theater acoustics

Discussion in 'Home Theater Projects' started by Bill Polley, Nov 13, 2003.

  1. Bill Polley

    Bill Polley Second Unit

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    Well, I am building my theater now. Using the "golden ratios" for room dimensions, I am going to build my room to be 7'-3" high, 11'-7 1/4" wide, and 16'-10 3/4" long. The screen will be 96"X56" and I am planning to simply have 2 rows of 4 theater seats in the room. From what I have read, the dimensions I have chosen are best (as a ratio of 1:1.6:2.33) for minimizing room peaks and nodes. In conjunction with the room size, I also will be using my trusty BFD to further smooth the room response.

    These measures should really tame the bass response (hopefully to nearly ruler flat!), but what about the rest of the spectrum. I have read that room reflections are what generally muddy the sound of even the best systems, so how should I treat my walls. Do I leave the front wall solid, to allow reflections towards the seating, and then deaden the ceiling, side and rear walls to eliminate reflected sound? Do I leave a portion of the side walls un-deadened to allow some reflected sound?

    I had Kind of leaned towards the first idea. If you are outside, the sound only comes from the one direction. Any sound that should sound reflected, such as bounced off a building or the lively reflections in a gymnasium, should (I would think), be re-created by the surround speakers and therefore I would want all walls deadened. Then I wonder if I can get good width and depth imaging if the front wall is deadened. I am leaning towards a stiff front wall that will reflect sound, much like an ampitheater. Perhaps the front wall, and the first four feet of the side walls and ceiling should be reflective. This would provide for a reflective depth, width, and height, without being so far out as to allow first reflections to be directed at the listening positions.

    I have read that there should be a balance between reflective and absorptive materials, but I have never read any good "tried and true" facts. Anyone have any good actual designs that are proven to sound great?
     
  2. Wayne A. Pflughaupt

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    Bill,

    I like parts of both plans. The amphitheater analogy is a good start, but it is outdoors after all. I think a better analogy would be a well-designed concert hall. Like the amphitheater it typically has a lot of reflective surfaces where the performers are. But the audience area has combinations of soft panels for dampening, and irregular surfaces for dispersion.

    Bringing that home, you could keep the front where the speakers will be live and reflective, as you’ve noted. I don’t think there would be any need to dampen the ceiling as long as you had wall-to-wall carpeting on the floor, soft seats, etc. – at least that has been my experience.

    That leaves the vertical surfaces. I think I would be more concerned about the back wall than the side walls. About the worst thing you can have is the sound from the front of the room bouncing off the back wall and headed towards the front again. Dampening the back wall might make the room too “dead.” Thus I lean towards dispersion. This is pretty easy to accomplish in say, a living room, where things like book cases, etc. accomplish dispersion for “free,” as it were. I expect a dedicated room will have none of that, so what you could do, instead of building the back wall flat like the others, is build it out in a zig-zag or saw-tooth pattern. This would break up and disperse the sound instead of reflecting it back to the front.

    These things might very well get you the acoustics you want without any treatment for the side walls at all. However, if the room still sounds too reflective, you could then add a series of dampening panels to the side walls.

    My knowledge of acoustics is rudimentary. You might want to send a PM to HTF member Terry Montlick. He’s an acoustics specialist.

    Regards,
    Wayne A. Pflughaupt
     
  3. Bill Polley

    Bill Polley Second Unit

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    Once again, thanks Wayne. After doing a bunch of research last night, the advice from the "experts" that came up often was to actually deaden the FRONT wall, the rear wall, and the side walls in a band at about ear level. This apparently only allows reflected sound to reach the listener after several reflections, thus lowering the SPL of the reflective sound. First and second reflections will all but be eliminated. Yet the room will remain neutrally "live" sounding. This assumes carpet on the floor. I will probably take it a step further and deaden the spaces between 2 or 3 ceiling joists where the angle of first reflection to the listening area occurs.
     
  4. sridhar

    sridhar Auditioning

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    Hi,

    To re-create a large venue theater in a small room, accoustic diffusers are installed above ear level and accoustic absorber are installed below ear level.

    You could checkout the following links for more info,
    @ auralex.com

    This thread on hometheatertalk.com has some great info,

    hometheatertalk.com/ubb/Forum1/HTML/006838.html
     
  5. Mike Co

    Mike Co Agent

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    A common studio techniques is non-parallel walls facing each other, including ceiling & floor.
     
  6. Bob Hill

    Bob Hill Stunt Coordinator

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    Just thought I would throw in some dimensional ratios that help decrease the number of standing waves in a room.

    A. .79 1.00 1.26
    B. .80 1.00 1.25
    C. .62 1.00 1.62
    D. .67 1.00 1.80
    E. .62 1.00 1.44
    F. .80 1.00 1.20
     
  7. Terry Montlick

    Terry Montlick Stunt Coordinator

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    Hello Bill,

     

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