John Sayers' Recording Studio Design Forum Archive

Hi,

After "Soundman2020" pointed out in another thread that my below discovery (for me it is anyway) may actually warrant a new thread (loose interpretation of events), I did a little more research and found that yes, I think I agree with him. I cannot find it mentioned anywhere else or with similar products with the same performance data.

You see, us Aussies simply "put up" with paying significantly more and getting significantly less with a lot of things, and acoustic materials and insulation is certainly one of them. So recently I've begun to investigate bass trapping again after many years not bothering, and this is simply because my wife and I have moved into a new home, double brick no doubt, and that means hard, parallel surfaces that are highly reflective at all frequencies - bugger!

Now, things are slightly better now than they were 10 years ago when all of the current DIY sound absorbing panels were gaining in popularity. Though, in a lot of ways things haven't changed as well - namely pricing, lack of availability, lack of products with verifiable testing data, etc., etc.
And then I stumbled upon this - http://www.insulation.com.au/products-1 ... -wool-slab
Here's the datasheet - http://fletcherinsulation.eprospect.com ... _Wool_Slab

Exert from datasheet
NRC Data: Coefficients at Frequencies per ASTM C 423 (125hz onward in whole octaves)
192kg/m3 50.8mm 0.40 0.79 0.78 0.94 0.94 0.87 0.85
96kg/m3 50.8mm 0.36 0.79 1.15 1.04 1.01 1.04 1.00
96kg/m3 101.6mm 1.15 1.17 1.18 1.03 1.06 1.08 1.10

In bold is the data that grabbed my attention.
Does this material seriously have an NRC rating of 1.15 for a 100mm thick panel on a wall? I've gone over the calculations, used the comprehensive data list and am yet to find anything that performs this well at 125hz.
The closest material I can find with similar results is 6 inches of 703 plain, with a rating of 1.19 at 115hz. I would be curious as to what the performance of that very dense (192kg/m3 nominal) version would be at 75 or 100mm's in thickness, as I think there would be further improvement at even lower frequencies due to its high density, but beyond that thickness I predict the higher density would hinder rather than help.

Now, as an engineer, I am fully aware of the variability of testing procedure (even when testing in accordance to a standard, trust me, standards often mean nothing - speaking from first hand experience with clients and vendors alike), but these numbers must prompt investigation. But I am not an expert in compressed fibrous materials; I am yet to investigate the maths behind absorption flow resistivity, so what I'd like is to open the discussion to those that are/do understand, to have a look and either tell me something is very wrong with the data, or confirm that we might have something here...

OR

It could be a case that this material is so ludicrously expensive, hazardous or irritating that the positives simply get drowned by the negatives - but I cannot find anything on this specific product anywhere else, and I am yet to get a quote. It doesn't appear to be the standard type of rockwool everyone else offers, it's not compressed fibreglass, and it is not polyester - does it use more recycled materials than standard rockwool? Maybe it uses less and the purity provides these acoustic benefits...(?) I have no idea, but I'm hoping a few of you on within this highly esteemed community may know, or at the very least can prompt an interesting discussion so that we can work it out...

Furthermore, I'm struggling to understand how a material with so much density can be remotely effective at low frequencies, when I was under the impression that around the 50kg/m3 mark was more ideal than 96 (and thus why 703 for bass trapping is typically better than 705), and especially 192! That just seems outrageous...

The more I read, the less concise the information appears to get. It is like the NRC values provided by most companies do not correctly represent the materials effectiveness when say, filling a corner for controlling deeper bass frequencies. It's almost as if it doesn't actually matter beyond a certain point unless testing a stand alone piece off material in an isolated room. So these values just seem to have reduced importance at this stage.

Then on the flip side, I'm struggling to find empirical evidence that suggests going lower (for fibreglass specifically) than the standard 48kg/m3 is better for bass trapping. I'm' reading many posts stating roughly 30 - 40 is best for these "triangle chunks".

I've spent the better part of my day-off looking into this, at one stage I had chrome running something like 50-60 tabs across 4 instances, each going down separate search paths, soaking nearly all of my ram; so it is not for a lack of trying... If someone has a link specifically to some sort of testing completed comparing, specifically, two large bass traps, one with 703 and one with 705, of exactly the same design in a controlled room, that would be all I need. Until then I'm going to start organising frame construction and fill these after I'm 100% certain I'm getting the best bang for my dollar absorber. Thanks.

Does this material seriously have an NRC rating of 1.15 for a 100mm thick panel on a wall?

No. It has an NRC rating of 1.1, according to the published specs. The "1.15" number you are looking at is the alpha, or coefficient of absorption, for 125 Hz, which is used to arrive at the NRC rating, but is not the NRC rating.

I am fully aware of the variability of testing procedure (even when testing in accordance to a standard, trust me, standards often mean nothing ...

Very true! as in this case, for example, where they mention that the sample was tested to ASTM-C423 standard, but fail to tell you which under SUB-PART it was tested! There's a huge difference between testing it as "A" or as "E", yet they don't tell you which. That's like saying that a car got 30 MPG consumption rating, but forgetting to tell you if it runs on gasoline, diesel, jet fuel, or orange juice! (OK, so I exaggerate a little...).

but these numbers must prompt investigation.

I'm not sure why you are so concerned about them. I don't see anything out of the ordinary. It would help to know if that is "A" or "E" testing, but either way the numbers are reasonable.

I'm struggling to understand how a material with so much density can be remotely effective at low frequencies, when I was under the impression that around the 50kg/m3 mark was more ideal than 96 (and thus why 703 for bass trapping is typically better than 705), and especially 192! That just seems outrageous...

But you are trying to compare two different types of fiber (mineral wool, and fiberglass), while using only an indirect indicator (density).

Optimum density for absorption in the wall cavity of an MSM system is obtained with mineral wool that has a density of around 50 kg/m3, or with fiberglass that has a density of around 30 kg/m3. Two different materials, with different characteristics.

In reality, it is not the density of the material per se that matters: That's just a handy indicator, but not accurate. The actual characteristic that matters is "gas flow resistivity", which represents how the material reacts to gasses flowing though it (duh!). Different materials have different resitivities, but manufacturers seldom bother publishing this spec, since it means nothing for the main purpose of their products: thermal isolation. It is also pretty esoteric, and even most people who do want the product for acoustic absorption would not know what to do with it if the saw a figure of 10,000 MKS Rayls for a product (or whatever). So most manufacturers never even bother measuring that, and the few that do don't bother publishing it. Instead, the ones who do want to sell their product for acoustical purposes get it tested under ASTM-C423 and publish those figures, which are somewhat more meaningful.

Fortunately, for each type of material there is an approximate relationship between gas flow resistivity and density. The relationship is very rough, not linear, and varies by manufacturer, but is close enough to be usable as a general rule of thumb. However, if a manufacturer uses a different method from everybody else, such as a different binder, different compression, different facing, different fiber selection, different processing, etc., then their product can indeed have characteristics that don't fit the "density / gas flow resistivity" rule.

It is like the NRC values provided by most companies do not correctly represent the materials effectiveness when say, filling a corner for controlling deeper bass frequencies.

NRC rating is not really such a useful number. Two products can have identical NRC numbers but very different performance. The individual alphas are a bit more use, since they at least gives you some idea as to how it reacts in each frequency band. But even then, the testing was done in a specific, controlled environment with well established characteristics: unless your room is identical to the test lab room, you will not get the same results. Just like a speaker can have perfectly flat response when tested in an anechoic chamber, it will never have flat response when actually placed in a real-world room. The same here: the table of coefficients ("aplhas") found in a test lab will not match the actual results you get in your room. And the case you mention is a perfect example: If you take a slab of that material and place it diagonally across the corner, then you are no longer even using the same test conditions, so it certainly will not perform in the same way as suggested by the alphas.

It's almost as if it doesn't actually matter beyond a certain point unless testing a stand alone piece off material in an isolated room. So these values just seem to have reduced importance at this stage.

Well, the alphas still give you a rough indication of how the material will perform, and do allow you to compare materials, to a certain extent. For example, just using that same chart, clearly you would not want to use their 25.4mm 64 kg/m3 product for a bass trap, but it would be pretty good if you needed to only absorb some highs, due to the room being too bright. On the other hand, their 101.6mm product of the same density should make a pretty good broadband absorber that goes down to pretty low frequencies, and covers the entire spectrum fairly evenly. From that point of view, it should be as good as, or maybe even better than, 4" 703 with "A" mounting.

Then on the flip side, I'm struggling to find empirical evidence that suggests going lower (for fibreglass specifically) than the standard 48kg/m3 is better for bass trapping. I'm' reading many posts stating roughly 30 - 40 is best for these "triangle chunks".

The "standard" number for fiberglass is 32 kg/m3, not 48 kg/m3: the latter is for mineral wool (generic, in both cases).

If someone has a link specifically to some sort of testing completed comparing, specifically, two large bass traps, one with 703 and one with 705, of exactly the same design in a controlled room, that would be all I need.

The OC catalog has that information. Here's the relevant page:
As you can see, plain 703 is better for both mountings.

In general, lower density is better for low frequency absorption. Compare, for example, how 4" 701 holds up in the above table: It outperforms both 4" 703 and 4" 705 across the board for E-405 mounting, which is probably the closest to a diagonal corner trap... But this is a set of bell curves, obviously, that vary by frequency for each product, so there is an optimum value that rolls off for higher and lower density, for each frequency, and each mounting.

It's all a bit confusing, for sure! No argument there. Some people even suggest using two or three layers in bass traps, with a more dense layer on front, and less dense layers going back, while others recommend the opposite (less dense on front, more dense going back). In reality, the differences aren't that great, so just filling the entire corner in the "Superchunk" style seems to give the best performance.

And one more issue to confuse you: all those measurements and tables and coefficients and ratings only really measure normally incident sound: but with real-world rooms, a lot of the sound is not normally incident!

By the way, please check the forum rules one more time: you are still missing something!

- Stuart -

Thanks for your taking the time to reply mate.
But what of the 192kg/m3 material? It appears to have a very good alpha value at 125hz, and this is what spiked this whole thing. If 50mm provides 0.4, then surely 75mm could do a pretty good job.
Here's my thinking; a 50 - 100mm panel absorber/bass trap assister, with 25/75mm of this 192kg/m3 mineral wool and 50/25mm of gap could provide better performance when mounting these panels behind the speakers or on the backwall behind the listening position (which is directly behind my head, unfortunately).

That's really the crux of this - assuming a superchunck would soon fill the corner, would this material be the best thing suited for a 50-100mm absorber? It seems to be significantly better than most other materials for at the very least providing some assistance to bass response, as well as providing good broadband absorption.

PS
Location fixed in my profile too - I did see that part in the rules and just forgot to go back...

I'm currently trolling gearslutz now, as similar to this forum, there is a wealth of knowledge on various types of bass traps and acoustic materials - but it's just too much to consume in such a short period of time, such that it is becoming difficult to retain...