Rock wool vs. Acoustic cotton vs. OC703

[BoomieMCT]

Is there any real difference between Rock wool, Acoustic cotton and OC703? Is one more efficient then the other or more efficient at different frequencies? Where can I find info on this?

[bpape]

Check out Bob Golds' site - www.bobgolds.com and look under the coefficients link.

If all else is equal (thickness, mounting, and density), the cotton will have a slight edge in the lower frequencies if we're speaking of 1 and 2" 3lb density materials. As you get thicker, the different materials tend to be more readily avialable in different densities so it's harder to compare. The best solution depends on the application and what specifically you're trying to accomplish.

For instance, 6" of OC703 is pretty effective and even across the board assuming an A mounting flat on the wall. 5.5" of 1.5lb acoustical cotton will do just as well over most of the band but has a hump in absorbtion around 250hz.

Bryan

[cellexjohn]

That's a great link bpape. I have been looking for the specs on John's Manville as it is the only thing I can get around here.

Thanks
John

[bpape]

It's kind of a hobby for Bob. It's a nice reference when you want to compare things. Don't get too hung up on tiny differences as some were done in different labs and certainly on different days. They're also only as good as the manufacturers' specs.

In general, the JM814 is pretty much a direct replacement for OC703 inch for inch. When you're looking to buy, remember that while local is generally cheaper, sometimes the local guy wants a ton more - and local you have to pay sales tax. Just make sure to do all the homework to really compare total costs.

Bryan

[cellexjohn]

My local supplier only carries 1" I believe. When he took me back to where they store it I only saw the 1" 475mf. I'll have to look a little closer next time. This product had 1/2' overlap all around the edges. There was male and female pieces.

John

[blownrx7]

Acoustical Cotton????
Another material to consider?
I'm having a hard enough time finding OC703. Where do you get acoustical cotton?

So, for a 2" or 4" thick 2'x4' absorber, which is the best material?

blownrx7 wrote:

So, for a 2" or 4" thick 2'x4' absorber, which is the best material?

They are all about the same. Mounting and placement are far more important than the material.

When an architect asked me what product to use, I always said the same thing "whatever you think will look good. I will adjust the balance and placement after you pick a product/s"

I like recycled cotton. Its just better for the environment.

[bpape]

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

However, you are correct that the cotton is the superior product - not just for the environment - but because it performs better. That said, the other products are less expensive and in an application where the superior absorbtion at some of the bands may not be desired based on overall balance, the other materials may be a better choice.

Bryan

[blownrx7]

Thanks for the responses.
I assume you can obtain Acoutstical cotton from the same insulation supply houses?
TIA

[bpape]

No - sorry. Some places can get the 1.5lb Ultra Touch wall insulation which works well for bass control in thickner installations. The 3lb and 6lb cotton is proprietary and pretty much only avaialbe from a few dealers.

Bryan

bpape wrote:

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 material’s 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.

[bpape]

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

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?
Thanks

[Seb]

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.