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This topic comprises 3 pages: 1 2 3
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Author
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Topic: Blown Subwoofer
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 07-11-2005 09:22 PM
quote: Bobby Henderson
I think the real term to apply to that sound system design is "cheap."
I didn't respond to John's remark about Frank Worthington but probably should have. The problem with "blaming" him is, 1, he did not post and claim anything and 2, we do not know the circumstances that lead to someone deciding on the current set up. Sure, it could have been due to cheapness, as you say. But did Frank recommend more sub/amp and the customer said "no way!"? Did Frank, knowing his customer, only spec a single 18" sub (one of the most powerful) so as not to lose a sale and "please" his customer. Or did Frank tell his customer that this set up was proper and all he needed...the customer trusting his consultant (Frank) just did what was recommended by a trusted contractor. It could have been a combination of the above too.
Unless one knows what transpired, one can not assign blame (or flame) to anyone.
quote: Bobby Henderson
But do they put in the sound systems beefy enough to do the job?
The problem there, I've found, comes in two forms...one, the exhibitor doesn't want to spend the money and just wants to boast "digital", as you say. The other, and seemingly most prevelent is a complete lack of understanding of the science of sound systems.
When selecting lenses, they go to one of the "slide rule" devices or the venerable Schneider Lens Calculator (many Kudos to Dwight Lindsey and the people of Schneider for developing this valuable tool and GIVING it away). People don't just order an 80mm lens because it is a 200-seat theatre. You'd laugh at someone that ordered lenses just based on seat count.
As mentioned...people tend to have their formulas on xenon lamp size versus screen size. I did know one guy that speced 2000-watt lamps for 35mm and 4000-watt lamps for 70mm. It may be a rule of thumb for him, but it makes no sense in reality.
When it comes to sound systems what is the exhibitor to do? Speaker manufacturers don't make it easy either. They classify their speakers in terms of generic room sizes (Large, Medium and Small)...sometimes they assign seat ranges. Then how is one to know how big an amp to use?
Then there is the fact that sound moves in a logarithmic way...it isn't intuitive to most. The room is only 20-feet longer but it takes much more power to amplify. 3-dB doesn't sound that much louder but DAMN it takes twice as much power...and what is this dB business? there are suffixes and it seems to change depending on contex. Digital added 12dB to the output requirements of the stage channels and full bandwidth to the surrounds. The subwoofer has to deal with 22dB more than analog and most didn't have subwoofers in the Dolby-A era. 12dB means that if you needed 100-watts in the analog era, you now need 1,585 watts for digital. It is a huge change. A JBL 4675 speaker in a 65-foot room only running Good old Dolby-A could get away with about 22-watts. Many people ran on Kelmar mono amps that had 50-watts or 25-watts for many years in theatres without blowing things out. Dolby SR added another 3dB to the mix so stage channels needed 94dB. 50-watts still did you in a small theatre. There wasn't much head room there but it could work. I remember the QSC 1400 starting to go in just about everywhere around here. It was 200-watts a channel, reasonably priced 3U tall and worked for just about every room. Take that same room and install digital and the power needed jumps to 348-watts. Whoops, the good old QSC 1400 doesn't cut it anymore, except bridged. 348-watts seems rather massive...especially to people that used less than 100-watts even in the old barn sized theatres. But where are the rule of thumbs? What is layperson to do?
About the only publication I've seen that tried to explain this to exhibitors/technicians was Dolby's little known "Technical Guidelines for Dolby Stereo Theatres." It didn't even have digital in its name. There were two "draft" publications. The red cover version in 1992 and the gray cover version in 1994. In this document there are the math formulas for those that want work their calculators (or slide rules if you are game) and there are "nomographs" that allow one to look at the distance, the sensitivity of the speaker and then figure the wattage needed. Dolby even listed the popular models of speakers (in the three main categories of Stage, surround, and subwoofer). The subwoofer nomograph is flawed in that it only has one figure on 110dB instead of 113dB.
The rational, as I recall (Sam Chavez would know better since he was with Dolby at the time) was two fold. Room gain due to the reverberation of the bass bouncing around the room would shave some needed power and also that the sheer size of the power was just so damn huge it would seem cooky to the average technician/exhibitor.
That aside...the Dolby document is the single best one I've come across for exhibitors/consultants to use to ensure a reasonably well designed system. How many Film-Techers have seen it or read it or use it? From what I can tell it is one of the best kept secrets and wasn't updated beyond 1994 (Too bad).
It is also too bad that no one ever made the "Speaker slide rule" like the various lens calculator slide rules...it probably would have done a lot to allow people to choose the right amount of speaker and power. Perhaps a manufacturer out there could make one. I have worked on an Excel spread sheet that will do the math but it is very primitive at the moment.
quote: Randy Stankey
The picture is just about as bright and clear as what the local Cinemark does with the same size screen and half the distance and twice the wattage.
Randy? Are you baiting me on this one? Do you really think there is a relationship between throw and light for a projection 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 07-12-2005 01:34 PM
quote: Randy Stankey
Help me out for a second on this question: Is it not possible that increased throw works FOR me in this case? Since the screen is the same size, a lens with a longer focal length is needed. That lens will have a lower maginification factor. And, less magnification produces a clearer picture. (Clearer? Sharper? What?)
You can't really draw any conclusions there. There are too many variables once one changes focal length. Not all lenses are created equal, even within a model line. When one is talking about a significant distance difference..say between 60 and 120-foot throws...one is almost always talking about a different lens design entirely.
For instance...if you had a 16' tall screen...for scope at 60-feet away, you would use a 65mm lens. From ISCO, that would be an Ultra Star PLUS (from Schneider a Super Cinelux)...if you were 120-feet away it would be part of the "Ultra" line from each manuafactuer. A much older lens design. The new short lens may be inherently sharper and collect more light than the longer lens. Then again, the longer lens might be more efficient due to its design.
Next, the screen comes into play. How much does it really behave like a matte-white screen (essentially 180-degree light scatter). Many "matte-white" screens are not matte-white in nature...I've measured gains as much as 20% and still termed "matte-white". Once directivity comes into play...the point of projection has an effect on how the light reflects off the screen. The longer throw will have the light come back towards the audience better, the shorter throw will have the light reflect off to the side walls giving the appearance of a hot-spot.
However, all things being equal (perfect screen, identical lens except for focal length, no significant dust particulate)...then no; projection throw has zero effect on brightness.
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Andy Summers
Master Film Handler
Posts: 397
From: Bournemouth Dorset United kingdom
Registered: Jun 2005
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posted 07-12-2005 09:25 PM
Hallo there all
Thank you so very much for inviting me to the forums Brad, however I have been unemployed from the game foe awhile and I can’t get my foot in the door and get back on the horse, anyway please bear with my spelling will you I’m not word perfect nor will I ever be.
Blow sub bass, eek not good there is another possibility the fact that motion picture films today are getting louder in there potential usage of there dynamic ranges, as for the JBL 4645 the Jedi blaster Lightsaber as I call her she’s sat on her side in my home cinema alone with smaller three-screen JBL control 5, as for the JBL 4645, you mentioned early Brad, about one could do the task in a cinema, and is this without any strain what so ever put on her, as its my total understanding that the more the better and with some custom made design implemented into this program and the projectionist advised on this as must don’t care a least bit about the sound side.
As one projectionist said to me many years ago you’re the only one that I know, that knows what to look for and listen for.
“Event Horizon” 1997 I remember this playing at the MGM cinema as it was called back in 1997 which is now under the ABC name and in partnership with Odeon Uk cinemas, anyway this loud popping sound was the sine of something’s wrong very wrong hear is came in during the LFE sub bass tracks, and to me it stuck out like a sore thumb and for years the ABC just let this carry on giving the cinema experience a let down, and one sub bass is just not going to cut the ice.
The peaks of popping are within the 25Hz and 40Hz ranges mostly, and maybe it was down to poor technical engineering that was rushed and not tested out properly, and the amplifier is under powered as well, what I think it really comes down to is budget and all the money had been spent on the Dolby DA 20 Dolby Digital decoder which is inline with the classic Dolby CP-200.
So I think its down to experience not ignorance Brad its not his fault we are only human and life is short, all I can say it was just bad luck, I read on the in-70mm life with THX that the HF gave up the ghost on the film “Eraser” and also I remember reading an article on home THX that someday the amplifier is going to clip and give up the ghost, it like what NASA says a rocket has to explode at some point it’s the odds on it, though that may sound crawl it the same that can be said about cinema and the home cinema market something as to give sooner or later.
However with my home THX I always do a rehearsal before playing to friends, and sometimes making customized EQ adjustments if the mix is to bright sound on the mid highs and high frequencies, but that is very rare, anyway with the 7 years I’ve been running the JBL 4645 with a different sub bass driver with edge ribbon surround and not the edge foam surround, which will give in time as you may know, and is a pain in the a$$ to put right and sticky too, if you get my meaning, but doable.
Going back to the ABC I saw “War of the Worlds” there last week and may go and see and hear it again this time I’ll remember to take the SPL with me as it’s a question of science, those tripods are loud and mean, I guess they looked at the sub bass with envious eyes and slowly and surely drew their plans against the JBL 4645 KABOOM, sorry my poor attempt in humour.
Though the ABC screen #1 sub bass was taken out with the Matrix which does not surprise me at all, the JBL 4675-A are still intact and sound fantastic for there age the JBL 2360-A HF sensitivity is around 130db if I’m not totally mistaken the edge it as is good the front row gives around 105dbc weighting though I believe she can reach 110dbc weighting at best.
Anyway that’s that I sound worse than a Hollywood actor picking up his award.
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[ 07-13-2005, 01:08 AM: Message edited by: Brad Miller ]
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