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Are acoustic panels stackable?
Old 16th June 2010
  #1
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jsbeeth's Avatar
 

Are acoustic panels stackable?

This may be an epically stupid question, but I came into some 2" thick OC 703 panels, and as 2" is not thick enough to provide significant bass trapping, I was wondering if simply stacking the panels 2 or 3 times together would have an identical acoustic effect as a 4" - 6" panel. I assume they would, but I wasn't sure because the 2" panels are framed and hence there would be an inch or so of separation between the actual fiberglass segments when stacked. Obviously I could just remove the existing frames and make new thicker panels, but would that even matter? Thanks!
Old 16th June 2010
  #2
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Stacking multiple unbacked pieces would perform as a single piece of the total thickness. Stacking multiple pieces with a one inch airspace between each will not to my understanding.
Old 16th June 2010
  #3
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I have never tested a air gap between sheets but I would think your ok.
Old 16th June 2010
  #4
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Quote:
Originally Posted by Glenn Kuras View Post
I have never tested a air gap between sheets but I would think your ok.
has anyone tested that?

-Danny
Old 16th June 2010
  #5
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gaps between panels is roughly the design concept of "hangers"... those seem to preform very well.
Old 16th June 2010
  #6
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My understanding is for LF trapping, you need a rigid substrate within the absorbent material for "hangers".

T
Old 16th June 2010
  #7
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OP

Stacking should be just fine. Some people glue them together with SprayTac or similar specialist glue. These glues throw out a lacy spiders web of fast drying glue. They are used to attach batts to Ducts or FRK to batts and so on.
Simply squeezing into your frames is probably perfectly fine. However I would not leave airgaps in there. The impedance change would generate slight reflections reducing the power of your panel.
For the same reason I would not use mix panels of different material or density.
DD
Old 17th June 2010
  #8
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Quote:
Originally Posted by tonio View Post
My understanding is for LF trapping, you need a rigid substrate within the absorbent material for "hangers".

T
good point.
Old 17th June 2010
  #9
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Wisdom or...

Well there is indeed a view that changes of density within a sandwich will cause reflection. However is that good or bad? If the wave is reflected back and forth between these layers, will it have to traverse longer more tortuous paths. Getting medieval now heh. I'm with Glenn, Test Test Test.
I also wonder is there a way to use stepped impedance to encourage the wave inward, conversely giving it a difficult escape route when it bounces off the boundary within. A lobster trap.
Speaking of which, Newell does use hangars, arranged in arrays to create such a lobster trap. It would appear that these work as a system, empirically designed, but the science to explain the behaviour is catching up.
DD
Old 17th June 2010
  #10
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Quote:
Originally Posted by DanDan View Post
Well there is indeed a view that changes of density within a sandwich will cause reflection.
It's not a view DD, it's a fact. State it will confidence heh

Changes of acoustic impedance will cause a reflection of sound energy, a transmission of energy and a loss of energy
Old 17th June 2010
  #11
SAC
Registered User
 

...to throw gasoline on that fire...


As far as it being 'fact', the effects of the transverse across differing acoustical boundary impedances is well understood and indeed based on physics.

And while a few have suspected the later point (but lacked the necessary and sufficient proof!), and a few more have had the audacity to openly postulate such a heretical and blasphemous notion that acoustics is indeed (supposedly) posited on a physics basis, we only make reference to that 'black art' when ABSOLUTELY necessary at last resort if a subjective emotional basis is insufficient to sway the masses - and never if 'math' is involved.

Too often I get the impression that this forum is akin to an engineering firm where no one is allowed to refer to (or acknowledge) established mechanical models (let alone wave mechanics in compressible and non-compressible fluids) and mathematical quantification is NEVER allowed as we debate and design the latest suspension bridges and earthquake tolerate high-rises.

But then, what is nice is that with no objective measure of performance, subjective feelings rule and reality becomes a function of whatever one believes it to be. (Based on the etymology of the term "to believe" which appropriately evolves from the root meaning "to wish something true".)

Old 17th June 2010
  #12
SAC
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As far as what is happening in regards to multi-layer absorption, the transfer matrix is rather simply laid out in 1.5.1 of D'Antonio & Cox's Acoustical Absorbers and Diffusors, 1st ed. (fig. 1.7, p 19 of the 1st Ed.; or fig 1.9, p.23 of the 2nd Ed.). One need not dig out or find an obtuse mechanics text (you know the one's that tend to utilize ...wait for it... numbers! :-S ).



And if one is really sharp you can evaluate whether or not there is a resultant inversion of phase of the incident and reflected waveforms depending upon the subsequent acoustical density of each region encountered as well. Sounds like lots of fun! (Actually its very simple - you just have to keep track of it through all of the transitions!)

And actually, the comparison of energy lost via multi-layer transversal and single thick layer of porous material could be as well - and not that difficult! IF it were worthwhile to do so!

The problem is that we are dealing with a relatively inefficient passive absorber functioning in an area of relatively low velocity near a boundary - which is actually the wrong place to be using such a 'tool'. But we suffice as it serves our rather low order purposes.

In such a scenario, there is the reflectance of energy with each boundary crossed, whereas in a contiguous porous system of uniform density and acoustical impedance, there is minimal directional reflectance other than the incident surface itself. Thus, while such additional boundaries can conceivably increase absorptive losses, this is done with an increase in the degree of reflected energy with each boundary transition (which must then traverse each boundary once more over its return path...).

Now, is the reflected energy of sufficient energy content that it will traverse the various layers and exits the multi-layered surface with sufficient energy to make a real perceptible difference? Probably not if you are more than a few inches from the surface!

And why debate this based upon unsubstantiated subjective feelings (read: guesses)? Calculate it! (Basic 'dimensional analysis' will give you the rough final relationship without even needing to use numbers...as mass air mass air transitions are rather simple.)
Or is everyone simply waiting for the proverbial "other guy" - you know the one, with the name "they"..., as in "When are "they"...going to do the dirty work????" No wonder so many 'think' (I mean 'feel') that acoustics is such an amorphous unknown 'black art'. Hmmm...I wonder if Walt Kelly wasn't actually an acoustician????

My 'feelings'? We are spending dollars to determine if pennies can be saved.

First, for a given thickness, one would be better off simply cannibalizing half of the panels and doubling up the existing panels in order to make several 4 inch thick panels and, if desired, offsetting them another 4 inches from the wall. Heck, you only have to hang ONE panel! And if more are desired, simply buy additional OC703 and stuff the remaining frames (or do so from the start and simply add the additional material to all of the 2" thick frames to have twice the number of 4" thick panels).

But if anyone is intent to debate this further, my suggestion would be to instead take the same energy that is spent guessing and apply it more productively to measuring and calculating the design for a dual layer perforated resonant trap offering a more optimal and efficient trap for use in the high pressure region near the boundary surface where so many are instead more concerned with debating the performance of porous absorbers designed for high velocity regions deployed in low velocity - high pressure regions!
Old 17th June 2010
  #13
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If anyone can find the flow resistivity of OC 703 I'll do some calculations in Matlab, using Delany and Bazley and transfer matrix, i.e. the code in the back of the Cox and D'Antonio book. This will show the difference between having air gaps between each layer and not having gaps.

Actually for comparision any reasonable flow resistivity value will do... don't have time to do it right now though...

SAC for some reason I completely forgot the point that when meeting a change in impedence the reflected wave will travel back through the absorbent it has just travelled through, hence improving performance....kind of obvious

Also dual layer perforated resonant trap.....now there's a good idea! It would be good to see a few more of those kicking around this forum
Old 17th June 2010
  #14
Gear Guru
Obvious

Quote:
If the wave is reflected back and forth between these layers, will it have to traverse longer more tortuous paths. Getting medieval now . I'm with Glenn, Test Test Test.
Dange, My seeming lack of confidence was considered! Test trumps theory in my book, although I do like when all views agree.
I would love to see some number crunching. You would be a brave man to tackle the above though! I think it might be more useful if you could run the numbers for some common practice, thin panel, large airgap, and vice versa.
If you are that brave soul, how about a SSC vs a Straddle panel?

DD
Old 17th June 2010
  #15
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This has revived a question that other threads have piqued in the past...what about a continuously-variable-impedance absorber that provided a smooth gradient from the impedance of air to the impedance of wall? A variable-impedance solution seems very elegant and possibly very effective, but the fabrication might be difficult/expensive. Is there any research on this? I haven't seen/recalled it.
Old 17th June 2010
  #16
Gear Guru
Yes-ish

Brain, yes I have noticed a couple of examples. Newell, after Hidley and others perhaps uses Waveguides. Arrays of hangars, with angles and spacing too provide an easy entry. It gets a bit more tortuous inside the structures, slowing down the wavefront and giving it in general a hard time. When it is finished bouncing around in there it should hopefully have little energy for the return journey.

On a simpler note, the wedges in an anechoic chamber would appear to offer a graduated impedance.

DD
Old 17th June 2010
  #17
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Quote:
Originally Posted by DanDan View Post
SSC vs a Straddle panel?
What are those?
Old 17th June 2010
  #18
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Tips

SSC is the Studiotips SuperChunk, they asked for it to be referred to in these ways. A Straddle panel is an absorbent panel across the corner, 45 degrees.
This could be a Studiotips Corner Absorber, or a GIK or any other panel trap.
studiotips - tips on studio design, acoustics, and wiring
DD
Old 17th June 2010
  #19
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Quote:
Originally Posted by Brainchild View Post
This has revived a question that other threads have piqued in the past...what about a continuously-variable-impedance absorber that provided a smooth gradient from the impedance of air to the impedance of wall? A variable-impedance solution seems very elegant and possibly very effective, but the fabrication might be difficult/expensive. Is there any research on this? I haven't seen/recalled it.
Posiible and done all the time in anechoic chambers.

There are cheaper methods, in material, labour, and floor area that are usually used.

Andre
Old 18th June 2010
  #20
Gear Addict
 

Here's a cool animation of high speed to low speed (low density to high density) scroll down.

Reflection of Waves from Boundaries
Old 18th June 2010
  #21
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Some more goodies for related info.
Superposition of Waves

Old 18th June 2010
  #22
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Quote:
Originally Posted by tonio View Post
Here's a cool animation of high speed to low speed (low density to high density) scroll down.

Reflection of Waves from Boundaries
Nice animations. Thank you!

Strung out,
Andre
Old 18th June 2010
  #23
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so is it bad to make mix fiberglass panels from different manufacturers? specifically i have some oc 2" 703 and some knauf fiberglass insulation board with ecose. both are 3.0 pcf but make my room sound different from each other and seem to be more/less absorbent at different frequencies (the knauf seems to kill too much high end to me judging by the sound of it). i basically ran out of 703 and picked up 6 sheets for the knauf for cheap. would it be bad to have them in the same panel...or should i just get more 703 since that is the majority of what i have?

knauf (a mounting)
3.0 PCF
(48 kg/m3) Plain 2"
(51 mm) .29 .65 1.11 1.13 1.06 1.03 1.00

3.0 PCF
(48 kg/m3) Plain 4"
(102 mm) .95 1.11 1.17 1.07 1.07 1.06 1.10

oc 703 (from bob golds)

703, plain
2" (51mm) on wall
3.0 pcf (48 kg/m3) 0.17 0.86 1.14 1.07 1.02 0.98 1.00

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

the trouble spots in my room seem to be around 125 and 250...

in order to deal with it i ended up using 6" panels of 703 and 705 on the back wall and 6" panels of 703 in first reflection point. am planing to do 4" panels for 2nd reflection point which is why i am asking about this.

thanks,

ej
Old 18th June 2010
  #24
SAC
Registered User
 

Others may be able to comment more authoritatively on any discernible audible spectral differences...if there are significant discernible differences.

But for the most part, I would think you would be OK.

If
perceptual differences are an issue (and I am not sure that they are significant) you could address the issue by mounting pairs of like construction in a symmetrical manner.

Does this make sense? In other words, say you have a 703 based panel, a hybrid 703/Knauf panel and a Knauf panel; you could install 2 - 703 panels symmetrically, with one each on, say, the left and right wall, followed by one each of the hybrids on each wall and one each of the OC panels on both surfaces.

On the other hand, you could make them all 'equal' to each other by making them all hybrid panels with the OC and Knauf materials and they will all have the same character.

Hopefully I have sufficiently confused you - as I would hate to have to come back in order to try again!

BTW - ditto re: the nice links tonio!
Old 18th June 2010
  #25
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Thanks for the replies. Had no idea it would instigate such a thoughtful discussion! And now, it's getting a bit sciency in here for my bohemian constitution... heh
Old 20th June 2010
  #26
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Quote:
Originally Posted by jsbeeth View Post
And now, it's getting a bit sciency in here for my bohemian constitution... heh
Don't read this then heh

I did a comparision in Matlab of two different configurations:

-2in panel, 2in air gap, 2in panel, 2in air gap
-4in panel, 4in air gap

The difference is minimal.....

Note: In the 4in gap/panel one, the model treats 2 x 2in panels together as one 4in. In real life there will be a slight impedance change, probably tiny, as you go from one panel to the other.

DanDan, modelling the SSC and the straddle panel isn't as straight forward as what I've done above. Above is on axis absorption into a uniform thickness and infinitely sized panels/air gaps, with rigid termination. The fact the SSC and straddle panel are in a corner and neither has a uniform thickness of air gap or absorber makes it slightly tricker.....I could try average thicknesses but not sure how accurate that would be
Attached Thumbnails
Are acoustic panels stackable?-stacked-absorbers.jpg  
Old 25th June 2010
  #27
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Is there a theoretical answer about layering? Even if you adhere two panels together, the fibers are not intertwined like the manufactured panels are.
Old 25th June 2010
  #28
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Quote:
Originally Posted by TooManyToys View Post
Is there a theoretical answer about layering? Even if you adhere two panels together, the fibers are not intertwined like the manufactured panels are.
The acoustic interface between the panels is not significant.

Andre
Old 26th June 2010
  #29
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Thanks Andre.
Old 26th June 2010
  #30
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And what about drop ceilings!!?!?!?

We have older rooms that have suspended t-bar ceiling grids with 1"/6lb panels. On top of those panels are 2 layers of (I believe) R-13 which looks to be 7" in total thickness. From that point we have a 6" air gap to the real ceiling. After reading this thread, I'm wondering what anticipated benefits the additional R-13 provides??? Unfortunately, I don't know who specified the original "design" nor do I have access to original drawings. But I never really thought about it until reading these posts!!!!

Any quick thoughts???

Thanks for a very interesting exchange.

AB
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