> a waste in this case other than a common system sub. I haven't read the
 > AC-3 spec enough to know what is going on there, but suspect that the
 > same idea has been carried over. I do know Dolby Stereo fairly well and
 > am certified by them in Cinema.
     AC-3 has full bandwidth stereo surround channels.  However, AC-3
is also designed to be able to direct the bass in those channels to
either a single subwoofer, dual subwoofers, the main speakers, or 6
discreet subwoofers (or 5 full channel speakers and a sub).  It's
designed to be easily configured for the equipment you have or want
to use in those terms.  The question becomes, what is the *best*
configuration, if you're starting from the groundup?  The people at
THX, Audio, and Stereo Review all agree a single subwoofer for all
channels with a steep crossover at approx. 80Hz or so, along with
speakers capable of flat response to at least that low for the
remaining 5 channels will work best.  It is also generally agreed
that Dipoles are still the best way to go for Cinema surround
sound, even with stereo surround.
     The soundtracks are still mixed for theaters which generally have
diffuse surround sound (in the form of large arrays of direct radiators
which aren't really suitable for the typical small home living room
environment).  Direct surrounds, from the tests I've read, seem to
provide a similar level of performance for the "sweet spot" with AC-3,
however, but do a poorer job for the rest of the seats.  Future
music-only discs could make direct firing rear speakers more
desirable, however, but that remains to be seen at this time.
     Considering the AC-3 processor I'm looking at (the new DX-1 from
Lexicon) and its hefty price tag and the fact I still need to buy the
amps for the rear channels and the center channel along with a center
channel speaker and a new laserdisc player, it may be awhile before I
get my AC-3 setup completed (I want to do it right or not at all).
     However, from some limited testing I've done with the speakers I
intend to use for surround (I bought them when they were on sale and
used them as my mains with the sub until I got the AL-IIIs), I think
bipoles might be a happy medium between the extremes of dipoles and
direct radiators for surround sound, given AC-3.  The movie industry
might like diffuse, but I think stereo surround deserves a little more
directionality.  Bipoles still provide a "null" when placed directly
to the sides of the main listening positions and diffuse the sound
well, yet they're not nearly "as null" as dipoles.  I've tested them
(The Definitive BP-2s) with a regular stereo feed used in a surround
type configuration and when placed with rear driver just at the front
edge of the listening position, and either 1/3 the distance from the
top or bottom of the room, they imaged to the sides and the rear and
when fed a mono signal provided a diffuse "I'm not sure where it's
coming from" sound as long as I stayed in that plane, which sitting
on the couch, wouldn't normally be a problem.
     So, I'm pretty confident they'll make a nice surround speaker.
Unlike dipoles, the bass is reinforced by the extra driver, so these
are rated down to 60Hz, even though they're very small.  That'll let
me use a lower crossover point in the future if I decide I want to
make use of the AL-III's woofers again sometime.  Your average dipole
surround is designed to go down around 80-120Hz depending on which one
you buy, so that's cutting it pretty close.  I've seen some new ones
that change phase as they approach the bass region to lower the bass
response (going from dipolar to bipolar), but I don't like screwing
around with phase like that in midspectrum.  I think a few might
also just use an extra bass driver, but the less drivers and
crossover points, the better as well as far as I'm concerned.  Either
type of speaker should work well with AC-3 though.  I hear the
dipoles become pretty directional, despite their reputation, when
fed a stereo surround signal.  In the same vein, the AC-3/DTS
sound at the theater images well even though the speakers are
designed to provide diffuse sound with normal Dolby so I guess it
shouldn't be a suprise that di-pole surrounds behave little
differently.
 >  While talking about locating the sub close to the listening position
 > you said:
 > 
 > -> Hovever, for several listeners (if seated out of the same plane as
 > -> the primary listening seat...say on a recliner in the corner or
 > -> something), the bass will not be as good there overall as it would
 > -> be in a corner, which provides the best even spread.
 > Evenness of coverage is important, but I consider getting the flare of
 > the corner to help couple the bass to a lower frequency is important,
 > too.
     True, but if your sub is already capable of 15Hz, placing close
to the listening position will give you a boost in efficiency that
won't compare to putting it in a corner.  I would also want to lower
the crossover to 60Hz or so, personally.  The further you put the sub
from the main speakers, the larger the fringe area in terms of
frequency where bass becomes non-directional.  40Hz is much safer than
80Hz, but it's hard to find a surround speaker that will go down to
40Hz.  Even in stereo, you'll be limiting your speaker choices.  I
guess it depends a lot on how big your room is.  You can buy bigger
subs to a point, but after a certain room size, buying more and bigger
subs could "slow" the bass response more than putting the sub
near-field.  So, I would guess there's a trade-off depending on the
conditions of the room you're putting it in and of course on available
funds. 
 > -> However, as a recording engineer, I don't suppose you have to worry
 > -> about such a listener. 
 > 
 > Caught the humor there, but on the other hand, in addition to being
 > able
 > to hear the original work as it is in fact laid on the recording media,
 > while mixing down, it is every bit as important to hear the music in
 > any
 > likely environment it will be played back in to give the mix validity
     True.  However, if what's on the recording is "true" to the
event, then I think it's the consumer's job to play catchup.
There *are* a number of recordings out there that cater more to what
the recording industry might consider an "average stereo" to make them
sound better on that stereo (or even on FM radio).  I can name several
in my own collection that sounded decent on my old system (as far as
decent went in those terms), but now sounds simply *horrific*.  The
original FIREHOUSE album is a good example.  It has its bass in the
bass and kickdrum regions boosted so much it hurts my ears at the same
levels I play all my "good" stuff at while at the same time the vocals
and lead guitars sound like a shouting in a thunderstorm...barely
distinguishable.  Most of Billy Joel's albums sound "fine" on my car
stereo but pretty bloody awful on my main stereo.  I find a *lot* of
the stuff from the 1980's is that way.  Lately, the newer albums have
begun to sound a *lot* better on average for mass market discs.  Yeah,
I've got some Japanese CDs from 1985-89 that sound *fantastic*, but
they're the exception I'm afraid.  Lately, though, it seems like most
of the stuff I've gotten from 94-96 or so has been pretty decent
sounding.  (e.g. The Mission: Impossible soundtrack sounds pretty
darn good overall.)
     Pretty soon Mark won't be able to harp on American CDs so
much....:)
     I still say the CDs limiting factor has been the mix-down
and handling in the post production processes.  You wouldn't be
hearing all the complaints about "CD sound" if all CDs sounded as
good as they *could*.  I've heard some darn good ones and I'd *love*
to see an LP fanatic produce an LP to match those ones.  
 > work so that all sound good that has to be done, or even better, have
 > the film sound mix, the video mix, the dance mix, the CD mix etc..
     That's fine with me as long as they don't mass market the "FM
mix" and the "mass market boombox mix" as one and the same CD for
everybody which happens all too often, I'm afraid.  Fortunately, this
doesn't seem to be happening as often.
 | AmiQWK 2.9 - FREEWARE |
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