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Active Noise Control FAQ v1996-03-14

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Archive-name: active-noise-control-faq
Posting-Frequency: monthly
Last-modified: 1996/02/22
Version: 1996-03-14

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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

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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 typoís
1994/06/14  initial release

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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.  

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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!

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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.  

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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.  Donít 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

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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 todayís automobiles.  
Passive techniques work best at middle and high frequencies, and are 
important to nearly all products in todayís 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.

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Subject:  2.2. Is active control new?

The idea of active noise control was actually conceived in the 1930ís 
(see the Lueg patent mentioned below), and more development was done 
in the 1950ís.  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 1970ís and 
1980ís, 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.  

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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      /    \              /


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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.  

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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.  

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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!)  

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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.  

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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) ----------

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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 

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