Density of fiberglass

[goneten]

Hi guys,

I can get fiberglass boards at a really cheap price but the kicker is the low density (in the region of 36 kg/m3...) per 1 inch.

I've got a few Ready Acoustic bags left which take 4 inch boards so what I would like to know is, would this be effective for first reflection duty or should I be looking at higher density panels in the region of 68kg+ ?

I'm thinking that 4" thick panels at 36 kg/ m3 density would be more effective as a broad band solution compared to 1" thick 96 kg density ? Just need to know if I can use this density or not for first reflections and if not, please explain why.

BTW, the pricing for 96 kg/m3 panels per inch is almost FOUR times more expensive than the 36 kg/ m3 panels. If I had to use 4" of the less dense boards vs 4" of the more expensive 96 kg, would there a huge difference or subtle ?

Thanks.

[avare]

Originally Posted by goneten

I'm thinking that 4" thick panels at 36 kg m/3 density would be more effective as a broad band solution compared to 1" thick 96 kg m/3 density ? Just need to know if I can use this density or not for first reflections and if not, please explain why.

Yes you can use it. It is the better than the denser material for 4" thick absorbers.

Andre

[goneten]

Hi Avare,

Thanks for answering my question. Here is another one. How much better, in your opinion, is a 4" 36 kg/m3 panel vs 96 kg/m3 panel of identical thickness ?

Pricing for the 96 kg panels is like R100 per inch ($13) compared to R25 (slighter over $4) for the 36 kg/m3 panels.

[avare]

Originally Posted by goneten

Thanks for answering my question. Here is another one. How much better, in your opinion, is a 4" 36 kg/m3 panel vs 96 kg/m3 panel of identical thickness ?

Pricing for the 96 kg panels is like R100 per inch ($13) compared to R25 (slighter over $4) for the 36 kg/m3 panels.

I do not understand what you are seeking. The cheaper product is better. If it is 1% or 50%, it is better. 96 kg/m3 material at 4" thickness is starting to reflect low frequencies. It is not opinion, it is fact. Study the data on Bob Golds Aborption Coefficients. The data shown for OC series has 96 kg/m3 and 24 kg/m3 material. The 24 kg/m3 has slightly less absorption at 125 Hz, and is 25% less dense than the 30 kg/m3 material you are refering to. Note that the best is with 48 kg/m3 for 4" thickness.

What are you after?

Andre

[goneten]

I want to target first reflections only, but because my Ready Acoustic bags hold 4" worth of material I simply wanted to know how much more effective it would have been if I chose a more dense fiberglass board, you've answered that for me.

I currently have 4 GIK Tri-traps handling broadband absorption down low in each front corner, stacked floor to ceiling. I had two questions, both of which you answered. My second question was simply out of curiosity; I simply wanted to know how much more effective the more dense material would be if the thickness were identical. Although I am confused as to why 4" thick 96 kg/m3 material would start to reflect low frequencies. I assumed that it would absorb frequencies down low.

The only reason why I mentioned prices was to give you an indication of what they go for in South Africa and to illustrate how ridiculous the pricing is of the various densities. Not sure if the pricing structure is the same overseas.

Thanks again for all your help !

[avare]

Originally Posted by goneten

Although I am confused as to why 4" thick 96 kg/m3 material would start to reflect low frequencies. I assumed that it would absorb frequencies down low.

Do not worry. If acoustics was simple, God would not have created aspirin.
As porous material gets thicker, it starts to reflect sound. You saw that in Bob's absorption data. It is not obvious, although it does sort of make sense. Keep filling the holes, or making the holes smaller in a porous amaterial, and the material becaomes less porous.

The only reason why I mentioned prices was to give you an indication of what they go for in South Africa and to illustrate how ridiculous the pricing is of the various densities. Not sure if the pricing structure is the same overseas.

Thanks for the information on South Africa prices. The price of mieral wool etc, all other things being equal, should vary directly with the density, as that is the actual material that is being bought. HOWEVER, different regions have different factories close by, and raw materials. Also depending on the market conditions, like a building boom or slump, mprices vary. In my part of the world, it also varies with the tyope of marleting. Whether it is a home construction supplier, or industrial dealer. The end effect is that the price varies according to no percievable logical manner. It is sort of like acoustics, oh, it is a part of acoustics.

Andre

[Glenn Kuras]

I can't find the article right now but Ethan Winer did some home testing on 701 703 and 705, and found that even at 6" the 705 was slightly better at 6". Not saying avare is wrong but there seems to be a lot of opinions on the matter. The bottom line though is the less dense stuff works so you can save money and use with confidence.

[goneten]

Andre,

Thank you again for your advice, I appreciate it. Glenn, I'll see if I can find Ethans density report. Your acoustic panels are first rate. I just wish I could put another 4 Tri-traps in the room but unfortunately that isn't possible. Unless I straddle the ceiling-wall corners....

Nah...never mind.

[Glenn Kuras]

Found it.
Rigid fiberglass density tests

[avare]

Originally Posted by Glenn Kuras

I can't find the article right now but Ethan Winer did some home testing on 701 703 and 705, and found that even at 6" the 705 was slightly better at 6". Not saying avare is wrong but there seems to be a lot of opinions on the matter. The bottom line though is the less dense stuff works so you can save money and use with confidence.

The denser material in Ethan's (excellent) tests was starting to act like a membrane. Just from an acoustic perspective, I do not want to recommend a design that is (somewhat) tuned. Especially on a forum like this. It is almost a given that someone will think they are copying it while not, and get inferior results.

No opinions...

Andre

[Ethan Winer]

Originally Posted by avare

The denser material in Ethan's (excellent) tests was starting to act like a membrane.

I also think that's correct. And as you can see, the best results were with FRK type rigid fiberglass, which definitely adds a membrane effect.

--Ethan

________________
The Acoustic Treatment Experts

[avare]

Originally Posted by Ethan Winer

I also think that's correct. And as you can see, the best results were with FRK type rigid fiberglass, which definitely adds a membrane effect.

We are in total agreement. I received a pm from a friend who thought I was meaning that the denser material is of no value. I am sorry I gave that impression.

There is absolutely no doubt denser materials do something good when (like almost everything else in acoustics) they are used appropriately. I do not know enough of what is going with them, to be comfortable recommend that use. They are good in the right application. Unfortunately I do not know enough about them to be comfortable recommending them, with similar methods that I do understand providing predictable performance.

The same applies with FRK etc increading bass absoption. I have not seen and studied enough data to recommend them. It works, I just do not know to predict what it does.

Andre

[Sebastian Vera]

Oh, I think I'm misunderstanding somthing...

I looked the coefficient abssorptions tables of Owens RFG and found that 703 4"thick mounted on wall (I guess this means no air gap) has 0,84 at 125hz. Same FG same thickness but with 16" of air gap has 0,65 at that frq. I was specting to find more absorption because of the gap. Ethan specially Sugested the gap in order to trap waves with longer length.

I think that the lower absorption at 125 is a resoult of the lost of linearity cause of the air gap and that at lower frq there would be more absorption.

By the other hand I was surpriced of the 703 6" mounted on the wall, looks like really near a full band absorber I'm gonna try them for early reflection points!

[Glenn Kuras]

Got yea. thumbsup
The good news is between our lab data and Ethan's lab data you can see the improvement on the low end. But this may or may not have other factors (like a membrane) that could effect things.

[avare]

Originally Posted by Sebastian Vera

I looked the coefficient abssorptions tables of Owens RFG and found that 703 4"thick mounted on wall (I guess this means no air gap) has 0,84 at 125hz. Same FG same thickness but with 16" of air gap has 0,65 at that frq. I was specting to find more absorption because of the gap. Ethan specially Sugested the gap in order to trap waves with longer length.

I think that the lower absorption at 125 is a resoult of the lost of linearity cause of the air gap and that at lower frq there would be more absorption.

By the other hand I was surpriced of the 703 6" mounted on the wall, looks like really near a full band absorber I'm gonna try them for early reflection points!

From data souirces that I trust, gapping up to around 2:1 provides good, as in flat increase in low end absorption. A 16" gap means a 4:1 ratio.

Yes, 6" of 703 is pretty good. 4" with a 2" will provide similar results. In my part of world, f you have 6" available for depth, ~6" of Roxul Safe N Sound or similar product will provide the same acoustic performance at a fraction of cost.

Andre

[Alonso Contreras]

I am sorry to bring this up again, but I couldn't find a dealer for the OC703, all they have is Owens Corning QuietR Duct Liner in my place, which has about the same NRC of the 703 and also pretty good in the 125-500Hz range.

The thing is they have it in 24kg/m3 versus OC703's 48kg/m3 density.

What should I do then? Put more sheets? Maybe 6" to compensate?

[avare]

Originally Posted by Alonso Contreras

I am sorry to bring this up again, but I couldn't find a dealer for the OC703, all they have is Owens Corning QuietR Duct Liner in my place, which has about the same NRC of the 703 and also pretty good in the 125-500Hz range.

The thing is they have it in 24kg/m3 versus OC703's 48kg/m3 density.

What should I do then? Put more sheets? Maybe 6" to compensate?

What are you building? If they are deep, as in more than 200 mm, absorbers, the 24 kg/m³ will be just about perfect.

Andre

[Alonso Contreras]

Hi Andre, I need to build these:

[Audioactive]

i'm quite confused too about density and material, everybody seems to have his own idea about it,

It would be nice if somebody tells us what's good for what!

there is a grey area regarding what's the best density and width in relation to frequencies

[Audioactive]

Originally Posted by avare

What are you building? If they are deep, as in more than 200 mm, absorbers, the 24 kg/m³ will be just about perfect.

Andre

what about 48 kg/m³ x 100mm...will it be as effective on that freq range, more effective, or it suits different applications, like better attenuation on lower freqs and less attenuation on hig freqs ???

[avare]

Originally Posted by Audioactive

i'm quite confused too about density and material, everybody seems to have his own idea about it,

It would be nice if somebody tells us what's good for what!

there is a grey area regarding what's the best density and width in relation to frequencies

There is no confusion. It is subject that people who do not know what they are writing about like to writing about. this thread will explain the truth.

Andre

[avare]

Originally Posted by Alonso Contreras

Hi Andre, I need to build these:

Lovely drawing. ZERO dimensions. If they are ~100 mm thick, then the 40 kg/m³ material is not dense enough.

Andre

[Alonso Contreras]

The side walls have to be 4" and spaced arround 3" from the wall, so as the rear wall absorbers, which act as broadband.

For the front walls the corners I'm thinking of 6" to bass trap, and on the middle of the front wall a bass trap with slats on top, leaving air enclosed like a membrane in the window for isolation.

Thanks Andre, sorry if this was off topic. But I really wanna get this done.