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This topic comprises 3 pages: 1 2 3
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Author
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Topic: Room levels
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Frank Angel
Film God
Posts: 5305
From: Brooklyn NY USA
Registered: Dec 1999
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posted 09-17-2004 04:27 AM
Years ago when THX was first coming on the scene, I read somewhere, most probably in an SMPTE article or a white paper at one of the conventions, that THX's first preference for split surrounds was with a point-of-source configuration -- single, albeit hefty, speaker systems positioned in rear corners of the room. Multiple, smaller side speakers were THX's second choice for the surround source. Any one ever hear that? Unfortunately I can neither find any reference to that anywhere, or know of anyone who espouses that configuration. I know I read it because it was based on that concept that I put two JBL double bins with their 90* constant Q horns hanging in each corner of our big theatre, about 20ft from the floor and abandoning the smaller column arrays that were in the traditional "along the wall" configuration. I must say, the corner setup delivered a much greater sense of "surround," with localization coming from behind you rather than from the side of you. And of course with such big mutha bins, there was never any problem getting really deep, vibrate-your-gonads sound from the surround tracks, even with analog.
I also read an article, again, more than likely in one of the white papers that Dolby first submitted to the SMPTE in the very first days of their introduction of stereo optical, that "ceiling surrounds" were an acceptable surround channel configuration. The Brooklyn Museum has that setup, not by design, but by the fact that their PA uses ceiling speakers, elaborately delayed, for their non-film sound system. For film, there was no surrounds at all. I figured, cieling speakers were better than no surround at all, so I hijacked the PA to reproduce the surround channel when we played film. And of course it brought to mind that I recall Dolby saying it's OK. But again, I can't verify where that was ever stated. Anyone ever hear that? Now I am not talking about that recent idea of an overhead channel (for that ear in the top of your head). This goes back to the mid-70s when Dolby first talked about their new system.
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 09-17-2004 08:10 AM
Frank, I think your memory is playing tricks on you. Ceiling speakers never were regarded as an acceptable surroud channel due to beaming effects resulting from how a ceiling speaker projects. This is not to say that ceiling speakers were not used...but they were never recommended by Dolby or THX and I doubt you will ever be able to find any literature that states otherwise. You may wish to consult our search master, John P.
In Dolby's "Technical Guidelines for Dolby Stereo theatres" circa 1994...on page 20 section 2.3.2 paragraph 2, it states:
"Selection of a suitable type of loudspeaker, thought, demands of assessment of more than just power handling capability. Diffusion is a major requirement of surround channels, meaning that surround signals should never appear to come from a point source. This means that a large number of loudspeakers are always preferable to a few, regardless of power handling."
THX has never, for theatres, allowed a point source system and I doubt you will find any THX certified theatres that have just a pair of surround speakers in the rear. In fact, they tend to go to the extreme with speakers spaced about every 4-6 feet.
For the home, THX has traditionally proclaimed to use dipole speakers mounted on the sides of the room. The dipoles are aimed to point kinda forward and behind to create a diffuse surround field with just two units...however they do not resemble a point source either.
Steve
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Frank Angel
Film God
Posts: 5305
From: Brooklyn NY USA
Registered: Dec 1999
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posted 09-19-2004 12:14 PM
Brad, seems like yours is a VERY common complaint and has been discussed here before....you might want to do a search to see if it comes up and what the consensus was -- I can't remember exactly. Me, I don't worry about that magic Number Seven. To me, it's all relative with the room playing a BIG role in the perceived loudness, as does the soundtrack mix itself, as does even the age of the audience. That's why they put a fader there and didn't lock it down at a prefixed 85dB level. I guess in theory, there should be no need for a volume control -- just like in theory, there shouldn't be any need for a Framing Knob on the projector as some techies here espouse. But we live in an imperfect world. Forget about the #7. If you aligned your system correctly and it's too loud at that setting, don't fret (everything else being equal -- like clean sound without distortion)....just turn it down. In the end, it's the customers who paid for the tickets, not Ray Dolby (although he sure does get his cut indirectly, eh?). If you want the paying public to come BACK to buy more tickets, then the final issue is that it has to sound good to them, not meet some arbitrary technical benchmark. Of course that is technical heresy, and a techie will pull his hair out and go gray in order to figure out why he's getting sound that is too loud at what the manual says is the "correct" setting, but not to worry about him....that's his job -- it's what make him a techie. You just relax and set it where it makes your audience happy.
And Steve, thanks for the info. But I question the Dolby position circa 1994 -- obviously they did published that, but my recollection is from much earlier with the very first white papers that came out around 1975-76 when they first described the stereo optical system. They very well may have changed that concept down the line, but I do believe they mentioned ceiling configuration as an acceptable, although perhaps not their first choice, way of dealing with the surround channel. Just like in those first descriptions unveiling of the system, the paper said that the matrix they used was the Sansui Vario Matrix chip; after those initial papers, obviously that changed and you never heard them mention Sansui -- after that it was they who invented phase dependent matrixing. And I agree that the problem with ceiling speakers not giving adaquate coverage might be valid in 99% or the cases, but in this particular room, the PA was computer designed so that each speaker's optimum "cone" of coverage overlaps the next with mulitple delay zones so that each seat has uniform coverage. It's main fault is that I can't get them up to 85db without stressing them, so threshold compression keeps them from cracking at the loudest portions of the soundtrack whilst at normal levels the effect is every effective. It's either this or no surround at all, which is the way the installer left it when the museum said, no speakers on the walls (museums are notorious for that kind of "artistic" snobbery).
As for the point source configuration; I won't press that until I can find where I read it, but I know it was published because the system that we set up in the large theatre was based on that recommendation and second by the acoustical engineer firm of no less pedigree than Klepper & Klepper (redesigned Alice Tully Hall at Lincoln Center) installed it. Whether or not that early on (our system was installed in the early 80s) the discussion of point source was preliminary and THX may have subsequently modified or completely revamped its recommendations in favor of multiple sides, I will not dispute. In fact, I will even give you that point source might have been discussed in an SMPTE Journal article as an alternative to side surrounds with THX mentioned and I am linking the two, and I will agree that to that extent my memory may be playing tricks on me, but there is no doubt that there was such a discussion.
In any event, the corner surround have worked exceptionally well in our theatre, easily matching the SPL and characteristics of the screen channels and localizing surround from behind the audience when it needed to be dominant as in a plane fly-by; yet when the mix was intended to be difuse ambience and it was mixed subtily, the corner surrounds presented that effect quite nicely too. It especially became apparent how well corner surrounds recreate rear stereo with digital tracks. When a plane flies from the right side of the screen to BEHIND the audience left, it sounds exactly like that -- like it has flown behind them, rather than vaguely to the left SIDE of them. Localization is in the back.
We also extended the corner surrounds to the back wall with two speaker clusters (six custom boxes each with URIE drivers) directly behind the audience. That insures that the center rows get enough surround to localize it -- again, from BEHIND them and not from the side of them. We will be able to use these center clusters for the EX surround channel, should we ever get the decoding hardware -- although my gut tells me that an extra outboard CAT150 should be able to decode center surround easily enuf.
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