Posted by T-Rex (A) on September 06, 2006 at 04:20:04

Hello,

A friend of mine is building a cinema room. He's going to put a pair of huge (and I mean HUGE) speakers in a rather modestly sized ~15X15ft room. All the walls as well as the ceiling are stuffed with rock wool, but I told him he'll probably still have huge bass issues. Other than having some type of external traps in the corners, he's left (in a rare moment of clarity) a big recess in the ceiling, a rectangle about 6X9, which is about 2ft deep.
Question is - what's the best way to utilize this recess. Build a bass trap? What kind? Or maybe use some of the space for diffusing or absorbing elements?
Guidance will be appreciated.

Thanks all,
-T

• Re: Ceiling trap - T-Rex 01:15:50 09/10/06 (0)
  In Reply to: Ceiling trap posted by T-Rex on September 6, 2006 at 04:20:04:

  Thank you for your insights, Tumara Baap

  -T
  
  

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• Re: Ceiling trap - Tumara Baap 00:47:50 09/10/06 (0)
  In Reply to: Ceiling trap posted by T-Rex on September 6, 2006 at 04:20:04:

  Another thing that comes to mind is that whenever you remove an existing drywall, or add it to construct a closet for example, you'll profoundly affect the inherent distribution of axial mode frequencies. If the change veers towards a new cuboidal shape (for example adding two more feet to a nine foot ceiling of an eleven by twelve foot room), you'll worsen bass acoustics. Conversely you can greatly improve things if the change steers you away from a cubish shape.

  Tumara Baap
  
  

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• Re: Ceiling trap - Tumara Baap 10:15:17 09/06/06 (0)
  In Reply to: Ceiling trap posted by T-Rex on September 6, 2006 at 04:20:04:

  I'll soon be doing a similar room. The ceiling recess is best stuffed for purposes of low frequency absorption. For a room that size, bass distortion will be the main issue. My ceiling has joists spaced 24 inches apart. On these joists I plan to lay down some 2 inch thick mineral fiber boards. And above that I'll push in fluffy R-38 insulation where possible (ceiling recess is crisscrossed with ventilation ducts, cables and gas pipes). I don't think you really need to have insulation stuffed up all the way... particle velocity is essentially zero at any wall boundary surface and little absorption will take place there. Most of the absorption should take place at the mineral fiber boards, about a two foot drop from the upper floor boundary. Before dressing things up, this should be an opportunity to run any HDMI or CAT-5 cabling in the ceiling space. I'm not sure how to finish up the ceiling, as I'm afraid a simple breathable cloth covering may sag over time, and there's the issue of using trim and crown molding to cover up the countless staples. Maybe using something rigid would be easier e.g. GOM covered 1 inch 703 affixed to the joists with snap-on rotofast anchors. A concern with all this treatment is the ceiling would make the room too dead (although the room will have hardwood floors). At the very least having diffusion interspersed in the ceiling should make the room more natural sounding. An idea was to use a few FSK/FRK 703 pieces but to drill holes so as to afford Binary Amplitude Diffusion (I hope the grids by Trevor Cox on his website are an accurate representation of an actual BAD panel). Or one could even use the BAD Arc panels from RPG. I think BAD panels offer only modest diffusion but are better than a straight reflector or absorber. BAD's should simply add to the absorption by the insulation above them.
  Tumara Baap
  
  

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