From: chrisc@gold.gvg.tek.com (Chris Christensen)
Newsgroups: rec.audio
Subject: Re: Surround speakers--Out of phase vs. tradit
Date: 7 Jan 93 19:50:26 GMT
Organization: Grass Valley Group, Grass Valley, CA

In article <lkmhgeINN8i5@exodus.Eng.Sun.COM> klein@napali.Eng.Sun.COM writes:
>I am experimenting with surrounds myself after having bought a new
>ProLogic receiver (Denon ACR-2000).  I have looked into the ProLogic
>theory and tried to wangle as much info as I could about THX from LucasFilm
>(not very rewarding) and looking at THX speakers in stores.

>The info points in the following general directions:

>A. the best surround effects are achieved when *ALL* speakers have similar
>		sonic qualities, particularly frequency response
>B. surround speakers should set up a non-directional "ambience" effect
>		that surround the listeners from the rear and/or side.

I must interject here.....  THX it's concept, implementation and
folklore is widely mis understood.

THX is nothing more then a set of specifications.  The THX scheme has
been commercialized like "Dolby B'  or "HXPRO"

Equipment that bears the THX symbol only meet Tom's specifications.
The use of bi-polar or di-polar or that ever description of that type
of speaker system doesn't make it THX.  It just meens that Tom OK'ed
the idea.

Now back to your discussion.  Both of your comments A and B are true
statements and i stand with you in your assertions (I will only stand
an arms length away because I am usually shot at a lot! :-)

>Achieving B in THX is typically done by having each surround speaker
>actually radiate the sound in opposite directions (front and back) out
>of phase with each other.  The ear apparently has great trouble
>locating the source of the sound when this is done.  Then populate the
>side and/or rear of the listening area with as many of these as you can
>(2 is typical; 1 alone actually works; fancy THX setups have more, like
>4-8 pairs in big theaters).

This is the paragraph that set me off, specifically the first sentence
"Achieving B in THX........."  Like THX is some sort of new found
level of consciousness!  

I apologize if I have come out of the gate strong, you are on the
right track.

I recently saw Aladdin in a theater that was set-up for Dolby Stereo.
The theater had side only speakers.......

I have posted and discused, at length, the merits of surround and what
the best implementation is.  I don't like side only speakers and
I didn't like the way the surround at this theater.  BTW the side
speakers in this theater were normal speakers, on the wall, pointing
at the audience; th wrong way to do it (see below).

I do like the way that the di-polar speakers work because they have
the best posiblity of making the surround part of of the home theater
work!  But not for, or in the way that people usually use them.

For the "normal" home theater,  I suggest that speakers be located on
the sides of the room approximately equal to the listening position.
These "side speakers" can be of the di-polar type or regular speakers
with the front of the speaker pointed to the back wall.  In adition to
that one or two (depending on the size of the room) identical speakers
placed on the back wall with the front facing toward the listening
position.

This configruration will provide the requirement B in your opening
statement.  It should also be noted that the rear wall would best
serve the theater enviornment if the wall was, acoustically, a
diffuser.  A wall to wall book case is kind of nice for this
application.

Now back to the "real" theater and what is done there.

In a large theater (deep) it is necessary to use speakers on the
sidewalls to carry the surround effect for the length of the room.
One of the speakers for this application is the dipolar because it will
reinforce the surround from the rear.  The dipolar speaker will create
a series of nulls at the location of each speaker as well as at the
space between the speakers.

If the theater go'er sits at a position in front of a dipolar speaker
s/he will hear surround from behind.  s/he will not hear the next
speaker up because of the cancellation from the one behind it!

And so the story goes, draw up a simple sketch for yourself,  use ='s
and -'s for in phase and outa phase.

It is my firm belief that the best implimentation of surround in a
deep room will incorporate side wall mounted speakers,  but they can
be conventionla speakers pointed forward and slightly towards the
center of the room.

-- 
Chris Christensen                  The opinions I express are my own,
chrisc@gold.gvg.tek.com            and sometimes they are wrong!
916-478-3419 FAX 916-478-3887      After all, I *AM* only human.
Newsgroups: rec.audio