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- From: ruckman@xis.com (Dr. Chris Ruckman)
- Newsgroups: alt.sci.physics.acoustics,comp.dsp,alt.answers,comp.answers,news.answers
- Subject: Active Noise Control FAQ v1996-03-14
- Followup-To: alt.sci.physics.acoustics
- Date: Fri, 15 Mar 96 01:50:30 GMT
- Organization: Vibration & Sound Solutions Limited
- Lines: 1152
- Approved: news-answers-request@MIT.EDU
- Expires: 1996/04/14
- Message-ID: <4iail7$jse@news2.cais.com>
- Reply-To: ruckman@xis.com (Dr. Chris Ruckman)
- NNTP-Posting-Host: dial37.xis.com
- Summary: This FAQ describes active noise control, a novel method for controlling unwanted sound and vibration.
- X-Newsreader: News Xpress 2.0 Beta #0
- Keyword: active control, noise, vibration, sound, anti-noise, cancellation, FAQ
- Xref: senator-bedfellow.mit.edu alt.sci.physics.acoustics:4343 comp.dsp:27995 alt.answers:16396 comp.answers:17601 news.answers:67137
-
- Archive-name: active-noise-control-faq
- Posting-Frequency: monthly
- Last-modified: 1996/02/22
- Version: 1996-03-14
-
- -------------------------------------------------------------
- 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 typoÆs
- 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/active-noise-control-faq
-
- * 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/active-noise-control-faq,
- 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. DonÆt 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 todayÆs automobiles.
- Passive techniques work best at middle and high frequencies, and are
- important to nearly all products in todayÆs 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 1930Æs
- (see the Lueg patent mentioned below), and more development was done
- in the 1950Æs. 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 1970Æs and
- 1980Æs, 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 ------------
-
-
- ----
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