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Density vs. GFR
Old 4 weeks ago
  #1
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Mark Alpine's Avatar
Density vs. GFR

Hello,

I posted this response to a tread in the subforum "Bass traps, acoustic panels, foam etc" where a few of you discussed which porous materials to use for bass traps. Density seemed to be one of the main concerns.

Can someone here tell me why density is so important? I though that gas flow resistivity is one of three important factors to consider (the others being material thickness and air gap). In the Porous Absorber Calculator (http://www.whealy.com/acoustics/Porous.html), density is not specified as an input at all...

Maybe density is somewhat correlated with GFR, but to my knowledge GFR, thickness and air gap is the only thing to consider when designing Bass Traps using porous materials.

I'm planning to to some panels myself:
Corner Bass Traps completely filled with Hemp (3000 rayls/m) and broadband panels 4-6" deep (maybe with membrane in the middle to prevent too much HF absorption) using an insulation material with approx 15000 rayls/m. I'm not done yet though, but I think that is somewhat how it will be at the end.

Interesting reading on this part of the forum btw. Most people are obsessed by the gear (like myself a while back :-) ), and tend to forget the room treatment.

Another question: I hear that the more bass trapping you have the better. I plan to add quite a few, but assume that one could only treat one corner (floor to ceiling), what kind of audible improvements could you expect?

/Cheers!
Old 4 weeks ago
  #2
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Quote:
Can someone here tell me why density is so important? I though that gas flow resistance is one of three important factors to consider (the others being material thickness and air gap). In the Porous Absorber Calculator (http://www.whealy.com/acoustics/Porous.html), density is not specified as an input at all...
It's not the density itself that matters, but rather the GFR, as you already know. However, not all manufacturers publish the GFR figures for their insulation products, so you can't always get the data you need. Fortunately, there's a rough relationship between density and GFR. It's not a simple linear relationship, it varies from one product to the next, and the relationship is different for different types of insulation (one for mineral wool, another for fiberglass, etc.). But it's still close enough that you can use the density to estimate the GFR... very approximately.

Below is a graph that shows the aprox. relationship.

Of course, if you have the atual measured GFR numbers, then use those, for sure! But if you can only find a product locally that has no published GFR numbers, then you can estimate from density using this graph, as long as you realize that it won't be 100% accurate.

- Stuart -
Attached Thumbnails
Density vs. GFR-gas-flow-resitivity-gfr-vs-density-mineral-fiberglass-insulation.jpg  
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