textfiles/bbs/KEELYNET/KEELY/geo180.asc

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The following are messages relating to phasing and where the energy goes.
------------------------------------------------------------------------------
From: Visor@globalcom.net
Subject: To phase or not to phase?
Date: Sat, 06 May 95 22:59:26 PDT
Organization: GlobalCom
Lines: 11

This is a simple question that was asked to me by a scientist working on a
problem dealing with geology. I gave a quick answer but thought it might be
fun to see what other would say.

"If two waves, sound or EM are combined out of phase with a net sum of 0, what
 happens to the energy in the waves?"
------------------------------------------------------------------------------
From: kplotkin@access5.digex.net (Kenneth Plotkin)
Subject: Re: To phase or not to phase?
Date: 7 May 1995 00:17:19 -0400
Organization: Express Access Online Communications, Greenbelt, MD USA
Lines: 25

In article <NEWTNews.10515.799826717.visor@globalcom.net.globalcom.net>,
 <Visor@globalcom.net> wrote:

[snip]
>" If two waves, sound or EM are combined out of phase with a net sum
>of 0, what happens to the energy in the waves?"

That was asked here not too long ago.  The only way that two waves could add
up to zero everywhere would be if their sources were coincident.  In that
case, for coincident out-of-phase sources, the energy is zero to begin with.
(Think of one-dimensional acoustic waves in a tube with a piston at the end.
Or a piston at either end.  Zero sum means zero pressure on the piston(s),
hence zero work.)

Local cancellation increases the energy elsewhere.  Think of a lens coating,
where the reflection from the coating is out of phase with the reflection from
the lens itself.  The net reflection is reduced, hence increasing the light
transmitted through the lens.

Arthur C. Clarke wrote a delightful short story (forget the title, but I
believe it's in his "Tales From the White Hart" collection) about the
consequences of sound cancellation where the net energy was not all preserved
as sound.

Ken Plotkin
------------------------------------------------------------------------------
Subject: Re: To phase or not to phase?
Date: Tue, 9 May 1995 05:11:02 GMT
Organization: Monash University
Lines: 47

In article <3oh5bu$sc1@quiknet3.quiknet.com> Steve Rickman
<stever@quiknet.com> writes:
>From: Steve Rickman <stever@quiknet.com>
>Subject: Re: To phase or not to phase?
>Date: 7 May 1995 00:47:39 GMT

>>
>>
>> This is a simple question that was asked to me by a scientist working
>> on a problem dealing with geology. I gave a quick answer but thought
>> it might be fun to see what other would say.
>>
>> " If two waves, sound or EM are combined out of phase with a net sum
>> of 0, what happens to the energy in the waves?"
>>
>>
>Boy, this comes up a lot.

>You cannot get perfect cancellation *everywhere* of two waves unless
>the sources are identical in directivity and content, 180 degrees
>out of phase and superimposed in space. The last condition is the
>key. If these otherwise perfectly cancelling sources are not in the
>same location, then there will be regions of cancellation and regions
>of reinforcement. If you integrate the energy density over the
>entire field then the result will be just twice what it would be
>if only one of the sources were radiating. Energy density, however,
>will be "lumpy," varying from point to point throughout the field.

>So can we actually superimpose the sources in space? Well, how do
>you do that? Suppose we're talking about sound and you want to put
>two speakers in exactly the same location. Since you can't physically
>do that, you choose the next best thing and drive one speaker with
>two perfectly cancelling electrical signals. Net result? The speaker
>cone does not move!

>But now at least we can see what happens to the energy. The amplifiers
>generating the two perfectly cancelling signals are forced to absorb
>it, turning it into heat.

>Well, that's a lot of hand-waving. If you want to direct your
>scientist (?) to a good introduction to this subject, refer him to
>"Active Noise Control," in IEEE Signal Processing Magazine,
>October, 1993.

>Steve
 What about 2 pulses one the inversion duringf overlap the energy is
 is diminsihing to zero (perfect overlap) and then increases again. So where
 does the energy go during this period?
------------------------------------------------------------------------------
From: acampane@postbox.acs.ohio-state.edu (Angelo Campanella)
Subject: Re: To phase or not to phase?
Date: Tue, 9 May 1995 05:37:00 GMT
Organization: The Ohio State University
Lines: 31

In article <NEWTNews.10515.799826717.visor@globalcom.net.globalcom.net>
Visor@globalcom.net writes:
>From: Visor@globalcom.net
>Subject: To phase or not to phase?
>Date: Sat, 06 May 95 22:59:26 PDT

>This is a simple question that was asked to me by a scientist working on a
 problem dealing with geology. I gave a quick answer but thought it might be
 fun to see what other would say.

>"If two waves, sound or EM are combined out of phase with a net sum of 0,
  what happens to the energy in the waves?"

The energy is reflected to be elsewhere.  For istance, it could be sent back
from whence it came.  Just in front of a mirror, the energy flow is the same
away from and towards the mirror.  In the case of standing waves, where the
pressure is zero (from opposite pressure waves cancelling), the velocity is
at is maximum; a transformation from potential energy to kinetic energy.

So that the "cancellation" is only in one (of many possible) energy form.

Ang.
      /\/\/\/\/\/\/\ Sound Technology /\/\/\/\/\/\/\/\/
------------------------------------------------------------------------------
From: andy@moose.mv.com (Andy Borsa)
Subject: Re: To phase or not to phase?
Sender: usenet@mv.mv.com (System Administrator)
Organization: RF Design Consultant
Date: Wed, 10 May 1995 21:36:47 GMT
Lines: 13

In article <D8CHsL.73z@dio.dod.gov.au>, jrw@dio.dod.gov.au (Jerry Williamson)
says:
>
>consider that the average apmlitude of a sine wave is zero, it is only the
>insternaious apmlitude that causes the effects that can be measured. So if
>you add zero to zero you get .... zero. think about that one for a bit.
>
>                                jerry

Actually, the median over 1 cycle is 0.  The heating power (RMS) is (0.7071 x
peak)^2/R.  If that weren't true then we'd have no need for all those power
transmission lines.

    Andy Borsa -- !!!The universe is discretely analog!!!
------------------------------------------------------------------------------
From: Steve Rickman <stever@quiknet.com>
Subject: Re: To phase or not to phase?
Date: 7 May 1995 00:47:39 GMT
Organization: Quiknet Information Services
Lines: 39
>
>
> This is a simple question that was asked to me by a scientist working
> on a problem dealing with geology. I gave a quick answer but thought
> it might be fun to see what other would say.
>
> " If two waves, sound or EM are combined out of phase with a net sum
> of 0, what happens to the energy in the waves?"
>
>
Boy, this comes up a lot.

You cannot get perfect cancellation *everywhere* of two waves unless
the sources are identical in directivity and content, 180 degrees
out of phase and superimposed in space. The last condition is the
key. If these otherwise perfectly cancelling sources are not in the
same location, then there will be regions of cancellation and regions
of reinforcement. If you integrate the energy density over the
entire field then the result will be just twice what it would be
if only one of the sources were radiating. Energy density, however,
will be "lumpy," varying from point to point throughout the field.

So can we actually superimpose the sources in space? Well, how do
you do that? Suppose we're talking about sound and you want to put
two speakers in exactly the same location. Since you can't physically
do that, you choose the next best thing and drive one speaker with
two perfectly cancelling electrical signals. Net result? The speaker
cone does not move!

But now at least we can see what happens to the energy. The amplifiers
generating the two perfectly cancelling signals are forced to absorb
it, turning it into heat.

Well, that's a lot of hand-waving. If you want to direct your
scientist (?) to a good introduction to this subject, refer him to
"Active Noise Control," in IEEE Signal Processing Magazine,
October, 1993.

Steve
------------------------------------------------------------------------------
From: IANM@staff.monash.edu.au
Subject: Re: To phase or not to phase?
Date: Tue, 9 May 1995 05:11:02 GMT
Organization: Monash University
Lines: 47

In article <3oh5bu$sc1@quiknet3.quiknet.com> Steve Rickman
<stever@quiknet.com> writes:
>From: Steve Rickman <stever@quiknet.com>
>Subject: Re: To phase or not to phase?
>Date: 7 May 1995 00:47:39 GMT

>>
>>
>> This is a simple question that was asked to me by a scientist working
>> on a problem dealing with geology. I gave a quick answer but thought
>> it might be fun to see what other would say.
>>
>> " If two waves, sound or EM are combined out of phase with a net sum
>> of 0, what happens to the energy in the waves?"
>>
>>
>Boy, this comes up a lot.

>You cannot get perfect cancellation *everywhere* of two waves unless
>the sources are identical in directivity and content, 180 degrees
>out of phase and superimposed in space. The last condition is the
>key. If these otherwise perfectly cancelling sources are not in the
>same location, then there will be regions of cancellation and regions
>of reinforcement. If you integrate the energy density over the
>entire field then the result will be just twice what it would be
>if only one of the sources were radiating. Energy density, however,
>will be "lumpy," varying from point to point throughout the field.

>So can we actually superimpose the sources in space? Well, how do
>you do that? Suppose we're talking about sound and you want to put
>two speakers in exactly the same location. Since you can't physically
>do that, you choose the next best thing and drive one speaker with
>two perfectly cancelling electrical signals. Net result? The speaker
>cone does not move!

>But now at least we can see what happens to the energy. The amplifiers
>generating the two perfectly cancelling signals are forced to absorb
>it, turning it into heat.

>Well, that's a lot of hand-waving. If you want to direct your
>scientist (?) to a good introduction to this subject, refer him to
>"Active Noise Control," in IEEE Signal Processing Magazine,
>October, 1993.

>Steve
 What about 2 pulses one the inversion duringf overlap the energy is
 is diminsihing to zero (perfect overlap) and then increases again. So where
 does the energy go during this period?
------------------------------------------------------------------------------