time -- a tricky problem indeed. On the other hand, you can say that the control system changes the way the system "looks" to the disturbance, i.e., changes its input impedance. Consider the following analogy: Picture a spring-loaded door, one that opens a few centimeters when you push on it but swings shut when you stop pushing. A person on the other side is repeatedly pushing on the door so that it repeatedly opens and closes at a low frequency, say, twice per second. Now suppose that whenever the other person pushes on the door, you push back just as hard. Your muscles are heating up from the exertion of pushing on the door, but end result is that the door moves less. You *could* say that the door opens and that you "anti- open" it to "cancel" the opening. But that wouldn't be very realistic; at least, you would not actually see the door opening and anti-opening. You would be more accurate to say that you change the "input impedance" seen on the other side of the door: when the other person pushes, the door just doesn't open. (The spring-loaded door is supposed to represent the spring effect of compressing air in a sound wave. This is not a perfect analogy, but it helps illustrate impedance coupling.) In some cases, destructive interference and impedance coupling can be two sides of the same coin; in other cases destructive interference occurs without impedance coupling. The difference is related to whether the acoustic waves decay with distance traveled: Sound from a speaker hanging in the middle of a stadium decays (is less loud) at a distance because of "spherical spreading." The sound energy is spread out over an increasingly large area as you get farther away. Go far enough away and, for all intents and purposes, the sound decays completely down to nothing. On the other hand, sound in a "waveguide" such as a duct can travel long distances without significant decay. If a control system actuator is close to the disturbance source, destructive interference and impedance coupling can both occur. But what about when the actuator is far away from the disturbance, so far away that any wave it creates decays completely down to nothing before reaching the disturbance? There can still be destructive interference near the actuator, even though the actuator cannot possibly affect the impedance seen by the disturbance. Example: the tiny speaker in an active control headphone will not affect the impedance seen by a cannon firing a mile away, but it can create destructive interference within the headphone. - - - - - - - - - - - - - - - - - - - - - - - Subject: 2.6. When does active control work best? Active noise control works best for sound fields that are spatially simple. The classic example is low-frequency sound waves traveling through a duct, an essentially one-dimensional problem. The spatial character of a sound field depends on wavelength, and therefore on frequency. Active control works best when the wavelength is long compared to the dimensions of its surroundings, i.e., low frequencies. Fortunately, as mentioned above, passive methods tend to work best at high frequencies. Most active noise control systems combine passive and active techniques to cover a range of frequencies. For example, many active mufflers include a low-back- pressure "glass-pack" muffler for mid and high frequencies, with active control used only for the lowest frequencies. Controlling a spatially complicated sound field is beyond today's technology. The sound field surrounding your house when the neighbor's kid plays his electric guitar is hopelessly complex because of the high frequencies involved and the complicated geometry of the house and its surroundings. On the other hand, it is somewhat easier to control noise in an enclosed space such as a vehicle cabin at low frequencies where the wavelength is similar to (or longer than) one or more of the cabin dimensions. Easier still is controlling low-frequency noise in a duct, where *two* dimensions of the enclosed space are small with respect to wavelength. The extreme case would be low-frequency noise in a small box, where the enclosed space appears small in all directions compared to the acoustic wavelength. Often, reducing noise in specific localized regions has the unwanted side effect of amplifying noise elsewhere. The system reduces noise locally rather than globally. Generally, one obtains global reductions only for simple sound fields where the primary mechanism is impedance coupling. As the sound field becomes more complicated, more actuators are needed to obtain global reductions. As frequency increases, sound fields quickly become so complicated that tens or hundreds of actuators would be required for global control. Directional cancellation, however, is possible even at fairly high frequencies if the actuators and control system can accurately match the phase of the disturbance. Aside from the spatial complexity of the disturbance field, the most important factor is whether or not the disturbance can be measured *before* it reaches the area where you want to reduce noise. If you can measure the disturbance early enough, for example with an "upstream" detection sensor in a duct, you can use the measurement to compute the actuator signal (feedforward control). If there is no way to measure an upstream disturbance signal, the actuator signal must be computed solely from error sensor measurements (feedback control). Under many circumstances feedback control is inherently less stable than feedforward control, and tends to be less effective at high frequencies. For a concise comparison of feedforward vs. feedback control, see Hansen, IS&VD 1(3). Bandwidth is also important. Broadband noise, that is, noise that contains a wide range of frequencies, is significantly harder to control than narrowband (tonal or periodic) noise or a tone plus harmonics (integer multiples of the original frequency). For example, the broadband noise of wind flowing over an aircraft fuselage is much more difficult to control than the tonal noise caused by the propellers moving past the fuselage at constant rotational speed. Finally, lightly damped systems are easier to control than heavily damped ones. (Damping refers to how quickly the sound or vibration dies out; it should not be confused with "dampening", which describes whether the system is wet!) - - - - - - - - - - - - - - - - - - - - - - - Subject: 2.7. What is adaptive active control? Adaptive control is a special type of active control. Usually the controller employs some sort of mathematical model of the plant dynamics, and possibly of the actuators and sensors. Unfortunately, the plant can change over time because of changes in temperature or other operating conditions. If the plant changes too much, controller performance suffers because the plant behaves differently from what the controller expects. An adaptive controller is one that monitors the plant and continually or periodically updates its internal model of the plant dynamics. - - - - - - - - - - - - - - - - - - - - - - - Subject: 2.8. What are some typical applications? The most successful demonstrations of active control have been for controlling noise in enclosed spaces such as ducts, vehicle cabins, exhaust pipes, and headphones. Note, however, that most demonstrations have not yet made the transition into successful commercial products. One exception, active noise control headphones, has achieved widespread commercial success. Active headphones use destructive interference to cancel low-frequency noise while still allowing the wearer to hear mid- and high-frequency sounds such as conversation and warning sirens. The system comprises a pair of earmuffs containing speakers and one or more small circuit boards. Some include a built-in battery pack, and many allow exterior signal inputs such as music or voice communications. Used extensively by pilots, active headphones are considered indispensable in helicopters and noisy propeller-driven aircraft. Prices have dropped in recent years, and now start around US$650 for active pilots headsets. (See Section 2.11 for information about an active control conversion kit available for US$100.) Another application that has seen some commercial success is active mufflers for industrial engine exhaust stacks. Active control mufflers have seen years of service on commercial compressors, generators, and so forth. As unit prices for active automobile mufflers have fallen in recent years, several automobile manufacturers are now considering active mufflers for future production cars. However, if you ask your local new car dealer about the active muffler option on their latest model, you will likely receive a blank stare: no production automobiles feature active mufflers as of this writing. Large industrial fans have also benefited from active control. Speakers placed around the fan intake or outlet not only reduce low- frequency noise downstream and/or upstream, but they also improve efficiency to such an extent that they pay for themselves within a year or two. The idea of canceling low-frequency noise inside vehicle cabins has received much attention. Most major aircraft manufacturers are developing such systems, especially for noisy propeller-driven aircraft. Speakers in the wall panels can reduce noise generated as the propeller tips pass by the aircraft fuselage. For instance, a system by Noise Cancellation Technologies (NCT) now comes as standard equipment on the new Saab 2000 and 340B+ aircraft. The key advantage is a dramatic weight savings compared to passive treatments alone. Automobile manufacturers are considering active control for reducing low-frequency noise inside car interiors. The car stereo speakers superpose cancellation signals over the normal music signal to cancel muffler noise and other sounds. For example, Lotus produces such a system for sale to other automobile manufacturers. Unit cost is a major consideration for automobile use. While such systems are not at all common, at least one vehicle (currently offered only in Japan) includes such a system as a factory option. The following list of applications for active control of noise and vibration was compiled by Colin Hansen and is used by permission; see IS&VD 1(2). The list includes topics which are currently being investigated by research groups throughout the world. ---------- begin quote from C. Hansen, IS&VD 1(2) ---------- 1. Control of aircraft interior noise by use of lightweight vibration sources on the fuselage and acoustic sources inside the fuselage. 2. Reduction of helicopter cabin noise by active vibration isolation of the rotor and gearbox from the cabin. 3. Reduction of noise radiated by ships and submarines by active vibration isolation of interior mounted machinery (using active elements in parallel with passive elements) and active reduction of vibratory power transmission along the hull, using vibration actuators on the hull. 4. Reduction of internal combustion engine exhaust noise by use of acoustic control sources at the exhaust outlet or by use of high intensity acoustic sources mounted on the exhaust pipe and radiating into the pipe at some distance from the exhaust outlet. 5. Reduction of low frequency noise radiated by industrial noise sources such as vacuum pumps, forced air blowers, cooling towers and gas turbine exhausts, by use of acoustic control sources. 6. Lightweight machinery enclosures with active control for low frequency noise reduction. 7. Control of tonal noise radiated by turbo-machinery (including aircraft engines). 8. Reduction of low frequency noise propagating in air conditioning systems by use of acoustic sources radiating into the duct airway. 9. Reduction of electrical transformer noise either by using a secondary, perforated lightweight skin surrounding the transformer and driven by vibration sources or by attaching vibration sources directly to the transformer tank. Use of acoustic control sources for this purpose is also being investigated, but a large number of sources are required to obtain global control. 10. Reduction of noise inside automobiles using acoustic sources inside the cabin and lightweight vibration actuators on the body panels. 11. Active headsets and earmuffs. ---------- end quote from C. Hansen, IS&VD 1(2) ---------- - - - - - - - - - - - - - - - - - - - - - - - Subject: 2.9. Are all 'active headphones' the same? No. Two types are often called "active," but only one actually uses noise cancellation. For the sake of discussion, let's call the two types "active headphones" and "amplified earmuffs". Active headphones rely primarily on noise cancellation for low- frequency quieting. In some, the earmuffs themselves provide relatively little passive noise reduction. In others, the earmuffs provide as much passive reduction as possible, using noise cancellation to get even better performance at low frequencies. In any case, the unit includes a microphone *inside* each earcup to monitor the "error" -- the part of the signal that has not been cancelled by the speakers. A pilot's headset also includes a microphone boom to transmit the pilots voice, and an input jack to transmit communication signals into the earcups. The noise cancellation works best on tones or periodic noise like that from an aircraft propeller. Amplified earmuffs are quite different, as they do not use noise cancellation at all. A heavy passive earmuff attenuates as much noise as possible. Microphones on the *outside* of the unit pick up sounds that would ordinarily be heard by the ears. These microphone signals are then filtered before being played by speakers inside the earcups. The most common filtering is to mute loud, impulsive sounds such as gunshots; amplified earmuffs are therefore becoming quite popular at weapons firing ranges. (Example: the popular Peltor Tactical 7-S retails for around US$130.) Amplified earmuffs have also been suggested for use by sufferers of tinnitus ("ringing of the ears"), a condition that can be aggravated by loud noises. But amplified earmuffs should not be confused with true active noise control headphones. A new product has recently come to market: the Andrea Anti-Noise Computer Headset. This product includes an earpiece with a boom- mounted microphone, and is used to filter out background noise from voice signals recorded by the microphone. Details on this product will be included in a forthcoming posting; in the mean time, interested readers should contact Andrea directly and mention this FAQ. (Andrea Electronics Corporation, 11-40 45th Road, Long Island City, NY 11101, USA, phone 1.800.442.7787). - - - - - - - - - - - - - - - - - - - - - - - Subject: 2.10. What are the benefits of active control? The many practical benefits of active control technology are not all obvious at first glance. The main payoff, of course, is low- frequency quieting that would be too expensive, inconvenient, impractical, or heavy by passive methods alone. For example, the lead-impregnated sheets used to reduce aircraft cabin propeller noise impose a severe weight penalty, but active control might perform as well with a much smaller weight penalty. Other possible benefits reflect the wide range of problems on which active control can be applied. For instance, with conventional car mufflers the engine spends extra energy to push exhaust gasses through the restrictive muffler passages. On the other hand, an active control muffler can perform as well with less severe flow restrictions, thus improving performance and/or efficiency. Additional benefits include: * increased material durability and fatigue life * lower operating costs due to reduced facility down-time for installation and maintenance * reduced operator fatigue and improved ergonomics Of these, the potential for reduced maintenance and increased material fatigue life have received new emphasis in the last few years. In the long-term, however, benefits may extend far beyond those mentioned above. The compact size and modularity of active systems can provide additional flexibility in product design, even to the point of a complete product redesign. - - - - - - - - - - - - - - - - - - - - - - - Subject: 2.11. What was that short story by Arthur C. Clarke? Arthur C. Clarke's short story entitled "Silence Please" appeared in his 1954 collection "Tales from the White Hart" (reprinted in 1970 by Harcourt, Brace & World Inc., New York). In it, Harry Purvis recounts the tale of the ill-fated "Fenton Silencer," an anti-noise device that goes disastrously awry. In the tradition of Clarke's other works, the story itself is entertaining and well-told. Strictly speaking, however, the basic premise requires some poetic license regarding the physics of sound cancellation. Well-informed readers must rely on their "willing suspension of disbelief" to overlook the inconsistencies. [Easy for me to say, with the benefit of over fourty years' hindsight! CR] - - - - - - - - - - - - - - - - - - - - - - - Subject: 2.12. How can I do a simple, inexpensive active control demo? Because active control employs some interesting physics, readers often ask how to construct a simple, low-cost demonstration as a student project or for instructional purposes. Here are three possibilities: First, the hard way: it is possible to construct an analog feedback controller using op-amps, capacitors, speakers, and other parts available from any electronics supplier. While simple in concept, constructing such a demonstration requires a pretty solid foundation in acoustics, electronics, and control theory, and is well beyond the scope of this FAQ. [Please DO NOT ask the author for instructions. CR] A second approach is much more powerful and flexible, but only if you have a budget on the order of US$2000 or so: the EZ-ANC from Causal Systems. This comprehensive kit includes hardware, software, and a complete theoretical/user's manual. (See Section 3.2 for contact information, or check out their web page: http://www.io.org/~causal/cs/csdir01.htm) A third alternative is much less expensive, but not as flexible: the "ANR Adapter" from Headsets, Inc. The ANR Adapter is an add-on kit that transforms an ordinary passive pilot's headset into an active noise control headset. The kit costs only US$100; you supply the headset. The makers claim roughly 22 dB attenuation from 20 Hz to 700 Hz. If you simply want a demonstration in which you flip a power switch to hear active noise control at work, this kit may be for you. (See Section 3.2 for contact information. For a review of the product, see the following magazine article: Picou, Gary, "Low-Rent ANC," The Aviation Consumer, vol.25, No.7, MAY 01 1995, p.10-12.) ============================================= Subject: 3. Finding more information - - - - - - - - - - - - - - - - - - - - - - - Subject: 3.1. What is the active control newsletter? An informative newsletter about active control is published monthly. "Active Sound & Vibration Control News" describes itself as "An independent publication focusing on Research and Development in the field of Active Sound and Vibration Control (AS/VC) among Industry, Universities, and Government." The current price is US$419/year. Interested readers may contact the publisher for a free sample. Published by: Tech Pubs Inc., 8858 Blue Sea Drive, Columbia, Maryland 21046 USA voice 410.381.9359, fax 410.381.5843 - - - - - - - - - - - - - - - - - - - - - - - Subject: 3.2. What companies produce active control products? Some readers may wish to contact vendors for product literature. The following companies, LISTED IN ALPHABETICAL ORDER, produce active noise control products. No endorsement of any kind is implied by inclusion in this list, nor is this meant to be a complete list. There are many other companies that produce system components or are involved in active control research and development -- *far* too many to list here. The companies listed below are *only* companies that produce commercially available products intended specifically for active noise control. Please suggest others as appropriate! * Active Vibration Control Instrumentation, PCB Piezotronics, Inc., 3425 Walden Ave. Depew, NY 14043-2495, phone 716-684-0001 * BBN Acoustic Technologies, 10 Moulton Street, Cambridge, MA 02138- 1119, phone 617-873-3960, fax 617-873-3776, e-mail oliphant@bbn.com (Robert W. Oliphant) * Causal Systems Pty Ltd., P.O. Box 100, Rundle Mall, South Australia 5000, Australia, phone 61.8.303.5460, fax 61.8.303.4367, e- mail chansen@aelmg.adelaide.edu.au (Colin Hansen), Web http://www.io.org/~causal/cs/csdir01.htm * Digisonix, Inc., 8401 Murphy Drive, Middleton, WI 53562-2243 USA, phone 608.836.3999, fax 608.836.5583 * dSPACE Inc., 26677 W. Twelve Mile Road, Southfield, Michigan 48034, 810.354.1694 * Headsets, Inc., 2330-B Lakeview, Amarillo, Texas 79109, USA, phone 806.358.6336, fax 806.358.6449, Paige Brittain, President. * Noise Cancellation Technologies, Inc., Headquarters: Stamford, Connecticut, 203.961.0500 (Joanna Lipper). Engineering facilities: Linthicum, Maryland, USA, 410.636.8700 * Sennheiser electronic KG, D-30900 Wedemark, Germany * Also: Bose, David Clark, Peltor, Sony, others - - - - - - - - - - - - - - - - - - - - - - - Subject: 3.3. What universities teach active noise control? Some readers may wish to contact universities regarding curricula that include active noise control. Many universities teach active noise control (primarily at the graduate level). The following schools, LISTED IN ALPHABETICAL ORDER, have reasonably extensive graduate research programs in active noise control. No endorsement of any kind is implied by inclusion in this list, nor is this meant to be a complete list. [Editor's note: Please help me add to this list, especially universities outside the USA. CR] * Delft University of Technology, Delft, Netherlands http://www.tudelft.nl/home.html * Duke University, Durham, North Carolina, USA http://www.duke.edu/ * Georgia Institute of Technology, Atlanta, Georgia, USA http://www.gatech.edu/ * Norwegian Institute of Technology, Trondheim, Norway http://www.unit.no/ * Massachusettes Institute of Technology, Cambridge, Massachusetts, USA http://web.mit.edu/ * Northern Illinois University, DeKalb, Illinois, USA http://www.niu.edu/ * Old Dominion University, Norfolk, Virginia, USA http://www.odu.edu/ * Pennsylvania State University: The Graduate Program in Acoustics, Penn State University, PO Box 30, State College, PA 16804, Phone (814) 865-6364, Fax (814) 865-3119 http://www.acs.psu.edu * Purdue University, West Lafayette, Indiana, USA http://www.purdue.edu/ * RWTH Aachen, Germany http://www.itm.rwth-aachen.de/ * Southampton University, Southampton, England http://www.soton.ac.uk/ * Technical University of Denmark, Denmark http://www.dtu.dk/dtu/dtu.html * Technical University of Berlin, Germany http://www.tk.tu-berlin.de/ * Technical University of Erlangen, Germany * Technical University of Munich, Germany * Technical University of Stuttgart, Germany * University of Adelaide, Adelaide, South Australia, Australia * University of Goettingen, Germany * University of Hamburg, Germany * University of Karlskrona/Ronneby, Ronneby, Sweden http//hk-r.se/isb/research.html * University of Salford, England * Universite de Sherbrooke, Sherbrooke, Quebec, Canada http://www.usherb.ca/index.html * Universite de Technologie de Compiegne, Compiegne, France http://www.univ-compiegne.fr/ * University of Utah, Salt Lake City, Utah, USA http://www.utah.edu/HTML_Docs/Campus_Info.html * Villanova University, Philadelphia, Pennsylvania, USA http://www.vill.edu/ * Virginia Polytechnic Institute & State University, Blacksburg, Virginia, USA http://www.vt.edu/ - - - - - - - - - - - - - - - - - - - - - - - Subject: 3.4. How can I learn more via Internet? Besides the FAQ you are now reading, there are several Internet resources dedicated solely to active control. Two of the best are the home pages for Digisonix and Causal Systems Limited, both of which contain plenty of technical detail for those who want more than this FAQ provides. These two excellent resources may be found at: http://www.io.org/~causal/cs/csdir01.htm http://www.mailbag.com/users/dgsnx_mr New since last time: http://www.mailbag.com/users/dgsnx_mr Other URLs that at least mention active control: http://www.elen.utah.edu:80/~douglas/EE620_Winter96.txt http://www.signal.se/ http://www.magi.com/~blairc/ancp.html http://cac.psu.edu/~lnl/aiaa96/aiaa96.html http://helmholtz.ecn.purdue.edu/OtherPages/Bernhard.html http://www.arl.psu.edu/general/expert1.html/ http://kirkof.psu.edu/cav/ http://www.fie.com/web/fed/nas/prog/naspgbmx.htm http://www.sm.luth.se/~gunnarh/ http://mecmac3.tm.chiba-u.ac.jp/documentation/movic/MOVIC96E.html http://baloo.dc.luth.se/depts/mt/ene/articles/rigg/A_cab.html http://www.larc.nasa.gov/tops/Exhibits/Ex_V-611/Ex_V-611.html http://leoleo.mme.tcd.ie/Groups/SAV/asanca.html http://www.mme.tcd.ie/~m.carley/Acoustics/acoustics.faq http://www.acs.psu.edu/Acoustics.html http://www.mech.kuleuven.ac.be/pma/annual93/2_11b.html http://sun-valley.stanford.edu/users/howjo/mace.html http://web.mit.edu/org/a/avlab/www/vl.home.html http://www.yahoo.com/Science/Acoustics http://www-gaus.gme.usherb.ca/axeact_a.html Here are some other resources that deal with general acoustics and vibration topics: * The Acoustics FAQ is now available, thanks to Andrew Silverman: http://www.mme.tcd.ie/~m.carley/Acoustics or, in the US, ftp://rtfm.mit.edu/pub/usenet/alt.sci.physics.acoustics/Acoustics_FAQ * If you have access to USENET newsgroups, check out the following: news:alt.sci.physics.acoustics (general acoustics) news:comp.dsp (digital signal processing) * Check out the new home page of the Acoustical Society of America: http://asa.aip.org * Penn State University has an excellent acoustics home page: http://www.acs.psu.edu * If you have access to e-mail, you can subscribe to the International Sound & Vibration Digest by sending e-mail to yanas@eng.auburn.edu. - - - - - - - - - - - - - - - - - - - - - - - Subject: 3.5. Are there short courses about active control? Some readers may wish to contact universities or vendors that teach short courses on active noise control. There are many. Some are listed below. [Please help me expand this list. CR] Title: "Implementing active control: Designing & integrating active sound & vibration control systems" Instructors: R.J. Bernhard, L.J. Eriksson, L.R. Miller, and H.K. Pelton Contact: Digisonix, Inc., 8401 Murphy Drive, Middleton WI 53562-2543 USA, Fax 608.836.5583, Phone 608.836.3999 (information@digisonix.com) Next offered: 7-8 May 1996, Detroit, MI (register by 19 April 1996) Title: "Active control of sound & vibration" Instructors: A.H. von Flotow, C. Fuller, and S. Elliott Contact: Flotow & Associates, 1750 Country Club Road, Hood River OR 97031-9641 USA, Phone 503.387.2288 Next offered: 27-29 March, 1996, Alexandria, VA (register by 10 March 1996) Title: ?? Instructors: ?? Contact: The Graduate Program in Acoustics, Penn State University, PO Box 30, State College, PA 16804, Phone (814) 865-6364, Fax (814) 865-3119 (http://www.acs.psu.edu) Next offered: - - - - - - - - - - - - - - - - - - - - - - - Subject: 3.6. References from the general literature Listed below are a handful of articles from popular sources, i.e., non-technical magazines that you might find in a public library. If you know any other good articles, please e-mail references to ruckman@oasys.dt.navy.mil or post them on news:alt.sci.physics.acoustics. Note: %A=author, %B=book title, %C=city, %D=date, %I=publisher, %J=journal, %K=keywords, %N=number, %P=pages, %T=article title, %V=volume, %X=comments %A Picou, Gary %T Low-Rent ANC: For a hundred bucks and a couple of evening's work, build your own noise-canceling headset. %J The aviation consumer. %D MAY 01 1995 v 25 n 7 %P 10-12 %X Describes the ANR Adapter, an add-on kit that you can use to add active noise control to almost any pilot's headset ($100, you supply the headset). %A Higginson, Steven %T First-Class Communications %X To decide which headset/intercom system would best serve your needs, settle down with our 1995 buyer's guide. %J Plane & pilot %D FEB 01 1995 v 31 n 2 %P 47 %A Wilhelmsen, George R. %T Noise Jammer: The Telex ANR 4000 headset uses proven electronic noise-neutralizing technology to protect your ears %J Plane & pilot %D APR 01 1994 v 30 n 4 %P 56 %A Lert, Peter %T "It's Still Too Quiet Out There" %X Improvements for the Bose headset %J Air progress %D JAN 01 1994 v 56 n 1 %P 12 %A Antonoff, Michael %A Rick De Meis %T Noise Reduction: Quiet in the Sky %J Popular Science %D Dec 1994 %X Cabin-wide noise suppression system %A Foster, Edward J. %T Switched On Silence %J Popular Science %D 7/94 %V 245 %N l %P 33 %X Active noise control headphones %T Saab 340Bs get active antinoise system %J Aviation week and space technology %D MAY 09 1994 %V 140 %N 19 %P 55 %X Standard feature gives Swedish firm a jump on competitors %A Jerram, Mike %T Lotus aims to silence airplanes. (Lotus Engineering develops antinoise control system) %J Flying %P 42 %D March 1993 %V 120 %N 3 %X Lotus Engineering has spent 10 years to develop an active noise control for its cars and is now applying the same principles to aircraft. The effectiveness of the Antinoise system the company has developed is evaluated. %A Mecham, Michael %T Active noise control cuts aircraft emissions. %X The German Research Establishment's (DLR) Acoustics Division has developed a simple procedure to reduce general aviation aircraft noise. The active noise control (ANC) procedure, which involves modification of the propeller and exhaust systems, also reduces pollution. %J Aviation Week & Space Technology %P 63 %D Nov 2 1992 %V 137 %N 18 %A Adcock, Ian %T Lotus adaptive engine mounts. (Lotus Engineering technology to combat car noise) %J Motor Trend %P 72 %D May 1992 %V 44 %N 5 %X Lotus Engineering is developing two technologies to combat automobile noise and vibration. Adaptive Noise Control systems cancel noise by generating sound waves of opposite frequencies. Active Engine Mounts consist of hydraulic engine mounts that counter vibration. %A Mayersohn, Norman S. %T Hear no evil %J Popular science %D APR 01 1992 %V 240 %N 4 %P 84 %X The roar of a garbage truck; the whine of a lawn mower. These annoying sounds and others may soon be nullified by active noise cancellation systems. - - - - - - - - - - - - - - - - - - - - - - - Subject: 3.7. References from the technical literature The articles listed below are textbooks and technical journal articles not usually carried by public libraries. There is a huge and rapidly expanding body of technical literature on active control, with hundreds of papers published annually. The handful shown here describe active control in general terms and/or provide lists of references. If you know any other good articles, please e-mail references to ruckman@oasys.dt.navy.mil or post them to alt.sci.physics.acoustics. One of the best technical references to date is the book by Nelson and Elliott, listed first. Two of the most recent are the articles by Hansen, listed second and third, that were published recently in the electronic journal "International Sound and Vibration Digest." Note: %A=author, %B=book title, %C=city, %D=date, %I=publisher, %J=journal, %K=keywords, %N=number, %P=pages, %T=article title, %V=volume, %X=comments %A Nelson, P.A. %A Elliott, S.J. %B Active control of sound %I Academic Press %C London %D 1992 %X well-done textbook and reference, good bibliography. %A Hansen, C.H. %T Current research in active control of noise %J International Sound & Vibration Digest %V 1 %N 2 %D Nov 12 1994 %K active control, review %X published in electronic journal, good summary of new research %A Hansen, C.H. %T Overview of active noise control systems %J International Sound & Vibration Digest %V 1 %N 3 %D Jan 26 1995 %X compares feedforward vs. feedback control %A Elliott, S.J. %A Nelson, P.A. %T Active Noise Control %J IEEE Signal Processing Magazine %V 10 %N 4 %D October 1993 %P 12 %A Widrow, B. %A Stearns, S.D. %B Adaptive Signal Processing %I Prentice Hall %C Englewood Cliffs, New Jersey %D 1985 %X classic reference on the LMS control algorithm %A Stevens, J.C. %A Ahuja, K.K. %T Recent advances in active noise control %J AIAA journal %V 29 %N 7 %D July 1991 %X good bibliography %A Elliott, S.J. %A Nelson, P.A. %D August, 1990 %T The active control of sound %J Electronics & Communication Engineering Journal %P 127-136 %X general review of active control %A Lueg, P. %D 1936 %T Process of silencing sound oscillation %J U.S. Patent No. 2 043 416 %X generally considered the first published work on the subject, although Lueg's German patent application predates it by a few weeks %A H.F. Olson %D 1953 %T Electronic sound absorber %J Journal of the Acoustical Society of America %V 25 %P 1130-1136 %X another early reference Copyright (c) 1994,1995,1996 by Christopher E. Ruckman ---------- end of the Active Noise Control FAQ ------------ ---- InterNet: ruckman@xis.com HeyYouNet: Chris Ruckman, Ph.D. VoiceNet: 703.683.0293 FaxNet: 703.683.0304 SneakerNet: VSSL, 809 N. Royal Street, Alexandria VA 22314 USA User Contributions:
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Active Noise Control FAQ v1996-03-14
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