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Help understanding whether thickness of material affects GFR?
Old 24th June 2020
  #1
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Help understanding whether thickness of material affects GFR?

Hi,
Bit of a noobie question here, planning out my first DIY acoustic treatment project. Attached is an image I found on this forum that's shown up in a few threads which shows the GFR numbers published for each of Knauf Earthwool's products.
I'm just trying to make sense of the difference in GFR for the products that seem to only differ in thickness. It was my understanding GFR is irrespective of the material's thickness? For example if the product reports a GFR of 9000 MKS Rayls at 50mm, would it not still be 9000 at 100mm?
In this chart I'm also seeing the 14kg/m3 Ultra Acoustic Wall batt in the 50mm thickness has a higher value (14500) than the 75mm thickness variant (12400). Perhaps someone got the numbers mixed around.

Apologies if this is a tiring subject. I'm trying not to get overwhelmed by how much i'm reading, and how much different information I'm finding as well. I'm running the numbers into http://www.acousticmodelling.com/porous.php and I want to be sure whether varying the thickness of a given material (stacking layers of the insulation together for instance) affects the Pa.s/m2 value.
Attached Thumbnails
Help understanding whether thickness of material affects GFR?-earthwool_gfr.png  
Old 24th June 2020
  #2
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avare's Avatar
 

GFR is like alcohol content. It is same in a mickey or a fifth.

As materials are compressed to thickness, the material near the surface gets compressed more than in the middle. Because thinner material has less middle material, the fibrs are closer together and hence greater resistivity.
Old 25th June 2020
  #3
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Quote:
Originally Posted by avare View Post
GFR is like alcohol content. It is same in a mickey or a fifth.

As materials are compressed to thickness, the material near the surface gets compressed more than in the middle. Because thinner material has less middle material, the fibrs are closer together and hence greater resistivity.
Thank you very much for your response! That's good then, if I understand you correctly, this means I could stack together multiple pieces of Earthwool to reach a desired thickness at a desired air flow resistivity.

I've attached some numbers I ran from this chart using the Earthwool, the green would be a Super Chunk corner trap situation at 30cm depth, and the blue would be my panels at the early reflection points, which won't be able to achieve quite as much low-end absorption.
Attached Thumbnails
Help understanding whether thickness of material affects GFR?-airflowresistivity_chart.jpg  
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