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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).  


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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.  

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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]

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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

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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

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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/


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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:  

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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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