🚀 go-pugleaf

Archive-name: active-noise-control-faq
Posting-Frequency: monthly
Last-modified: 1995/04/11
Version: 1995/05/08

-------------------------------------------------
Frequently Asked Questions:  Active noise control
-------------------------------------------------

SUMMARY:

The FAQ you are now reading discusses active noise control, a 
novel application of basic physics that permits noise control not 
achievable by other means.  What is an FAQ, you say?  The 
Internet supports thousands of "newsgroups" -- discussion forums 
covering every imaginable topic.  An FAQ (Frequently Asked 
Questions list) is a summary designed to answer specific 
questions that arise repeatedly during discussion in the 
newsgroups.  This particular FAQ was written for the newsgroups 
alt.sci.physics.acoustics and comp.dsp, which focus on acoustics 
and digital signal processing, respectively.  This FAQ has four 
purposes:

  *  Provide concise answers to common questions about active 
noise control, thereby minimizing redundant messages 
and reducing network traffic.  
  *  Dispel popular misconceptions about what active noise 
control can and cannot do, and disseminate accurate 
information in an easily understood format.
  *  Refer interested readers to magazine articles, technical 
references, and other sources of information on both 
active noise control and general acoustics topics.  
  *  Help stimulate public interest in acoustics.


CONTENTS
1. Introduction
  1.1. Administrative trivia
  1.2. What's new in the Active Control FAQ
  1.3. Finding the most recent FAQ
  1.4. Contributors
  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. Are there different kinds of active control?
  2.3. Is active noise control like noise masking?
  2.4. How can adding sound make a system quieter?
  2.5. When does active control work best?
  2.6. What is adaptive active control?
  2.7. What are some typical applications?
  2.8. What are the benefits of active control?
  2.9. Is active control new?
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. Administrative trivia

Copyright (c) 1994,1995 by Christopher E. Ruckman

All rights are reserved.  Permission is hereby granted to use, 
copy and distribute this unmodified document for any purpose 
EXCEPT PROFIT PURPOSES, provided that both the above Copyright 
notice and this permission notice appear in all copies of the FAQ 
itself.  Reproducing this FAQ 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, unless upon explicit prior written 
permission of the author. 

Disclaimer:  This document does not necessarily represent the 
opinion of the US Government, nor anyone else except the author.  
Specifically, 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 FAQ "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 at your own risk.  

- - - - - - - - - - - - - - - - - - - - - - -
Subject:    1.2. What's new in the Active Control FAQ

     Version:   What was added/changed:
  *  04/11/95   added info on newsletter
  *  03/03/95   cross-posted to *.answers
  *  02/24/95   expanded introduction, revised format, added basic 
		definitions
  *  02/23/95   added new references from popular literature; added 
		info on short courses
  *  01/24/95   cross-posted to comp.dsp
  *  12/22/94   revised list of applications
  *  12/12/94   added new references
  *  10/04/94   expanded description of destructive interference 
		vs. impedance coupling; corrected several typos; 
		made minor revisions throughout
  *  06/14/94   initial release.

- - - - - - - - - - - - - - - - - - - - - - -
Subject:  1.3. 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:  subscribe to alt.sci.physics.acoustics, comp.dsp, 
sci.answers, alt.answers, comp.answers, or news.answers
  *  Anonymous ftp:  rtfm.mit.edu in 
/pub/usenet/news.answers/active-noise-control-faq
  *  Email: mail-server@rtfm.mit.edu (send 
usenet/news.answers/active-noise-control-faq)
  *  WWW:  (soon, stay tuned)

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 them to 
alt.sci.physics.acoustics.  In particular, please contribute the 
following:
  *  Universities with active control research programs
  *  Companies that produce active control products
  *  Interesting references from the general literature
  *  Comments from readers who do not know much about acoustics

This FAQ may be cited as:

Ruckman, C.E. (1995) "Active Noise Control FAQ," Usenet 
news.answers. Available via anonymous ftp from rtfm.mit.edu in 
pub/usenet/news.answers/active-noise-control-faq. 

- - - - - - - - - - - - - - - - - - - - - - -
Subject:  1.4. Contributors
The following people contributed to the discussions upon which 
this FAQ is based:  
  *  ruckman@oasys.dt.navy.mil (Chris Ruckman)
  *  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)


- - - - - - - - - - - - - - - - - - - - - - -
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 propagate in three dimensions, and  
  *  the wavespeed 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 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 
both frequency and wave speed:  lower frequencies have longer 
wavelengths and faster wave speeds.  

In this context, noise is simply *unwanted* sound.  In other 
words, you should answer "NO" to the old trick question "When a 
tree falls in the forest and nobody is there to hear it, does it 
make any noise?"  You might question whether or not the tree 
makes "sound" when it falls, but the sound cannot be "noise" 
unless someone hears it and finds it offensive.  


=============================================
Subject:  2. General discussion of active control

- - - - - - - - - - - - - - - - - - - - - - -
Subject:  2.1. What is active control of noise/vibration?

The question is usually posed in the following form:  "I heard 
about a new noise control technology called active something-or-
other ... can I use it to make my house quiet when the neighbor's 
kid plays 'Black Sabbath' on his electric guitar?"

The buzzword for 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.  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 as follows:  

  *  The plant is the physical system to be controlled, such as a 
headphone and the air around it, or air traveling down 
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, shakers, or 
piezoelectric elements.  
  *  The controller is a signal processor (usually digital) that 
tells the actuators what to do based on the sensor 
signals and some knowledge of the plant dynamics.  

Analog controllers may also be used, although they are somewhat 
less flexible and thus more difficult to use.

- - - - - - - - - - - - - - - - - - - - - - -
Subject:  2.2. 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; 
examples are mentioned below.  On the other hand, if the noise is 
radiated 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.  

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.  

- - - - - - - - - - - - - - - - - - - - - - -
Subject:  2.3. 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 noise to either  a) make noise less noticeable, 
or  b) reduce the chance of overhearing conversations in 
adjoining rooms.  In active noise control, the actuators seek not 
to mask offending sound but to eliminate it.  Masking increases 
the overall noise level; active control decreases it.  

- - - - - - - - - - - - - - - - - - - - - - -
Subject:  2.4. 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:

Suppose you're standing before a spring-loaded door, one that 
opens a few centimeters when you push on it but closes 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:  2.5. 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 decays; it should not be confused with "dampening", 
which describes whether the system is wet!)  

- - - - - - - - - - - - - - - - - - - - - - -
Subject:  2.6. 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:  2.7. 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.  Used extensively by pilots, 
active headphones are considered indispensable in helicopters and 
noisy propeller-driven aircraft.  Originally selling for $1000 or 
more some years ago, some active headphones may now be purchased 
for under $200.  

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 at the 
locations of the drivers and passengers ears.  Lotus produces 
such a system for sale to other automobile manufacturers; unit 
cost is a major consideration for automobile use.  One vehicle 
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:  2.8. 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:  2.9. Is active control new?

The idea of active noise control was actually conceived in the 
1930s (see the Lueg patent), 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:  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?

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 
components of active noise control systems, and even more 
companies and universities that conduct research and development 
programs in active control.  The companies listed below are 
companies that produce commercially available products intended 
specifically for active noise control.  Please suggest others as 
appropriate!  

  *  Active Noise and Vibration Technologies, Inc.  [Editorial 
note: ANVT is no longer in business. --cr]  
  *  Active Vibration Control Instrumentation, PCB Piezotronics, 
Inc., 3425 Walden Ave. Depew, NY 14043-2495, phone 716-
684-0001
  *  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)
  *  Digisonix, Inc., 8401 Murphy Drive, Middleton, WI 53562-2243 
USA, phone 608.836.3999 (Pat Lyke, Marketing)  
  *  dSPACE Inc., 26677 W. Twelve Mile Road, Southfield, Michigan 
48034, 810.354.1694
  *  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, Sony, others

- - - - - - - - - - - - - - - - - - - - - - -
Subject:  3.3. What universities teach 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
  *  Duke University, Durham, North Carolina, USA
  *  Georgia Institute of Technology, Atlanta, Georgia, USA
  *  Norwegian Institute of Technology, Trondheim, Norway
  *  Massachusettes Institute of Technology, Cambridge, 
Massachusetts, USA
  *  Old Dominion University, Norfolk, Virginia, USA
  *  Pennsylvania State University, State College, Pennsylvania, 
USA
  *  Purdue University, West Lafayette, Indiana, USA
  *  University of Salford, England
  *  Southampton University, Southampton, England
  *  Technical University of Denmark, Denmark
  *  University of Adelaide, Adelaide, South Australia, Australia
  *  Universite de Sherbrooke, Sherbrooke, Quebec, Canada
  *  Universite de Technologie de Compiegne, Compiegne, France
  *  Virginia Polytechnic Institute & State University, 
Blacksburg, Virginia, USA
  *  (In Germany):  TU Berlin, TU Munich, TU Stuttgart, RWTH 
Aachen, U Goettingen, U Hamburg, TU Erlangen, and 
others...


- - - - - - - - - - - - - - - - - - - - - - -
Subject:  3.4. How can I learn more via Internet?

Other than the FAQ you are now reading, there are no Internet 
resources dedicated solely to active control.  

There are some resources that deal with general acoustics and 
vibration topics:

  *  If you have access to USENET newsgroups, check out the 
following newsgroups:
	alt.sci.physics.acoustics (general acoustics)
	comp.dsp (digital signal processing)
	rec.audio (audio reproduction equipment; also see 
		other rec.audio.* newsgroups)
	alt.binaries.sounds.misc (recorded sounds; also see 
		other alt.binaries.sounds.* newsgroups)
  *  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?

Short courses and seminars, offered periodically by a variety of 
sources, provide a quick way to learn more about active control.  
There are many.  Some are listed below.  [Editor's note:  Please 
help me expand this list.  CR]

Active Control of Sound & Vibration
Flotow & Associates, 1750 Country Club Road, Hood River, Oregon, 
97031-9641, USA

- - - - - - - - - - - - - - - - - - - - - - -
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 
alt.sci.physics.acoustics.

Note:  %A=author, %T=title, %J=journal, %D=date, %V=volume, 
%N=number, %P=pages, %X=comments

%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 Swiss firm a jump on competitors

%T Developments to watch 
%J Business week 
%D JUN 07 1993 
%N 3322 
%P 103 

%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 Robinson, Gail M.
%T DSPs hit the road: from active suspensions to acoustic sound, 
going digital means real-time dynamics. (digital signal 
processors in automobiles) (Roll Out The '93s])
%J Design News
%P 128
%D Oct 5  1992 
%V 48 
%N 19
%X Automobile manufacturers are researching and applying the use 
of DSPs in automobile entertainment/cellular phones, engine 
control, anti-lock brakes, active suspension and noise 
cancellation. Results of their efforts are described.

%A Morantz, Alan
%T The quiet revolution. (active noise control system)
%J Canadian Business
%P 55
%D June  1992  
%V 65  
%N 6
%X Macmillan Bloedel Ltd. employed a unique method to control 
noise from the vacuum pump exhaust system of its pulp and paper 
mill in Port Alberni, Canada. They sought the services of Nelson 
Industries Inc. in installing an active noise control system.

%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.

%A Sedgwick, John 
%T Cut out that racket 
%J The Atlantic 
%D Nov 1991 
%V 268 
%N 5 
%P 50 

%T Infiniti Q45 
%J Motor trend 
%D MAR 01 1991 
%V 43 
%N 3 
%P 76 
%X Active suspension ups the mother lode

- - - - - - - - - - - - - - - - - - - - - - -
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, %T=title, %J=journal, %D=date, %V=volume, 
%N=number, %P=pages, %X=comments

%A Nelson, P.A.
%A Elliott, S.J.
%B Active control of sound
%P 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 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 by Christopher E. Ruckman
---------- end of the Active Noise Control FAQ ------------
InterNet: ruckman@oasys.dt.navy.mil     HeyYouNet: Chris Ruckman, Ph.D.
VoiceNet: 301.227.1768                  SneakerNet: Building.3.Room.352C
StampNet: CDNSWC.Code.725.Bethesda.MD.20084.5000   FaxNet: 301.227.4405

-- 
InterNet: ruckman@oasys.dt.navy.mil     HeyYouNet: Chris Ruckman, Ph.D.
VoiceNet: 301.227.1768                  SneakerNet: Building.3.Room.352C
StampNet: CDNSWC.Code.725.Bethesda.MD.20084.5000   FaxNet: 301.227.4405