Rock wool vs. Acoustic cotton vs. OC703 - Page 2 - Home Theater Forum and Systems - HomeTheaterShack.com

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post #11 of 14 Old 02-13-07, 07:32 PM
altenburgpj
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Re: Rock wool vs. Acoustic cotton vs. OC703

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bpape wrote: View Post
I'd have to respectfully disagree. Each material can be made to work for a certain situation but they're not at all the same.

2" 3.5 lb mineral wool
0.26 0.68 1.14 1.13 1.06 1.07

2" OC703
0.17 0.86 1.14 1.07 1.02 0.98

2" 3lb Acoustical Cotton
0.35 0.94 1.32 1.22 1.06 1.03

Bryan
I have to respectfully disagree too. As Bob Golds says on his web site "Differences in coefficients of less than 0.15 are not significant." Not to mention the fact that absorption coefficients greater than 1.00 are possible only because the materialís edges are open to the noise in the test chamber, but not included in the surface area calculation. Therefore thick materials will always act differently in the real world than in the test chamber especially if they are mounted in such a way that their edges are not exposed to the room. If you cover an entire wall floor to ceiling and edge-to-edge, you have no exposed edges, and the numbers you have quoted will not apply. So I do believe that mounting and placement are more important. This is especially true when they are placed at a point to prevent a first reflection from your front speakers, which is an important factor in maintaining proper imaging of your sound field.
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post #12 of 14 Old 02-14-07, 06:15 AM
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Bryan Pape
 
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Re: Rock wool vs. Acoustic cotton vs. OC703

Believe what you want. You're welcome to your opinion.

Differences of less than .10 should probably be taken with a grain of salt - especially as you approach 1.0. However, when I see 2 materials and one is DOUBLE the other at 125Hz (and I've confirmed this in real world appciations, not in a test chamber) then I pay attention.

You are correct in that numbers over 1 are not applicable when covring an entire surface. Once again I'll state that the numbers should not be taken as absolutes, but as relative values for purpose of comparison. In addition, you make the basis of your argument on these being 'thick' materials. I hardly consider 2" 'thick'.

Lastly, In a typical design, there are a lot of edges of absorbtion exposed - not just fronts. So, don't be so quick to dismiss the benefit that the measurement anomoly shows. I use this to my benefit all the time in doing soffit based bass absorbers. Take 12" of material 24" wide. Build a false soffit. With your theory, I'd only consider the 12" of thickness over the 24" width. However, I also get a 12" high by 24" thick absorber on the SIDE of the soffit.

I'll agree that where you put treatment and how much of what kind and thickness is critical to proper treatment of a room. I was merely taking exception to your statement that 'they're all the same - doesn't matter' - it does.

Bryan

I am serious... and don't call me Shirley.

Bryan Pape
Lead Acoustician/Owner
Sensible Sound Solutions
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post #13 of 14 Old 03-02-08, 12:08 AM
monkeyboy1
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Re: Rock wool vs. Acoustic cotton vs. OC703

What would be the best material type for this application? I have a 2 soffitt overhangs which conceal ductwork, but they run the entire width of the theater, in two locations. They are only 2.5" lower than the rest of the ceiling. I would basically need 2" triangles, and what would be my best bet to fill them with, and for that matter, to construct the triangles out of?
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post #14 of 14 Old 03-02-08, 04:32 AM
Seb
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Re: Rock wool vs. Acoustic cotton vs. OC703

I think that most Standard test procedures actually require the edges to be blanked off. The reason for getting abs coeff over 1.00 is more to do with the limitations of the Sabine formula assumptions.

I'd also be cautious about comparing test results from different labs and possibly different test standards.

Is there a link for more info on "acoustical cotton"? I am interested in the concept of recycling cotton since it requires so much water to produce in the 1st place. I'd be a bit concerned about its fire performance though.
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