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Mixture of absorber materials
Old 12th March 2013
  #1
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Mixture of absorber materials

I think I have read that ideally one shouldn't mix materials in broadband absorbers. I can't for the life of me figure out why and I'm pretty damn sure that it is incorrect. However I need confirmation of my cloud situation before I go ahead and build it.

My entire ceiling is covered with 175mm of pink fluffy, no air gap. In the cloud positon will go a standard size cloud of 90mm deep 32 kg/m3 do rockwool. To my thinking that's a nice gap of 7 inchs or so to the hard ceiling boundary, but even better that gap is full of pink fluffy. Surely the pink fluffy in the gap could only be of benefit, right? Unless I'm missing something big in all my research. Extra bass trapping hopefully, but at least adequate for ceiling reflections? Thanks.
Old 12th March 2013
  #2
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I would plow fearlessly forward. You are going to have quite a bit of absorption on your ceiling and if you feel the room is too dry when finished adding absorption, just add some reflective surface such as slats or diffusion. in terms of mixing broadband materials, I am not too concerned personally. If you have a link, please post it. Perhaps the person stating it was concerned with making an absorber that is too thick for its GFR and thus reflects?
Old 13th March 2013
  #3
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Ditty,

The Porous Absorber Calculator will accept up to 4 different materials in a given calculation. I see no problem with mixing materials if it's easier to construct whatever. I just don't see why you would need an additional "cloud" if your "entire ceiling is covered" already. That constitutes a cloud in my book.

"Extra bass trapping" if with the same materials may best be achieved elsewhere in the room....Additional factors, etc.....
Old 13th March 2013
  #4
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Ditty, is your ceiling covered in plasterboard too (below the pink fluffy) ?
If so, then you'll probably need that extra cloud at first reflection point.
Old 13th March 2013
  #5
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Rick,

No plasterboard, it's open ceiling cavity completely filled and exposed to the room.

John, thinking a cloud anyway as I'm not sure 175mm of pink fluffy is adequate for reflections from the ceiling.
Old 13th March 2013
  #6
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Quote:
Originally Posted by dodittydada View Post
Rick,

No plasterboard, it's open ceiling cavity completely filled and exposed to the room.

John, thinking a cloud anyway as I'm not sure 175mm of pink fluffy is adequate for reflections from the ceiling.
John is correct, your whole ceiling is already a huge cloud. Adding to that wouldn't make a difference, i think. Wool doesn't reflect, it absorbs.
Old 13th March 2013
  #7
Gear Guru
Reflections

I am quite sure this whole ceiling exposed fibre is a good thing. I have always liked the 'sound' of overhead treatment.
It is however not particularly thick, nor does it have much density, no drum head effect, no corner, no airgap. I suspect not great at LF. EDIT Fine for HF of course.
So I can see reasons why it might be beneficial to add denser thin panels underneath, perhaps even with an air gap.
I have seen tests showing that Space Couplers increase the LF absorption of the fibre above/behind them by 45% in the 125Hz band.
Jeff Hedback and John Brandt use these overhead frequently.
DD
Old 13th March 2013
  #8
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I see trying to re-invent the wheel. You have 175 mm depth for porous absorber. At that depth use Roxul Safe n Sound or equivalent (~10k rayls/m GFR, ~40 kg/m³ mineral wool). Unless properly designed with a porous absorber calculator, like the one linked in the the thread, the results will be worse than an homogenous absorber, AND more expensive.

Economically homogenous,
Andre
Old 13th March 2013
  #9
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I can't seem to find the Flow resistivity (Pa.s/m2) for R13 (or R11) pink fluffy fiber glass wool (Owens Corning), would anyone have that data at hand by any chance ?

Thanks

P.S. I've looked all over their site plus elsewhere...in vain.
Old 13th March 2013
  #10
Gear Guru
Fluffy

You may not find it. Depends on how much it is fluffed out.
Might be some clue in this big archive Common Gas Flow Resistivity numbers.
I am not entirely convinced about the lack of benefit of a more rigid layer.
Take a look over at the Amsterdam Mastering build, and ponder what the Homatherm is doing there. A layer of rigid stuff like this can begin to exhibit some panel or drum head type of behavior. This is not in the common calculators, but be assured it can and has been calculated.

DD
Old 14th March 2013
  #11
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avare's Avatar
 

Quote:
Originally Posted by DanDan View Post
I am not entirely convinced about the lack of benefit of a more rigid layer
You know my thoughts Dan; this is for others.

I did not write that mixng densities is bad. Doing it without a methodical manner will just about guarantee less than optimal, as in best use of space and material costs, results. BBC has used mixed densities. There is an RD report detailing an absorber with them. There is a technical paper written by a PhD now working in the abyss of Arup describing just about everything BUT the actual algorithms he used to design absorbers for an anechoic chamber with mixed density. Note that is a post-graduate professional.

With out being able to provide any reasonal assurance of results, as an acoustics professional, I can not recommend such construction based on guesses.

The OP can do whatever he wants. He has the caution that he is heading for uncharted waters.

Well charted,
Andre
Old 23rd May 2013
  #12
Gear Addict
 

Quote:
Originally Posted by avare View Post
I see trying to re-invent the wheel. You have 175 mm depth for porous absorber. At that depth use Roxul Safe n Sound or equivalent (~10k rayls/m GFR, ~40 kg/m³ mineral wool). Unless properly designed with a porous absorber calculator, like the one linked in the the thread, the results will be worse than an homogenous absorber, AND more expensive.

Economically homogenous,
Andre


Is this still considered a good value to use for Safe n Sound in a PAC?

Thanks!
Old 24th May 2013
  #13
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Quote:
Originally Posted by jeff_free69 View Post
Is this still considered a good value to use for Safe n Sound in a PAC?
What is a PAC?

Andre
Old 24th May 2013
  #14
Old 24th May 2013
  #15
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avare's Avatar
 

Quote:
Originally Posted by jeff_free69 View Post
Is this still considered a good value to use for Safe n Sound in a PAC?
I do not understand the question. I gave the GFR value for Safe N Sound so that people could use modelers like PAC.

Andre
Old 24th May 2013
  #16
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Quote:
Originally Posted by avare View Post
I do not understand the question. I gave the GFR value for Safe N Sound so that people could use modelers like PAC.

Andre
Yes, I was just checking if there was any further discussion /threads about the GFR value.
(like there was for OC 703 eg NASA's values, 17000 vs 27000 etc etc).

I am probably building some 12" thick traps to kill a null around 85Hz and wanted to see what the options are.

Thanks for the confirmation!

(too bad GFR info isn't included in Bob Gold's compendium)
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