6" 705 frk vs 6" 703 frk which is better???

[ejsongs]

hey,

i know that both will work but which is better for absorbing low end, 6" 705frk or 6" 703 frk?

from what i've read thus far the density of the panels is less significant the thicker the panel. i am guessing the 703frk would edge out the 705frk. what say you gurus?

also not to ethan...could you please explain how to interpret your graphs on your density report. not sure what i am looking for.

which is better for corner mounting?

specifically i've got 3 6" traps on the back wall of my room (2x705 and 1 703). i have the 6" 703 in the corner. is it ok to mix and match these panels?

thanks,

ej

[Ethan Winer]

I'm pretty sure 705 is a little better than 703 at six inches thick, but only a little better. Since it costs twice as much, let price be your guide.

What you're looking for is the decay times at each mode peak. From my Density Report:

"Note that the tests described here do not give any numbers!
All analysis is done visually, looking at how the modal ringing decay time
is reduced (the "mountains" come forward over time) at each mode
frequency, and also by seeing how the Q of each mode is lowered thereby
making the peaks broader."

Yes, it's fine to mix densities.

--Ethan

[Abel]

I am pretty confused now...

Ethan's density test suggests that 6" OC705-FRK is better then 6" OC703-FRK for frequencies below 120Hz.

but, the Insualtion FAQ at basstraps.net insists that OC703 is superior for thick panels.

which is true?

I understand that basstraps.net is affiliated with readyacoustics.com
and Ethan represents realtraps.com

[Glenn Kuras]

Originally Posted by Abel

I am pretty confused now...

Ethan's density test suggests that 6" OC705-FRK is better then 6" OC703-FRK for frequencies below 120Hz.

but, the Insualtion FAQ at DIY BASS TRAPS - Index insists that OC703 is superior for thick panels.

which is true?

I understand that basstraps.net is affiliated with readyacoustics.com
and Ethan represents realtraps.com

Considering that Ethan has tested it I would go with him. I think though Ethan will be the first one to say that both really are going to work close so if funds are low then go with more 703.

[Ethan Winer]

Right, I actually tested it.

But the differences are small enough to not be 100 percent conclusive. I think they are! One of these years I hope to do another test using two or three times more material to make the differences even more pronounced.

--Ethan

[Glenn Kuras]

Originally Posted by Ethan Winer

Right, I actually tested it.

But the differences are small enough to not be 100 percent conclusive. I think they are! One of these years I hope to do another test using two or three times more material to make the differences even more pronounced.

--Ethan

And hoping it is in a lab!

[Ethan Winer]

Originally Posted by Glenn Kuras

And hoping it is in a lab!

Nah, I'll get much more reliable data in the "lab" room at our factory. A lab is not as repeatable at very low frequencies as REW in a small room. In this case I'm looking for comparative data, not absolute sabins of absorption. So repeatability is more valuable than knowing the sabins.

--Ethan

[Glenn Kuras]

Just 3 weeks ago you said you wanted to test both. In fact you stated you would not be apposed to teaming up for this, which I am totally for BTW. With all due respect testing in your room at the factory is great but only tells you what is going on in that room. But I do respect your testing and think the one on density is done quite well.

[Ethan Winer]

The main problem with lab tests at very low frequencies is the microphone is in constant motion. That helps at mid and high frequencies, though only for C423 random incidence tests. Corner bass traps absorb mainly on their front surface only. But the real issue is the moving microphone. I explain why a small room test is actually better than a lab test (at very low frequencies) in detail in my article from S&V magazine:

Alternative Test Methods for Acoustic Treatment Products

With ETF or FuzzMeasure or REW, every time you run a sweep you get the exact same result. This is not the case for lab tests, at least not down at 60 Hz and below.

Edit: I just spoke with my contact at IBM's lab, and he said he has data on exactly what we're discussing. He promised to send me something conclusive "soon" showing the measurement spread across successive runs at different very low frequencies.

--Ethan

[Abel]

Originally Posted by Ethan Winer

I just spoke with my contact at IBM's lab, and he said he has data on exactly what we're discussing. He promised to send me something conclusive "soon" showing the measurement spread across successive runs at different very low frequencies.

this would be excellent!

[Glenn Kuras]

Originally Posted by Ethan Winer

Edit: I just spoke with my contact at IBM's lab, and he said he has data on exactly what we're discussing. He promised to send me something conclusive "soon" showing the measurement spread across successive runs at different very low frequencies.

--Ethan

That would be great as Riverbank lab pretty much explained the moving mic and why that is ok. I really don't have time to find the thread but this was benn explained. Basically the mic is moving in 6 different spots in the room to take into account for the room modes. That is part of the reason it does so well.
I will say we are getting off subject so lets move on.

[Ethan Winer]

Originally Posted by Glenn Kuras

That would be great as Riverbank lab pretty much explained the moving mic and why that is ok.

Yes, moving the microphone is okay - and necessary! - above 100 Hz, though you still will have some variances. But it's not "okay" below 100 Hz. That's the whole point. And that's also why most labs are not certified below 100 Hz. At those very low frequencies the lab room's response is dominated by highly positional modal ringing, so it's not true reverb anymore. IBM's lab uses a moving microphone, which is what the ASTM standards call for. Now, I admit I don't know everything about reverb room testing, but I do have the C423 Standards. That document suggests a few ways to ensure random incidence, but the main one seems to be:

10.1.2 If a moving microphone is used, the microphone path shall be at least 0.75 m from any surface of the room or test specimen.

Otherwise they expect a number of baffles to be moved instead:

It is strongly recommended that some of these panels be mounted on a rotating shaft or otherwise kept moving, presenting, in effect, a room that continually changes its shape.

Whatever. I stand by my assertion that a "home made" test can give perfectly reliable results when the goal is a comparison of different absorbers. A lab is needed only when one needs absolute sabins that are also certified.

--Ethan

[Glenn Kuras]

hat's the whole point. And that's also why most labs are not certified below 100 Hz.

Neither is yours below or above and that sir is the point.

Now, I admit I don't know everything about reverb room testing, but I do have the C423 Standards.

Right and really think both our companies need to gain more info. I sent Frank on a fact finding mission and will have him post his findings. Everything you are saying is right but there is a lot more to it.

[Abel]

Originally Posted by Ethan Winer

Whatever. I stand by my assertion that a "home made" test can give perfectly reliable results when the goal is a comparison of different absorbers. A lab is needed only when one needs absolute sabins that are also certified.

I would argue that home test results MAY NOT translate well to other rooms.
Like with speakers, that may be very revealing and precise, yet the mixes made on them do no necessary translate to the majority (average) of systems.

[Abel]

and there is this whole "entry impedance" thing, that I really don't understand.

[Glenn Kuras]

BTW this is about the point of time when 99% of the people on GS's eyes start to glaze over and view us acoustic guys are a bunch of nerds.

The Nerd,
Glenn

[Weasel9992]

Originally Posted by Ethan Winer

Yes, moving the microphone is okay - and necessary! - above 100 Hz, though you still will have some variances. But it's not "okay" below 100 Hz.

Mmmm...kinda. Even though they're not certified, that doesn't mean that the results are unreliable. It simply means that there's a higher error range, that's all. By "higher" I mean that it goes from like 6% above that to maybe 10% down to 63Hz. Below that it gets higher, but still not wildly higher...maybe 15%. That's pretty good data I'd say. Add to that the fact that the tests are being conducted and the data collected by extremely skilled technicians who do it all day long and I'd say you've got some very broadly applicable results that make it easy to extrapolate to external situations.

I mean...the efficacy and veracity of lab testing isn't really questionable. The methodology is well established with a number of years and many many thousands of tests behind it. I agree that more informal room tests of the sort we recommend all the time work well too...they're just not quite as accurate. As Ethan said, they're really not meant to be though...they're more like a snapshot just to let you know where your problem areas are likely to be.

Frank

[Glenn Kuras]

Frank,
Found your post on it.

Gap behind trap vs. thicker trap?

It's great working for GIK, where on any given day Glenn can have me hunting down answers on just about any topic on the planet. Your questions raised some curiosity, so I called RAL and talked to Dave.

Basically Dan was right about everything:

* There are no “official” method for measuring bass traps, but RAL has plenty of guidelines and history which provide a pretty solid framework for reliable testing methodology. In other words, there’s no real reason to doubt the results based on methodology.
* The reverberant chamber itself is diffuse within 1dB at any given certified frequency.
* The boom arm basically just puts the mic in any one of six locations continuously during the 80 decay-cycle testing procedure. Those readings are averaged at the end to give a comprehensive result. In other words, statistically it’s the same as taking the readings from six fixed positions in the room then averaging the results, which takes modal effects out of play with respect to results. This speaks to the implimentation of proper spacial averaging you were talking about.
* RAL is certified down to 100Hz.
* RAL’s testing error range is +/-6% down to 100Hz, and David estimated that it probably would increase to +/-10% by the time you got down to 63Hz, and something between 10-15% at 50Hz..

All of this is borne out in a real-world way by the curves we see on the RAL results our Monster Bass Trap and 244 panel. Given the physics at work and testing methodology employed it's exactly what we'd expect to see.

Hopefully this answers the questions. As Dan said earlier, there's really no reason to doubt the results since good methodology was followed and disciplined testing protocalls were observed. Any questions about procedure or reliablity could easily be addressed to Dave by phone I'm sure.

[Abel]

this gets more interesting...

so are there any certified test results for the 80-33Hz range
in case you want to have a sub in a smaller room?

[Abel]

I found this very interesting post by Scorpius:

When searching info about DIY bass traps from internet, one finds out that evertbody claims optimal wool density to be around 3.0 PCF (50 kg/m3). As density is somewhat propotional to flow resistance (flow resistance plays major role in sound absorption based on viscosity losses-> resistive, "passive" absorbers), 3.0 PCF has roughly approximating flow resistance of 30000 Nm/s4.

There is evidence in literature and in simulations, that flow resistance of ~30000 Nm/m4 is optimal only when wool is installed direclty on a wall, without any air gap, But in situations, wher air gap is behind wool, usually (this depends of depth of air gap) much lower flow resistivities deliver notably better absorption coefficient in lower frequencies.

We have made simulations with MATLAB code based on Delay-Bazley equations, see below:

Measurements seem to support this theory; I came across site where measured absorption coefficints of popular (american?) wool products are being displayed. Link.

Here popular Owens Coring 700 series wool shows evidence of this phenomena:

701, plain 4" (102mm) on wall 1.5 pcf (24 kg/m3) 0.73 1.29 1.22 1.06 1.00 0.97 1.15
703, plain 4" (102mm) on wall 3.0 pcf (48 kg/m3) 0.84 1.24 1.24 1.08 1.00 0.97 1.15 <--- optimal
705, plain 4" (102mm) on wall 6.0 pcf (96 kg/m3) 0.75 1.19 1.17 1.05 0.97 0.98 1.10

701, plain 4" (102mm) 16" air 1.5 pcf (24 kg/m3) 0.87 1.14 1.24 1.17 1.18 1.28 1.20 <--- optimal
703, plain 4" (102mm) 16" air 3.0 pcf (48 kg/m3) 0.65 1.01 1.20 1.14 1.10 1.16 1.10
705, plain 4" (102mm) 16" air 6.0 pcf (96 kg/m3) 0.59 0.91 1.15 1.11 1.11 1.19 1.10

I'd like to point out this because PCF 3.0 wool is constantly being adviced as the best wool in diy bass trap solutions (with air gap), but I beg to differ. I would suggest wool with density of PCF 1.0. If this is not available, then use wool woth PCF 1.5.

what do you guys think?

[Hypagen]

Originally Posted by Glenn Kuras

BTW this is about the point of time when 99% of the people on GS's eyes start to glaze over and view us acoustic guys are a bunch of nerds.

The Nerd,
Glenn

I disagree, I think 99% of GS people come here to get educated!

[Glenn Kuras]

Originally Posted by Hypagen

I disagree, I think 99% of GS people come here to get educated!

That is great but was talking in terms of what a lot of guys really want to know or understand about this stuff. I am glad you find it interesting.

[ejsongs]

i am both honored and flattered that two acoustic titans would contribute so much to a thread that i started.

this is quite educational for me and provides therapeutic reading whilst logic perpetually crashes!!!!

this is of extreme interest to me in that i have in my room:

4 gik tri traps
2 gik 244 panels
2 oc 705 6" panels with frk
1 oc 703 6" panel with frk
2 oc 703 2" panel plain

what i've noticed is that outside of the numbers 703, 705, and mineral wool (244 and tri traps) all have a unique sound to them.

i can get dramatically different results depending on what is place where and in what combinations...still searching for the right mix.

considering i have a small room (14x12x 8ft 6") knowing what will work the best is extremely important to me and others like me.

so please keep posting..we deserve to know the real deal.

ej

[avare]

Originally Posted by Abel

I found this very interesting post by Scorpius:



what do you guys think?

Scorpius is correct. There is more involved to getting desired performance from absobent material than just putting a gap behind it. Long has some good summary material in Architectural Acoustics derived from work by Ingaard, published in 1994.

Long's text is highly recommended as a serious summary (only 600+ pages) of the title subject.

Succinctly,
Andre

[AwwDeOhh]

Originally Posted by Glenn Kuras

BTW this is about the point of time when 99% of the people on GS's eyes start to glaze over and view us acoustic guys are a bunch of nerds.

The Nerd,
Glenn

AHH, my eyes are blurry! can't read, the nerds took my sight!

Actually i find this discussion very informative - as well.
Not that as the end user, i need to know all the knitty-gritty stuff... i'm the type that likes to learn new things, and why things work the way they do.
At the end of the day, all i know is: that the more traps i have in my relatively small room, the better recordings that come out of it!!!
It takes the 'room' out of the room. (which is especially great in my subpar recording space)... as the saying goes.. you can always add room into it after the fact.. but it's nearly impossible to take it out once it's recorded that way!

[Ethan Winer]

Originally Posted by Weasel9992

It simply means that there's a higher error range, that's all. By "higher" I mean that it goes from like 6% above that to maybe 10% down to 63Hz. Below that it gets higher, but still not wildly higher...maybe 15%.

Let's say the repeatability of a lab is +/- 6 percent at 100 Hz, and +/- 10 percent at 63 Hz, and +/- 15 percent at 50 Hz. I think you'll agree those figures are about right since I got them from you.

So that means that successive runs to test 703 versus 705, or a corner filled versus straddled by a 4-inch panel, can vary as much as 12 percent total at 100 Hz, 20 percent at 63 Hz, and 30 percent at 50 Hz. Again, this variance between runs is due entirely to using a moving microphone.

Let's further surmise that 705 absorbs 20 percent more than 703 at 50 Hz. I'm not saying it does, just using that as a hypothetical example. So we run a lab test of 703 and the result comes out 15 percent too high at 50 Hz. Then we run another test with 705 and this time the result is 15 percent low.

Okay, so now the "lab test" has told us that 703 absorbs 10 percent more at 50 Hz than 705, even though the 705 is actually 20 percent better. This is the problem in a nutshell. Versus using REW or Fuzzmeasure in a small room, where you'll get the exact same results every single time you test. You may not get to see the sabins of absorption, or even know exactly how much better the 705 is than the 703. But by looking at the response and waterfall plots it will be conclusive which density actually absorbs more at 50 Hz.

I mean...the efficacy and veracity of lab testing isn't really questionable.

Of course it is! We already know they can be off by as much as 12 percent total even in their certified range. And that's just for successive runs in one lab. When you compare the results of the very same material as measured in different labs, the variance might be even greater. I have in front of me an ASTM document that shows the variance among 16 different labs. Even at 1 KHz the spread for a given sample ranges from 0.95 to 1.08. This is document E33-1005, Table 5. I think that's about a 13 percent total variation. So when people look at the absorption numbers on Bob Golds' site and pick a material because it's 0.03 better than another brand, they're basing their decision on "noise" rather than meaningful data.

--Ethan

[Glenn Kuras]

Ethan,
I don't think anyone is doubting there could be a better way, well besides building a bigger test lab but all testing in your factory is only going to tell you what is happening in that room. You won't be able to translate it to another size room(s). BTW your 703 vs 705 example is guessing with worst case. NICE Needless to say I think your tests you have done showing density is great and do not find anything wrong with them.
We have a article coming out on this in the next few days. Frank has done a ton of reseach on this. I will forward a copy to you.

All the best

[Ethan Winer]

Originally Posted by Glenn Kuras

all testing in your factory is only going to tell you what is happening in that room. You won't be able to translate it to another size room(s).

Not so! If 705 absorbs better than 703 at 42 Hz, and 58 Hz, and 65 Hz, in my room, it will absorb better in all rooms at those low frequencies. And at all other low frequencies too. It's not like one material is better at 52 Hz but worse at 53 Hz.

your 703 vs 705 example is guessing with worst case. NICE

Of course, but the point is labs vary and that could happen. So you pay your $1,000 and end up with the wrong answer.

We have a article coming out on this in the next few days. Frank has done a ton of reseach on this. I will forward a copy to you.

Excellent, I look forward to it. If you'd like to run it by me first for a fact check before you publish, I'll be glad to do that. <half-kidding, ducking>

--Ethan

[Weasel9992]

Originally Posted by Ethan Winer

Excellent, I look forward to it. If you'd like to run it by me first for a fact check before you publish, I'll be glad to do that. <half-kidding, ducking>

Nah, that's okay. We want it to be right. <half-kidding, ducking>

Frank

[Glenn Kuras]

Not so! If 705 absorbs better than 703 at 42 Hz, and 58 Hz, and 65 Hz, in my room, it will absorb better in all rooms at those low frequencies. And at all other low frequencies too. It's not like one material is better at 52 Hz but worse at 53 Hz.

So if you test the 2 in your factory and there is no difference at a certain frequency in the test graph from nothing in the room then we can conclude nether did anything? Or if it only helped by 2 db, at a certain frequency, then that is what you will get in EVERY room? I have seen 8 traps help in one room 15 db and in another room (different size) 5 db at the same frequency.

Excellent, I look forward to it. If you'd like to run it by me first for a fact check before you publish, I'll be glad to do that. <half-kidding, ducking>

I really don't see any need to be insulting. So lets all try to learn instead of insulting each other. OK?