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Could someone help out interpreting material's gas flow properties
Old 18th February 2008
  #1
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Could someone help out interpreting material's gas flow properties

in other words, what does this mean and how does it relate to acoustical/absortion properties (and compared to for example ever so popular OC703). For example here is airflow resistivity properties of PAROC COS 5
( Pääsivu )

45 10^-6 m^2/sPa

http://www.paroc.com/spps/BI_attachm...ance_table.pdf

What does these figures actually mean?


This material was named by Paroc representative when I asked about rigid wool/fiber slabs which had density around 40-50 kg / m^2 although the guy could not give any ideas how the material works in acoustical sense.

I'm just looking for cheaper options than real acoustic rigid wool/fiber which is very expensive. I was thinking of putting these inside larger panels and use real acoustic (3,5cm think with fibreglass cloth) slabs, which I already have, as surfaces.

Thank you,

-Tomi H
Old 18th February 2008
  #2
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Fantastic! Almost all the time when we (the acoustic people) write about materials and their sound absorption, are really referring to the gas flow resistivity. You have the right numbers! The good news is that absorption is relatively independent of gas flow resistance in the ranges of common materials. 703 has a gas flow resistance of (depending on the phase of the moon when OC publishes data) between 16 000 and 24 000 . The materials you referenced will do fine!

Andre
Old 4th March 2008
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Quote:
Originally Posted by avare View Post
The good news is that absorption is relatively independent of gas flow resistance in the ranges of common materials.
No.
Old 4th March 2008
  #4
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Quote:
Originally Posted by Hannes_F View Post
No.
I have reading all of your posts here. You have great knowledge. Within the range that I wrote in this post, the effect on free field incident absorption is minimal. Playing with Chris Whealey's absorption calculator and various will show that.

What is the basis of your absolute comment?

Waiting to learn,
Andre
Old 4th March 2008
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Hi Andre,

sorry, maybe I did not get what you mean.

I am using material with low flow resistivity, around 5000 Pa*s/m². Cheap stuff.

Test these values in the Porous Absorption calculator:

Absorber thickness 100 mm
Air gap 100 mm
Start graph at 30 Hz.
Absorber flow resistivity 5000 or 16800 or 45000 Pa*s/m².



First, we see that the absorption is definetely different in the bass range.

Then, if we compare the 5000 Pa*s/m² with the 16800 Pa*s/m² curve we see that below ca. 135 Hz the absorption is better for the the more resitive material. Tests have shown this as well like Ethan Winer's site shows.

However if this leads to a "more is always better" thinking this could be an error. If you watch the curves closely you will see that there is a frequency range between 135 and 700 Hz where the fluffy 5000 Pa*s/m² material absorbs better. Moreover, the 45000 Pa*s/m² material aborbs worse than the 16800 Pa*s/m² material.

This is definetively counter intuitive. But if you think about it then denser material should be good for insulation, and this can include a good portion of backscattered energy.

Now enter an absorption width of 300 mm and a gap of 200 mm. Surprise - the 5000 Pa*s/m² is best!



If this is true then my conclusions are:

- If you are restricted to panels in the range of 10 cm or 4 " then material in the range of 16000 - 25000 Pa*s/m² is probably your best choice. With other words OC 703 or 705. (But do yourself a favor and don't ignore the demand for an air gap behind them).

- If you have more space then consider thicker absorbers with bigger gaps, filled with fluffy glass wool of 5000 Pa*s/m² or even less.

Now how relevant are those simulated values?

#1 Download the software ZORBA by Marshall Day Acoustics for a double check. It says the same if you activate "alpha normal". If you check their website you see that they verified their simulations with experimental data and the two are very close.

# 2 I am not finished yet with my studio but the traps that I installed so far behave exactly as they should according to these simulations.

My conclusions:
- More is not always better.
- It always depends (i. e. on the frequency range, the absorber width and position, the gap width)
- I try to know what I am doing. Read, read, read (I mean books, scientific articles, papers - not necessarily forum opinions).The links to the literature are everywhere. Start at the bottom of this site: http://www.bobgolds.com/AbsorptionCoefficients.htm . Read all the BBC articles about their acoustics tests.
- I try to understand the theory but also run tests in between.

Somebody who does not know what he/she is really doing may end up spending much money with mediocre results. Cruel but true :-)

Hope this helps
Hannes

Last edited by Hannes_F; 4th March 2008 at 03:42 PM.. Reason: Changed units from rayls to Pa*s/m²
Old 4th March 2008
  #6
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Great! We are getting somewhere good! I agree totally with the data you presented in the graphs. Those are based on normal incident sound, not diffuse field sound. BBC 1992-11 in figs 1 and 2 show calculated vs measured diffuse field absorption for rw 2 using 6,000 Rayls as the resistivity of the material. Fig 3 compares theoretical to normal incidence measured.

Without quoted and getting into details that may bore people here, I agree with you. The difficulty was that short answers are almost always wrong because the specific are not explained. I am referring to my initial response.

Thanks for the reference to Bob's site. Have you studied the list at the very end of site?heh

Andre
Old 4th March 2008
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Quote:
Originally Posted by avare View Post
Great! We are getting somewhere good! I agree totally with the data you presented in the graphs. Those are based on normal incident sound, not diffuse field sound. BBC 1992-11 in figs 1 and 2 show calculated vs measured diffuse field absorption for rw 2 using 6,000 Rayls as the resistivity of the material. Fig 3 compares theoretical to normal incidence measured.
Andre,

that are the two things I like about ZORBA: You can change the propagation model (but does not change too much) and calculate random incidence (diffuse field, will normally give higher values for absorption).

Thank you for your pm, tried to answer but it was bounced.

Yes, I read most of the documents at the end of Bob Gold's list.

Hannes
Old 4th March 2008
  #8
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Quote:
Originally Posted by Hannes_F View Post
Thank you for your pm, tried to answer but it was bounced.
My email address is [email protected]. Email me and I will send you some hopefully useful links.

Quote:
Yes, I read most of the documents at the end of Bob Gold's list.
Did you notice the first name on the acknowledgements list?

Andre
Old 4th March 2008
  #9
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Quote:
Originally Posted by avare View Post
Did you notice the first name on the acknowledgements list?
Ah, great. No, I wasn't aware of that but I am learning every day
Old 10th March 2008
  #10
NLP
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So, just air without mineral wool is even better
Old 11th March 2008
  #11
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Quote:
Originally Posted by NLP View Post
So, just air without mineral wool is even better
50 m air with just a cloth over it could be an interesting test and should be amazingly good indeed according to the ZORBA software. heh

Or, to put it even more to an extreme, use a tent outside. I have heard this is not a bad alternative for some projects.

Actually 'heavier is always better' is not true but 'lighter is always better' is also not true. What should be true is this: For every given absorber thickness and air gap combination there is an optimal flow resistance.
Old 12th March 2008
  #12
NLP
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What would be optimal absorber thickness, material, density and air gap for example ... 50Hz?
Old 12th March 2008
  #13
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Porous Absorber Calculator is your best friend.

I need to make a keyboard macro with that sentence.

Dammit, is it so difficult do download the bloody thing and insert some bloody numbers?

OK, here are some values as being written in my secret documents (no actually derived from Porous Absorber Calculator, hehe) for 48 Hz. Served on a silver tablet:

10 cm absorption, 10 cm gap, 16800 Pa*s/m²:
27 % absorption (equals a resulting dip of - 5.7 dB per wall for an uneven mode)

30 cm absorption 5000 Pa*s/m², 20 cm gap:
60 % absorption (equals -2.2 dB)

30 cm absorption 5000 Pa*s/m², 40 cm gap:
68 % absorption (equals - 1.7 dB)

60 cm absorption 2000 Pa*s/m², 50 cm gap:
85 % absorption (equals - 0.7 dB)

All values given for normal incidence. Porous Absorption Calculator is based on the formulas of Delany and Bazley and requires the gas flow resistivity to be between 1000 Pa*s/m² and 50000 Pa*s/m² (as is the case here).

Hannes

Last edited by Hannes_F; 13th March 2008 at 04:07 PM.. Reason: Correction of numbers
Old 12th March 2008
  #14
NLP
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Thank you, I have that calculator for more than a year, but simply I cant believe. Sorry.
Old 13th March 2008
  #15
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Hannes F,

What was your criteria for choosing the 3 rayls values?

Also, while this calculator seems useful, it seems very hard to find the acoustic impedance of many of the absorption products on the market to make any kind of informed comparison. Thoughts?
Old 13th March 2008
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Quote:
Originally Posted by NLP View Post
Thank you, I have that calculator for more than a year, but simply I cant believe. Sorry.
Hahaha ...

so after all the science during the last centuries we are back to believing.

Trapping basses with thick layers of low resistivity mineral wool has been a standard technique for professional studios since many years and in my room it also works, and very well. So whether you believe it or not it's your choice but doesn't change physics.

But - no problem, buddy! I am only sharing some food for thought, not trying to convince anyone or sell something. Somebody asked and I gave some hints, that is all.

Hannes
Old 13th March 2008
  #17
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Quote:
Originally Posted by Lissajous View Post
Hannes F,

What was your criteria for choosing the 3 rayls values?

Also, while this calculator seems useful, it seems very hard to find the acoustic impedance of many of the absorption products on the market to make any kind of informed comparison. Thoughts?
Lissajous,

5000 Pa*s/m² is the material that I can buy cheaply around my place.

16800 Pa*s/m² is a value I have read for Owens Corning 703. Although other numbers that I read are 25000 Pa*s/m² for 703 and 30000 for 705.

45000 Pa*s/m² is a value that was asked for by somebody in another thread.

Hannes
Old 13th March 2008
  #18
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That material what I was looking at has also 15 10^-6 m2/sPa (and 36 10^-6 m2/sPa ) version, So according to your graphs, the 15000 would perform better?

Pääsivu

I am about to build 6" and 8" deep panels to be mounted on the corners (wall-wall, wall-ceiling), panels size would be 120cm x 60cm. And I wil probably build 2 corner chunks, one 40cm x 80cm x 220cm and other could be ecven bigger since there in space.

So, any final words of advice? 45, 36 or 15 10^-6 m2/sPa?

Thank you very much the information you have provided.

-Tomi H
Old 13th March 2008
  #19
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Quote:
Originally Posted by esaias View Post
That material what I was looking at has also 15 10^-6 m2/sPa (and 36 10^-6 m2/sPa ) version, So according to your graphs, the 15000 would perform better?

Pääsivu

I am about to build 6" and 8" deep panels to be mounted on the corners (wall-wall, wall-ceiling), panels size would be 120cm x 60cm. And I wil probably build 2 corner chunks, one 40cm x 80cm x 220cm and other could be ecven bigger since there in space.

So, any final words of advice? 45, 36 or 15 10^-6 m2/sPa?

Thank you very much the information you have provided.

-Tomi H
Tomi,

all that will probably help you not much because corner treatment is not curing frequency response (only a few exceptions from that rule). Modal ringing, yes, frequency response no, sorry. See my other post of today in the same sub-forum.

Hannes
Old 13th March 2008
  #20
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Quote:
Originally Posted by Hannes_F View Post
Lissajous,

5000 Pa*s/m² is the material that I can buy cheaply around my place.

16800 Pa*s/m² is a value I have read for Owens Corning 703. Although other numbers that I read are 25000 Pa*s/m² for 703 and 30000 for 705.

45000 Pa*s/m² is a value that was asked for by somebody in another thread.

Hannes
Hannes,

I don't have Excel -- would you mind running 5000, 15000, 30000 Pa*s/m² at 10, 20, 30 cm with no air gap?

Thanks
Old 14th March 2008
  #21
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Lissajous,

you can download Open Office from OpenOffice.org: Home
It is free and can run the excel file. It will only run the first sheet but that will do all you need.

Hannes
Old 14th March 2008
  #22
NLP
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Quote:
Originally Posted by Hannes_F View Post
Hahaha ...

so after all the science during the last centuries we are back to believing.

Trapping basses with thick layers of low resistivity mineral wool has been a standard technique for professional studios since many years and in my room it also works, and very well. So whether you believe it or not it's your choice but doesn't change physics.

But - no problem, buddy! I am only sharing some food for thought, not trying to convince anyone or sell something. Somebody asked and I gave some hints, that is all.

Hannes
Oooo... like "blue or red tablet" thing...
Old 14th March 2008
  #23
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Quote:
Originally Posted by Hannes_F View Post
However if this leads to a "more is always better" thinking this could be an error. If you watch the curves closely you will see that there is a frequency range between 135 and 700 Hz where the fluffy 5000 Pa*s/m² material absorbs better. Moreover, the 45000 Pa*s/m² material aborbs worse than the 16800 Pa*s/m² material.
Very intersting topic!
Those simulation are a bit contradictory to the density tests Ethan made.
If 705 has about 45 kPA and 703 has about 17 kPa, the simulation indicates that 705 should work worse in the bass range but in the tests it never works worse. Not for 3" thickness and not for 6" thickness.

Greets
Old 14th March 2008
  #24
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Quote:
Originally Posted by Ydope View Post
Very intersting topic!
Those simulation are a bit contradictory to the density tests Ethan made.
If 705 has about 45 kPA and 703 has about 17 kPa, the simulation indicates that 705 should work worse in the bass range but in the tests it never works worse. Not for 3" thickness and not for 6" thickness.

Greets
Never?

Look at Bob Gold's absorption coefficients http://www.bobgolds.com/AbsorptionCoefficients.htm and you find:

125 Hz
1" on wall: 703 0.11 - 705 0.02
4" on wall: 803 0.84 - 705 0.75

Two of many examples I see at first sight.

Hannes
Old 15th March 2008
  #25
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Quote:
Originally Posted by Hannes_F View Post
Never?
No, but at least for corner traps.
The denser stuff doesn't work worse, even with increased thickness. There are diminishing returns, and thus at some point "more is not better", but 703 doesn't absorb more at 6" than 705, with ~ 45 KPa s²/m² for the latter.
It seems that the simulation is not accurate in the bass region and especially in the context of corner traps.
At least that's what Ethan's density tests show.

Greets
Old 15th March 2008
  #26
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Quote:
Originally Posted by Ydope View Post
It seems that the simulation is not accurate in the bass region and especially in the context of corner traps.
At least that's what Ethan's density tests show.
You are confusing apples with oranges with cherries.

The simulations are for normal incidence with plane waves.

The test data normally published is for random incidence in a diffuse field.

Ethan's tests are for in a corner focusing on modal frequencies.

Three different environments.

Welcome to the fun world of acoustics.


Andre
Old 15th March 2008
  #27
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Quote:
Originally Posted by avare View Post
Three different environments.
I think you make my point. A material's gas flow resistance has an influence on the performance of corner traps, but not the kind of influence the simulations would suggest.
Old 15th March 2008
  #28
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Quote:
Originally Posted by Ydope View Post
I think you make my point. A material's gas flow resistance has an influence on the performance of corner traps, but not the kind of influence the simulations would suggest.
Aargh! The absorption characteristics of a porous absorber are defined by the surface impedance of the material and the location relative to the velocity component of the frequencies. Corner absorbers are used for two acoustic reasons: all modes build up in corners, and the effect of being in a corner is that the effective absorption at modal frequencies is four times greater than on a wall. The gas flow resistance is still the controlling factor. Not very significant at the ranges of materials that we consider for absorption, but the controling factor in why we choose those materials to begin with.

Andre
Old 16th March 2008
  #29
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Quote:
Originally Posted by Ydope View Post
No, but at least for corner traps.
The denser stuff doesn't work worse, even with increased thickness. There are diminishing returns, and thus at some point "more is not better", but 703 doesn't absorb more at 6" than 705, with ~ 45 KPa s²/m² for the latter.
It seems that the simulation is not accurate in the bass region and especially in the context of corner traps.
At least that's what Ethan's density tests show.

Greets
I see your point.

Actually if I look at the measurements in Ethan Winer's density report I do not really see a significant difference between the 6 pieces 6" 703 and 705 (without paper). For the 12 pieces á 3" I see better modal ringing reduction for the 42 Hz mode with 705. However some of the higher modes seem to be a little better with the 703 material.

This is actually typical for the curve the Porous Absorption Calculator gives if we compare different gas flow resistivities. Often there is a tipping point that remains unchanged, and the absorption below that moves in the other direction than the absorption above.

Also if for a rough check we insert 6" as a average distance and 3" as absorber thickness, 16.6 kPa*s/m² for 703 and 25 kPa*s/m² for 705 then the absorption should be 23 % for 703 and 26 % for 705 (42 Hz). Here the calculation and the experiment show conformity.

I admit this does not really work for the 6" thickness. I suppose there are additional border effects, or the gas flow resistivities are a little different actually. Or the fact that there are these angled surfaces behind the panel that radiate waves into all directions changes the circumstances.

Nevertheless we should understand this:

This is false: Bigger gas flow resisitivity is always better for absorbing low frequencies.
This is false: Lower gas flow resistivity is always better for absorbing low frequencies.

This is true: For each combination of air gap, absorber thickness, chosen frequency and geometry (wall, wall-corner, corner-corner) there is an optimal gas flow resisitivity. If we go higher or lower from there, we loose absorption.

So I think that OC 705 is a good material for low transmission (for what it was designed in the first place). And it probably is one of the best materials (or even THE best material) for absorbing certain low frequencies - ... but only as long as you restrict yourself to relatively thin panels.

Think about this analogy: We fire with rubber bullets on targets made from different material. Let us say the thickness of the target is relatively thin and the target gives very low resistance, like wool or paper. Then most of the bullets will penetrate the target, bounce at the wall behind and penetrate again. No much effect.

Now we take a material that gives more resistance. With that we can catch more of the bullets, but quite a portion will be reflected.

If we now allow ourselves to have a thicker target then the lighter material may become more useful again. Because of its thickness, more bullets will be caught than before, but because the material is soft the reflection is very low. The overall absorption we can achieve with that material is better than with the denser material.



This is only a rough picture of course because sound waves can not only be reflected at the surface of the material but also inside and on the backside of the material. But you get the idea.

Since several vendors of acoustical treatment are reading here I would not be surprised if they come out with a GigaTrap or something alike that is 12" thick and filled with low resistance material. This would be not a bad idea and if you watch it happen remember where you read it first.

Actually I am thankful for your comment Ydope, because it while thinking about it I found a way to calculate better corner traps for my own needs. Thank you!

Hannes
Old 17th March 2008
  #30
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Quote:
Originally Posted by Hannes_F View Post
Actually if I look at the measurements in Ethan Winer's density report I do not really see a significant difference between the 6 pieces 6" 703 and 705 (without paper). For the 12 pieces á 3" I see better modal ringing reduction for the 42 Hz mode with 705. However some of the higher modes seem to be a little better with the 703 material.
Thanks for your reply. A good read.
I agree with your analysis. Comparing 6" thick 703 with 705 would probably show more detailed results if more material had been spread in the room. 705 seems slightly better, but not enough that it that it would justify a higher price IMO. At least for 6" thickness.
703 at 6" seems like a good compromise. You could go less dense and thicker, but that would need more space.
Thanks again.

Edit: BTW where do these values for 703 and 705 come from? I found no orignal data. If 703 has 16 kPa it seems likely that 705 has at least twice that value, because that's what I found with comparable materials.

Greets.
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