From: daniel@redwood.ppc.ubc.ca (Daniel Ouellet)
Newsgroups: sci.engr.control,sci.engr.mech,sci.physics,sci.engr.civil
Subject: Re: factory noise elimination?!
Date: 23 Nov 1992 17:42:16 GMT
Organization: University of B.C. Pulp & Paper Centre
Distribution: world
NNTP-Posting-Host: redwood.ppc.ubc.ca

In article <22NOV92211553@fnalp.fnal.gov>, boswell@fnalp.fnal.gov (Chris Boswell JHU CDF (708)840-3638) writes:
|> meyers@lorenz.ocean.fsu.edu (Steve Meyers) writes...
|> >announced an ability to reduce or eliminate noise associated with 
|> >industrial production with a technique that "uses electricity". 
|> >I guess their claim is they can make a factory quiet. Has anyone 
|> >heard about this? Is it true? 
|> 	Yes, this is true.  Sound waves are waves, and
|> can be interfered with, both constructively and destructively.
|> If I have a speaker system such that sound waves are exactly
|> out of phase with the factory motor, the sound waves are cancelled.
|> You can either do this right in front of your ear, or on a surface
|> around the noisy motor.  To do it well with today's audio technology,
|> the motor has to have a very periodic sound signature,
|> which most motors do.
|>   I am afraid I cannot find the articles I have about it,
|> but search the periodical guide in a library for
|> active noise elimination or electronic mufflers.

While active noise control is feasible in theory, it is in practice very difficult
to implement in situations where the sound field is complex. In most industrial
environments, it is difficult to characterize the sound field  precisely because
sources have a complex radiation pattern and you have to deal with reflexions
and diffusion due to obstacles, etc, etc,etc.

The idea to cancel the noise "right in front of your ear" is also nice in theory
but pretty hard to realize in practice. The sensitivity of the ear varies depending
on the direction from which the noise is coming. Its frequency response is not
uniform and varies between individuals. The ear has a "protection mechanism"
known as the acoustic reflex, which changes its acoustic impedance under
prolonged noise exposure. Also consider that if you want to cancel out a wave
that will hit the ear, you have to detect and characterize this wave before it 
reaches the ear, whatever the direction it is coming from.

I could go on and on, but I think this is enough to make my point. The problem
is a very complex one, and this thing is frequently brought up as a sensational
technique, but we are far from being able to apply it on a large scale basis to
industrial noise problems. I have heard of a few applications in simple cases
(not factories) but I cannot say if these were only rumors. I have not yet seen
a paper in a well recognized acoustics journal describing a commercial
application. I would suggest you post to alt.sci.physics.acoustics if you want
to get more feedback on this. There was a thread on this subject some time
ago.

-- 
----------------------------------------------------------------------
Daniel Ouellet, Research Associate
Pulp and Paper Centre, University of British Columbia
2385 East Mall, Vancouver, BC, Canada V6T 1Z4
Tel:(604)822-8240   Fax:(604)822-8563   Email: daniel@ppc.ubc.ca