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This topic comprises 2 pages: 1 2
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Topic: Crossover issue on JBL 8330A surround
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Scott Norwood
Film God
Posts: 8146
From: Boston, MA. USA (1774.21 miles northeast of Dallas)
Registered: Jun 99
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posted 10-22-2008 08:03 AM
So, how does one determine how many surround speakers of a given type are needed in a given room?
I'm assuming that it is more complicated that simply doing the math necessary to achieve a given SPL at all seats, since that does not take into account the need to achieve even coverage throughout the seating area. Is it?
Old surround systems from the 1950s usually use (or used) 70v lines, but I haven't seen this in a new system. Why?
And where does one learn about cinema sound system design in general? As in acoustical treatment type and placement, speaker type and placement, power requirements, etc. As far as I know, there aren't any books that discuss this specifically and in detail. I've heard a number of rooms with similar equipment and some sound great, while others sound lousy, so there is obviously more involved than simply installing quality speakers.
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Steve Guttag
We forgot the crackers Gromit!!!
Posts: 12814
From: Annapolis, MD
Registered: Dec 1999
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posted 10-22-2008 08:36 AM
Scott,
To some degree...as for power handling is strictly math. No one surround speaker is expected to cover the entire room.
A great document can be found here It was written in 1994 but it still applies today. The only thing to watch out for is that its subwoofer power recommendation is 3dB deficient.
The basic idea on surrounds is to either do the math or used the nomographs in the above linked document to determine the power needed given the surround speaker sensitivity. So, let say you determine that you need 750-watts per surround speaker channel...Then you look up your speaker's sensitivity (in watts/1-meter)...lets say it is 90-watts. That tells you that you need 8.3 speakers so you would need 9 speakers in this example.
If you are doing an "EX" surround channel and do the math...there is a very heavy demand on the rear channel speakers. Side speakers are only really trying to reach about half way across the room...which is often on the order of say 25-feet (for a 50-foot wide room)...rear speakers are try to reach about half the way down the auditorium...even for small theatres, this going to be a greater distance than the side speakers have to deal with. There are execeptions, of course, and theatres that have balconies have notably less demands on the rear speakers than ones on the sides (for the balcony section).
As for designing theatres...there are too many factors in play to come up with an all ecompassing book on how to do it. Often the looks of the theatre and the budget are in conflict with good movie presenation (form over function). There was a very good document from the technical side by SMPTE called EG-18. It was "Withdrawn" for very political reasons. It was, non-the-less accurate and would guide an architect into designing a proper theatre (techincally). I suggest obtaining a copy from somewhere if you are in the situation of designing a theatre.
The THX program, under Tomlinson Holman did quite a bit of research on how sound reacts in a theatre environment. Regardless of what members here feel about THX...it began with the idea of getting sound right and uniform from mixing stage to mixing stage and from theatre to theatre. That evil substance $$$$ seems to have gotten the best of it.
Generally the key is, regardess of appearence, keep reflections down (low reverb times) though one shouldn't go for a completely dead room, isolate the room from outside sources (vibrations) and to also keep its sound from getting into other spaces. The methods of controlling reverberation and isolation are many though.
Some of the biggest mistakes I see exhibitors make are lack of keeping the reverberation low by simple use of fiberglass...not just in the obvious places like on the walls and behind the screen...but also above the ceiling...that cavity above the ceiling, to the roof line is a huge resonate cavity...it shows up everytime on an RT60 test. Put at least 6" of fiberglass up there an dialog quality goes up...and your heating/AC costs go down...why heat/cool the space above the ceiling? Dumb. The area between the screen and the first row...deaden it...don't leave it all hard surface. That is the first area the sound will start reflecting. Keep the side wall insulation to BELOW the ear line...not above. Really deaden the rear wall where the speakers are pointing. You definitely don't want that projection room wall bouncing sound around...it will be more of an echo than mere reverberation due to the physical delay.
Build the THX wall...you don't need THX certification to do it. It will really improve the LF response and keep audio reflections from behind the screen down. Don't forget to put black ductliner on it and fit it to your chosen speaker systems.
Isolation is another HUGE failing in most cinemas. Too many common walls. Demising walls should be on the order of STC-75...which will shock most architects that are used to designing STC-45 to STC-55 walls...not nearly good enough for theatres.
Physically isolate each theatre...cut the slabs, booth walls, behind screen walls and use expansion joints. I've measure many theatres that use the traditional "staggard-stud" double drywall isolation wall...they suck. Typical FSTC numbers if this is the sole source of isolation is anywhere from the upper 40s to the low 60s...depending on how many flanking paths are there.
Again, there is no one-source I've found that will get you a well designed theatre...and all of that can be ruined anyway if poor equipment is chosen or some clown tunes your room poorly.
Steve
[ 10-22-2008, 09:38 AM: Message edited by: Steve Guttag ]
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