Gasflow resistivety Vs density.

[Gdupproductions]

How come that everytime when a person asks what density they need to buy for basstrappin or reflections points, that 50% says that density matters and other 50% says no get insulation with that amount off rayls/m3.


I've bought some insulation and i contacted knauf they said the density lays around 16 kg/m3 (probally useless some gone say). Tough i used the recommended absorbtion calculator and it gave me a result wich im actually satisfied with (please check atached pic).
Still it's confusing how many people say A while it's B.

Hopefully this thread can clear things out, im not a pro acoustician but for me it makes more sense that gasflow resistivety is more important then density.

[avare]

Originally Posted by Gdupproductions

How come that everytime when a person asks what density they need to buy for basstrappin or reflections points, that 50% says that density matters and other 50% says no get insulation with that amount off rayls/m3.

The short answer is that gas flow resistivity is what matters in determining absorption properties, but that data is very difficult to get. Density is used as an indicator of the air fow resistivity.

The complete answer is in this thread.

Long and complete,
Andre

[Audioactive]

it would be nice if somebody could try to tell "not acoustic peolpe" what gas flow resistivity means in "simple words" ..i understand it may be a difficoult task to simplify the concept, but ..i'd be cool if someone tries !

[Sebastian Vera]

Correct me if I am saying nonsense.
GFR (gas flow resistivity) is the relation between the density of the Fiberglas and the diameter of it's fibers.

Let's put it this way, if you compress the fibers (going denser) and increase the diameter of them too, you will increase the resistance so the air won't move easily. This means higher GFR

Thats why low density FG generally tends yo have low GFR

[Sebg]

Gas flow resistivity is a measure of how hard it is for air to pass through a sample of the material. Denser material usually means less space for the air to flow through and so higher flow resistivity. Ditto with finer fibres.

The Standardised test is done by sticking the sample in a tube, producing an air pressure differential across the sample and then measuring the air flow velocity that results.

Since 100mm thick material will present more resistance than 50mm thick material, the test results are reported as "per m" of thickness.

[Gdupproductions]

Originally Posted by avare

The short answer is that gas flow resistivity is what matters in determining absorption properties, but that data is very difficult to get. Density is used as an indicator of the air fow resistivity.

The complete answer is in this thread.

Long and complete,
Andre

So actually i diden't luckly screw myself by buying 16kg/m3 fibreglass ?.
As im satisfied with the numbers the calculator has given me. Or is it useless ? =/. hope not

[Audioactive]

i'm not sure i got it, but i think gas flow resistivity is a combination of mass and density, where more density = more resistivity to gas flow, which is not a good thing, so, the best is to have a mass of medium density that allows gas to go throu..

in other words, if a surface is to dense, and has a big mass, then sound will bounce.....if the surface is not too dense, and the mass is not big enough the sound will pass throu it, ...hit the wall, and still have the energy to pass throu it agan and come back, so...the best combination is a medium density which attenuate the sound, and enough mass to kind of trap the sound..

but i may be completely wrong.

[Gdupproductions]

Originally Posted by Audioactive

i'm not sure i got it, but i think gas flow resistivity is a combination of mass and density, where more density = more resistivity to gas flow, which is not a good thing, so, the best is to have a mass of medium density that allows gas to go throu..

in other words, if a surface is to dense, and has a big mass, then sound will bounce.....if the surface is not too dense, and the mass is not big enough the sound will pass throu it, ...hit the wall, and still have the energy to pass throu it agan and come back, so...the best combination is a medium density which attenuate the sound, and enough mass to kind of trap the sound..

but i may be completely wrong.

There for imma build 2 6 inch bass traps for the corners and 2 4 inch traps for refelction point.....tough the density is 16kg/m3 and has 5000 rayls. So im asking the pro acousticians for their toughts Before im gonna get more materials.

[mhch]

Sorry, I deleted my previous post by mistake ...

Actually for porous absorber, and if I understood it well, it's not mass displacement which slows down the gas flow, but the friction of gas molecules against the fibers.

That also explains why thickness impacts the absorption: the thicker the material, gas can rub against more length of fibers. The less resistive it is also means gas can easily go deeper into the material and so rub against more length. That's why it's better to use less resistive material when building very thick panels.

All of this doesn't take into account reflection of gas flow against the surface of the porous material. The denser it is, the more reflection, another aspect of the story which is described by the acoustic impedance parameter (again, if I understood it right)

[Audioactive]

according to the infos i've collected, 40kg/m3 ...20cm width ...it will make a better bass trap.

with 16kg/m3 you will need more than 6 inches to make it work, but i don't really know much about it, i relate on infos collected here and there, plus, air gap between panels and walls, according to the experts, will icrease the performance...basically because it will crate more mass for the sound to travel ...(i guess)..

[Gdupproductions]

That's why im planning on building Chunks.....going 8.26 inch deep (corner filled with fibreglass).
But i can't find nothing on the internet bout traps that been build with the density off 16kg/m3. That's my problem at the moment. I read allot off threads where our more expierienced acousticians say that the more dense/fluffy stuff are best used for chunks but tough.

Seems like nobody knows how effective they will be.

[mhch]

Originally Posted by Gdupproductions

That's why im planning on building Chunks.....going 8.26 inch deep (corner filled with fibreglass).
But i can't find nothing on the internet bout traps that been build with the density off 16kg/m3. That's my problem at the moment. I read allot off threads where our more expierienced acousticians say that the more dense/fluffy stuff are best used for chunks but tough.

Seems like nobody knows how effective they will be.

The fluffy wool is best for larger depth, 12-16" or so. use Chris Whealy porous absorber calculator to validate your choice of material.

[Gdupproductions]

Originally Posted by mhch

The fluffy wool is best for larger depth, 12-16" or so. use Chris Whealy porous absorber calculator to validate your choice of material.

I did, but it can't calculate abosbtion for superchunks.

[Gdupproductions]

....waisted 40$ on sh*t i can't even use. Terrific thumbsup

[mhch]

Originally Posted by Gdupproductions

I did, but it can't calculate abosbtion for superchunks.

It doesn't give the exact absorption for panels either since sound doesn't hit a panel at a single fixed angle ....

You can use the average depth of the SC triangle to get a first order idea of the absorption ...

[Gdupproductions]

Originally Posted by mhch

It doesn't give the exact absorption for panels either since sound doesn't hit a panel at a single fixed angle ....

You can use the average depth of the SC triangle to get a first order idea of the absorption ...

Okay i made a quick drawing off the triangle with the exact size im going to use. + extra pics.

0.10 at 40hz and 0.38 at 0.80.

[jhbrandt]

Gdup,

You are splitting hairs. 16-60kg/m3 is great for superchunk style traps.
Your problems will be more practical than theoretical.

NOTE: the lighter rock wool is extremely crumbly and will require horizontal support every 12" or so, or it will compress and droop. I would recommend building little shelves into the chunk trap so that the entire weight of the rock wool stack does not destroy the bottom layers.

Cheers,
John

[mhch]

Originally Posted by jhbrandt

Gdup,

NOTE: the lighter rock wool is extremely crumbly and will require horizontal support every 12" or so, or it will compress and droop. I would recommend building little shelves into the chunk trap so that the entire weight of the rock wool stack does not destroy the bottom layers.

Cheers,
John

I always wondered why not hanging fluffy stuff rectangles (or so) vertically, hooked up at the top of the superchunk, instead of putting triangular layers on top of each other, which indeed suffer from compression due their weight.

So instead of mass adding up for compressing the wool, mass would help the wool stay stretched. Unless of course, extension due to the weight tears the wool apart ?

Am I splitting fibers ?

[Gdupproductions]

Originally Posted by jhbrandt

Gdup,

You are splitting hairs. 16-60kg/m3 is great for superchunk style traps.
Your problems will be more practical than theoretical.

NOTE: the lighter rock wool is extremely crumbly and will require horizontal support every 12" or so, or it will compress and droop. I would recommend building little shelves into the chunk trap so that the entire weight of the rock wool stack does not destroy the bottom layers.

Cheers,
John

Thanks John.
Finally a anwser that vanishes my worries (partly).
I'm using fibreglass now (eeek rockwool ), and has a etra layer off naturual fibreglass on the back off it. So i think sagging wont happen this quick. Wel i can add an extra shelf every 15,74 inch becuz the height will be 62,99 total. So i can add 4 support shelfs.

[Gdupproductions]

Originally Posted by jhbrandt

Your problems will be more practical than theoretical.

Sorry ?.

[Supersilence]

i think he means that the compression of the material and the fact that is very crumbly would be the problem rather than if it can absorb enough.

[jhbrandt]

Yes. Thanks, Super, for the translation. -- we all need help sometime.

cheers,
John

[Gdupproductions]

Originally Posted by Supersilence

i think he means that the compression of the material and the fact that is very crumbly would be the problem rather than if it can absorb enough.

Oh okay in that manner....LOl

[Gdupproductions]

Originally Posted by jhbrandt

Yes. Thanks, Super, for the translation. -- we all need help sometime.

cheers,
John

Uhm john ?, Before i had more dense rockwool in my hands maybe it whas the double off this 16kg/m3 fibreglass and even the fibreglass is more rigid then the rockwool so i don't know....

Tough i'll take your advice and design a superchnk with more shelfs, cuz indeed a 63 inch superchunk with just 1 base-shelf......