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Discussion Starter · #1 ·
Does anyone have any good advice on Ceiling Clouds (size, thickness, etc.)?

I'm also very curious as to how big of an air gap should be present between the ceiling and the panel. I've seen advice from 4" to 12", but nothing which describes why one would be selected over the other.
 

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It depends largely on what you're trying to accomplish and the thickness of the absorbing material. Generally, the gains slow down when the gap gets bigger than the thickness of the material.

Bryan
 

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I suspect that this will be far more than desired, but hopefully this will provide a more complete 'method to the madness' into the concepts behind broadband porous absorptive panels used to control specular reflections...of which clouds are simply one example.

Energy that exhibits wavelengths longer than room dimensions manifest themselves as standing waves described by modal behavior models. These frequencies are dealt with either with velocity based porous bass traps, or with pressure based tuned resonant absorbers.

Energy that consists of wavelengths shorter than the room dimensions manifest themselves as specular reflections – focused discrete energy that can be modeled as focused rays or vectors able to be resolved in terms of gain and direction. Specular energy reflections that create problems (note, all reflections are not bad!) is typically deal with by the application of either porous velocity based absorption or with diffusion. Here we will focus on the use of absorption.

So, first, how are modal and specular energies related, and at what point (frequency) do we need to begin addressing them. (We will simply describe basic wave behavior here, what constitutes anomalous wave behavior and what reflections should be addressed is beyond the scope of this post and can be addressed in another thread…)

The ‘point’ at which the wavelengths are become larger than the room dimensions, or conversely, at which they all become smaller than the room dimensions is referred to as the Schroeder frequency after Manfred Schroeder (or on occasion one will see it described as the Davis frequency after Don Davis). Typically this ‘point’ is a region that defines a bandpass range, covering the dimensions defined by the 3 major dimensions of a space, where one will have a mix of modal and specular behavior. Or one may use it to describe the ‘limiting wavelength defined by the shortest dimension that will correspond to the highest frequency. Don’t get hung up on this, as the important issue is that one understands the conditions and nature of the change in the nature and behavior of waves.

Also note that the lower the frequency, the higher the energy content of the wave. And note that above ~10,000 Hz, the wavelengths and the energy content is so small that we normally do not bother with this except in very rare instances. Thus what needs to be addressed is the energy from the low frequency where the wave begins to act specularly as a focused reflected wave whose spatial behavior is generally modeled as a ray.

As such, absorptive panels used to control early arriving specular (focused) reflections that impact intelligibility, imaging and localization must be broadband on nature. They must adequately control all of the high energy content specular frequencies above the modal frequencies. Thus the critical aspect of an panel absorber used to control anomalous specular reflection behavior is their low frequency extension and our ability to control this.

Thus, in order to address the energy that is reflected specularly, the minimum thickness one should consider using porous absorption that is effective against velocity waves, is 4 inches thick with a 4 inch gap between the panel and the wall boundary. Thus, the total effective thickness is 8 inches.

(To give you an idea of the difference in effectiveness, a typical 2 inch thick porous absorber would become effective at ~1690 Hz at normal incidence and at ~1200 Hz at 45 degrees of incidence! …FAR too high as this ignores the preponderance of the high energy spectrum!!!)

For broad band absorbent panels, the best material density characteristics are ~3lb/ft^3 Fiberglas or 4 lb/ft^3 mineral wool.

The reason this configuration works is that porous absorption works on velocity – velocity that is greatest at the ¼ and ¾ point of a cycle (one wavelength). The velocity is zero at the boundary. Thus the gap serves to move the porous material out closer to the quarter wavelength point. And the further out one moves this, the longer the quarter wavelength and the lower the effective frequency. Also, as the wavelength increases, the amount of material required to effectively dissipate the energy increases as well, so the thickness of the porous panel will increase as well if maximum effectiveness is desired. But here we deal with the minimum required.

Thus, using simple quarter wave estimates, the low frequency extension of an 8 inch porous absorber at normal (90 degree perpendicular) incidence would be (8inches)(4) =32 inches = would be ~421 Hz. Figuring a 45 degree angle of incidence (and an acoustical impedance representing an ideal absorber), the effective thickness would be 11.3 inches corresponding to a quarter wavelength of 45.2 inches and an effective frequency of ~299 Hz. (This simplified model assumes that the material is ideal and 100% absorptive. More complex prediction models are available if you know the actual gas flow resistance of the material).

So, instead of making a panel the full thickness of the panel plus gap, we place the material where it is most effective closer to the ¼ wavelength point rather than in the gap area where the velocity is decreasing and the porous material correspondingly less effective; until it reaches the boundary where the velocity is zero and porous absorption ceases to be an effective absorber. Thus we take advantage of the nature of the behavior and obtain close to maximal utility while reducing the amount of porous material required. Think of it as an effective ‘free lunch’.

Note: in the world of porous absorption, more material is generally better. But also be aware that one cannot simply substitute denser material for less dense material and achieve the same effect! Also note that the actual effective character is referred to as the gas resistive value which takes into consideration much more detail regarding material structure than does simply density. But we are trying to keep things simple here for this explanation.
 

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Discussion Starter · #4 ·
Thanks for the awesome post! I appreciate the time and thought you put into this.

Given space issues, a 2" panel flush against the side walls at the first and second reflection points are about the best I can do.

I can, however, install a 4" to 8" thick absorber (with a matching air gap space) on the ceiling. Addition to this, I can install 'superchunk' style absorbers in the back corners (floor to ceiling on the side and back walls, back wall to ceiling between the side walls).

Given this, would the 2" absorbers on the side wall be a waste of money? If space allows, would it be better to include a 2" air gap behind these absorbers? I just don't think I'll have space for a full 4" absorber with corresponding air gap on these early reflection points.
 

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A 2 inch absorber with a 2 inch gap would function approximately as a 4 inch thick absorber with no gap - effective to ~850 Hz.

A 2 inch panel will function to ~1690 Hz... with SIGNIFICANT issues remaining! (Such panels are useful only for such relatively trivial issues as flutter echo...)

Not trying to sound flip, but its a matter of being effective or not. And what is not effectively treated is much more critical than what is.

Unfortunately while you may notice a small seeming improvement (primarily in removing high frequency energy rendering it feeling more 'dead' - hardly a pleasing sensation...), the higher energy frequencies below the effective range of the absorbers will STILL significantly degrade performance in terms of intelligibility, imaging, localization and comb filtering.

Acoustically the problems introduced by superposition will remain.

The use of ETC response for each individual source will clearly show the direct and specular reflection energy arrivals at a given spot in terms of their gains and arrival times. Unless you attenuate the early reflection energy a minimum of 15-20 dB SPL below the direct signal gain levels, the problems remain.
 

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Man it's great to have Bryan and SAC here for acoustic reference. It is (or at least it can be) a very complex subject, and having people who understand it this well is a huge help.

SAC, I just had a thought while reading your above post. ledgerdc seems to be in a similar position to me, with not a lot of room to build out thick panels. But what about using a combination of a thinner panel, an air gap, and another thinner panel? To clarify, I had planned to use the usual fabric panel wall treatments for a portion of my room, and will likely have the space for 1 inch of absorptive material behind these if required.

Now for the critical areas, if I could position a 2" panel (disguised to look like a movie poster, using luma's method here) in front of these with about a 1" gap, would that be more effective? Can the effects of multiple panels "stack" like that with a gap in between, or would the material have to be contiguous?
 

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A panel, an air gap, and a thinner panel will not offer an improvement.

As you will recall, the reason a gap works at all is because we are moving the porous absorption from the position of low velocity near the surface boundary out closer to a 1/4 wavelength position - where the further out from the boundary, the lower the frequency corresponding to the 1/4 wavelength.

Adding additional material 'within' that spacing effectively adds little, and does not serve to extend the extension of low frequency cutoff effectiveness. What you are proposing is effectively the same as if someone asked the effect of adding additional porous material into the gap - and the answer would be very little - you would effectively simply be adding labor and cost with little change in effective behavior.

Porous absorption is pretty simple in its rules of thumb. Thicker is better. The farther a substantial amount o material is located from a boundary, generally the lower its effective extension. And extending the panels' effectiveness to cover the full bandpass of high energy lower frequency specular reflections is a fundamental challenge to maximizing the effectiveness of broadband panels. And conversely, simply mitigating higher frequency energy is not a sufficient alternative to the nature of the challenge at hand.

Also, I will repeat the warning regarding printed absorber coverings.

Russ Berger and Richard Schrage have already commented on the 'myth of acoustically transparent materials'. And I will limit comments addressing valiant but fatally flawed methods in testing such samples posted elsewhere (except to point out a few concerns). But, assuming the cloth substrate is sufficiently porous, one must use a dye sublimation technique to print onto the material - effectively dying each thread rather than depositing a film that bridges the gaps between fibers and changes the acoustical impedance of he material boundary. This bridging film renders the material even less porous than it already is and renders the material significantly more reflective, especially at increased incidence angles. Cloth material is already more reflective than most suspect (and using a frequency response to evaluate the reflective nature of the material is not the proper test (as opposed to properly using an ETC response), nor is using a sample that is small relative to the wavelengths at hand combined with a reflective mount whose presence significantly impacts results...).

Contrary to what some may believe, playing the curmudgeon and dashing hopes is not as much fun as would be the alternative where one can be able to say that such ideas are great and will work just fine.
 

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Man it's great to have Bryan and SAC here for acoustic reference. It is (or at least it can be) a very complex subject, and having people who understand it this well is a huge help.
I too thoroughly enjoy, and appreciate the responses in this forum area. Thanks Guys :T
 

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Discussion Starter · #9 ·
I'd rather you play the curmudgeon and provide accurate information than play the enabler and lead me astray. :)

My hope is to get accurate information and then determine how close to that I can adhere. At least this way I am making an educated decision rather than blinding following intuition.

Along the lines of acoustically transparent materials, is there really any such thing, or are there any that come as a close approximation?

Also, I'm looking into some design ideas for the wall panels following your advice of a 4" thick absorber with a 4" air gap behind. Does the air gap behind the absorbing material need to be open on the sides? In other words, could I build a frame that was 8" deep, but only fill the outer 4" with an absorbing material? What about if the sides were framed open, but covered with "acoustically transparent" material?

You'll notice I'm trying not to assume anything here, so let the curmudgeon loose.:D
 

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Contrary to what some may believe, playing the curmudgeon and dashing hopes is not as much fun as would be the alternative where one can be able to say that such ideas are great and will work just fine.
And just when I had you pictured as some mean spirited old SOB who revels in stomping on our high fidelity dreams! :eek:lddude: lol

Along the lines of acoustically transparent materials, is there really any such thing, or are there any that come as a close approximation?
ledgerdc, according to my reading ( :nerd: ) nothing, not even air, is truly acoustically transparent. Although sound does need some sort of medium to be transmitted, which is why there is no sound in the vacuum of outer space.

Oh, and I think I've seen somewhere that open side panels were good for absorbers, because they reduce the reflection you would get from the sides of standard closed panels.
 

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As to whether cloth or any other material is truly transparent…

Whether we are addressing absorption, reflection or diffusion, we are referring to a dominant characteristic – and a generalization.

Now there is no need to panic, but there is a need to be aware of the more complex actual nature of material behavior.

The underlying principle in all materials is the acoustical impedance. And while I am not going to go into any great depth regarding this very important concept, suffice it to say that every real world material exhibits both absorptive and reflective components.

If the material tends to be more absorptive than reflective, we generally refer to it as an absorber, and conversely, if a material is more reflective than absorptive, we tend to refer to it as a reflector or a diffuser/scatterer. So far no one should be amazed at this description.

But it is often easy to ignore the less secondary nature of a material while focusing on its dominant characteristic.

A few examples…

We generally assume that absorbent panels used to control early arriving first order reflections are absorptive, while ignoring any residual reflections that may remain. And this can be a ‘dangerous’ practice. Thus it is always prudent to perform before and after ETC responses, not only to determine the original conditions which will dictate the choice and placement of appropriate treatment, but also as a follow-up ‘proof of performance’ check to verify that the placement actually mitigated the original concern. In doing so we verify that the absorption doe not present any residual reflection of sufficient energy gain to still present a problem. And folks might be surprised to discover that such a problem can indeed exist! But with awareness this issue can generally be easily resolved – provided one is aware of the potential and verifies performance.

Likewise with diffusion… A very significant issue regarding QRDs and other diffusers became apparent when we were first testing these devices when introduced. And this was that it was easy for someone to assume that a device comprised entirely of material generally assumed to be highly reflective would also be highly reflective. But what we discovered was that diffusors, while reflecting and in the process diffusing, had an inordinately high degree of energy loss (we might refer to such loses, however inaccurately, as ‘absorption’ here).

Thus, while we desired to use diffusion to simply all available energy back to a space in a diffuse manner, we discovered that much of that energy was being lost (absorbed) by the devices! Thanks to folks like Ron Sauro we now know the source and nature of such loss (edge diffraction). And as a result of such awareness we are now more careful in our designs and we try to minimize diffractive losses by trying to effectively diffuse signals only once in order to retain a more significant energy return.

Thus in each case, it pays not only to be aware of the dominant characteristic, but also to be aware of the existence and degree of the less dominant characteristics that might , in some cases, be of significant magnitude to cause an unintended issue. Again, generally this is not a reason to panic, but it IS an issue that one overlooks at their peril. And it is something that simple follow-up measurements e.g., the ETC response) will easily and quickly identify and quantify. And such ‘proof of performance measurements are indeed best practice.

I suspect many of you who are reading this post are saying – of course, that is obvious – and I hope that it is! Most things worthwhile often are, AFTER one is aware of them! So, while some may be bored with this, I hope that it will make others a bit more aware of the more complex reality behind our generalizations, and also I hope that it will provide an incentive for folks to investigate a few of the powerful tools (especially the ETC response) that we now have available to properly evaluate the performance of a space both before and after treatment to insure that one gets exactly what they assumed they would get…especially with absorption, as the residual reflections can, on occasion, be significant!
 

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A 2 inch absorber with a 2 inch gap would function approximately as a 4 inch thick absorber with no gap - effective to ~850 Hz.

A 2 inch panel will function to ~1690 Hz... with SIGNIFICANT issues remaining! (Such panels are useful only for such relatively trivial issues as flutter echo...)
Nice thread.

I don't understand those numbers. If you look at the graph below (I know it's small), you will see the absorption cofficient for a 1.2" panel (flushed or with airgap) and a 2" panel (light blue) flushed. Rockwool with 4 lb/ft^3 density.

The 1.2" panel with no airgap absorpes well down to 500 Hz (red line). Yes, I know the coffiecient isn't 1, but closer to 0.9, but isn't still that quite good?
The green line is a 1.2" panel with 2" of airgap and it absorps well down to 250Hz. The data is from the company that makes these panels.

Also, I have (mostly in the past) used some 2" panels flushed to the wall. And utilizing the ETC I was able to remove early reflections with them. Isn't that an indication that they work fine? I can't remember seing a difference in removing early reflections whether one uses panels that are 2" or thicker. The thicker ones will of course add bass/low midrange absorption which is a benefit, but I would assume one can get still get nice result by using more effective basstraps other places in the room and not using too many thin panels.
 

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The numbers he's referring to is for purely impedance tube type testing, with direct angle of attack of the wave, and assuming nothing appreciable below 1/4 wavelength - none of which are applicable for reflection panels.

Reflection panels more closely mimic standard type lab room testing with random angles of incidence.

That said, the point is still somewhat valid. Yes - you're addressing the reflections down to a certain frequency effectively and less so as you go down in frequency. For instance, it's not unusual for male voice to be in the 200's pretty easily and there, by the chart you posted, you're looking at about 0.6 coefficient (2" panel). Still doing something - just not as much as higher in frequency.
 

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The numbers he's referring to is for purely impedance tube type testing, with direct angle of attack of the wave, and assuming nothing appreciable below 1/4 wavelength - none of which are applicable for reflection panels.

Reflection panels more closely mimic standard type lab room testing with random angles of incidence.

That said, the point is still somewhat valid. Yes - you're addressing the reflections down to a certain frequency effectively and less so as you go down in frequency. For instance, it's not unusual for male voice to be in the 200's pretty easily and there, by the chart you posted, you're looking at about 0.6 coefficient (2" panel). Still doing something - just not as much as higher in frequency.

"None of which are applicable for reflection panels"...:rofl:

Nonsense. ...And IF I ONLY knew whereof I spoke!

Granted, we are IGNORING the effects of acoustical impedance and the REFLECTION component of the absorber (you see folks, while we generalize the performance of a material by calling it an "absorber', in fact it DOES have a reflective component! It is simply that we generalize and describe it by what is generally its larger function! But all one has to do is to make ETC measurements of a panel absorber to learn that they are NOT 100 % absorptive!)

But since my figures have been dismissed and deemed irrelevant to the discussion, here is a performance response modeled for substantially thicker panels based upon the gas flow resistivity (as density means nothing to the acoustical performance) and averaged for various angles of incidence in a program called Soundflow from AFMG.

You will note that the panels modeled are effectively 6 inches thick and 12 inches thick (being a 3" panel with 3" gap, and 6" panel and a 6" gap).

And lest one forget, the energy content of the reflections increases with the lowering of frequency, thus rendering the lowest octaves of the reflection MUCH more critical to control than the easily controlled lower energy content upper octaves! Thus the low frequency extension of the absorptive panels designed to control early arriving first order reflections is VERY critical! And mediocre half-way 'kind of' effective solutions are just that --- as they merely effectively EQ the higher frequencies which are minimally consequential with regards to imaging and intelligibility as they render the space 'dead', while they ignore the more critical lower frequency energy content.

And the application of treatment should for all practical effects be Neutral! It should address the entire passband of consequence equally...not treat the various regions of consequently unequally and effectively alter the sonic composition of the propagated sound within the space.

And without even resorting to D'Antonio, even Toole, in his latest tomb proselytizing averaged measurements, goes on to say that if absorption is used it should be at least 3-4" thick so that it can absorb effectively down to 300hz or so as thinner material only absorbs the upper frequencies and thus changes the spectrum of the reflected sound.




Pardon me as I go back and try to remember from which hearsay online forum I sourced my non-applicable notions...:sarcastic:

And then if we have time, maybe we can discuss the MAJOR industry wide testing scandal that has been exposed by research into the energy loss incurred in diffusors - the biggest story that you have NOT heard about - but which has tossed the existing process for absorption and diffusion into the crapper - not to mention the exposure of labs extrapolating measurements based upon scale models which are NOT valid! But its always fascinating to read quoted noise reduction ratings of materials for which the manufacturer cannot even find gas flow resistivity or porosity values!
 

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Ok. So basically you're saying those numbers from rockwool of Norway and other such numbers are invalid. Would you say the same about Bob Golds measurements?
http://www.bobgolds.com/AbsorptionCoefficients.htm

Will ETC show us what frequencies needs to be attenuated or do we do use something else as well? Again, I've not seen a difference in using thin or thick panels when it comes to treating early/specular reflections. They seem to both work equally. A waterfall would of course show a difference in the area of bass decay.
 

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Most of the NRC values are very generalized.

Tell me, what is the deviation in temperature during a day in a desert and night if I tell you the average temperature was 70 degrees F? Sounds rather pleasant, doesn't it...as you freeze at night and burn during the day.

What I am saying is that there are much better and more accurate methods of determining actual behavior rather than using the equivalent of tire ratings for acoustic absorption and reflective characteristics. Folks generally treat absorption as if is a purely resistive component, when in fact it has a substantial reactance!

And few can actually translate a .2 NRC coefficient to an actual quantized energy loss. Especially as it is an averaged value of little use for our purposes where we are concerned with absorption distributed across an entire bandpass, and not simply some Gaussian distribution (Bell curve for those not familiar with the term) that kind of covers the bandpass! The middle regions are not of primary interest! It is the lower extents where the majority of the energy is contained that are critical!

Refer to the D'Antonio and Cox Acoustic Absorbers and Diffusors for more detail, as well as an introduction to the various models used to model such behavior (listed below).

An ETC indicates the total energy arrival, which in this case would be relative to a particular incident angle. And note that it accounts for the energy reflected by all of the various components, be it the boundary, porous material itself, and covering, as they all will have a reflective component. This can be band limited but it rather misses the point, as the energy weightings in such a reflection environment are heavily skewed to the low energy as we are not dealing with band limited sources. In other words, we address all of the specular energy simultaneously. To do otherwise is to effectively EQ the room, which is NOT a goal of time based acoustical treatment - in fact, it would be considered, not as source of error, but a mistake. And the critical bandpass of concern is in the region from about 300 Hz to about 6 kHz - or you could say 10kHz if you want to round off...

The fact is, the higher frequencies are very easy to mitigate. In fact, one might say TOO easy, as they are quickly absorbed in a small room rendering it deader than we might like),while the challenge is to mitigate the lower, higher energy specular content frequencies.


And that is exactly what one is doing when they use material that is insufficient to absorb the full bandwidth of the reflected energy - such as what happens when a too thin absorptive panel is utilized! And in porous material, thickness does indeed matter! And its as close to a free lunch as we come is provided by utilizing a gap between massive boundary and panel equal to the panel thickness.


And based upon the very nature of velocity based porous absorption (where the gas flow resistivity, porosity and tortuosity are critical aspects - NOT the density) (...Not too surprising when one becomes aware that many of the manufacturers engineering departments literally don't even have those values as the products are primarily thermally oriented and employed only secondarily for their acoustical value in a relatively small niche market?)

Instead the various accepted mechanical models for porous absorption are Allard and Champoux, Delany and Bazley, Mechel, Dunn and Davy, Stinson, Beranek, Rayleigh, Wassilief, Allard and Champoux modified. The first two are the most commonly referenced.



The bottomline is that thin panels are not sufficiently absorptive over the full critical bandpass.

And the generalized rule of thumb for broad band absorbent panels, when sourcing commonly available materials in the common stated characteristics, is that the best material density characteristics are either ~3lb/ft^3 Fiberglas or ~4 lb/ft^3 mineral wool. And the minimum configuration that should be considered for a broadband panel is 4" thick with a 4" boundary gap.

That is from the physics perspective.

What one chooses to do for other reasons is beyond the scope of this discussion...
 

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Ok, instead of deleting the double post I will add one more notion sure to upset a few :devil:

Regarding absorptive panels...

It is common on many forums to watch as folks debate the issue of framed panels versus and unframed panel, balancing practicality with effectiveness; with the unframed camp asserting that due to the increased edge area that the degree of effectiveness is increased over that of the framed panel.

And at first, common sense tells you that this sounds quite reasonable. But what we actually need is a bit of 'un-common' sense!

As we have discovered, within the range of panels we are discussing - say a 2'x4' panel 4 " thick - the issue of the comparative effectiveness is effectively moot for a number of reasons.

One, directional incident sound is not uniformly distributed over the edge surface. And there is another effect which was not adequately researched until 2 years ago that uncovers the additional source of losses that render both techniques essentially equal in this particular application.

You see, with diffusors made of perfectly reflective material, one might expect the energy to be reflected, albeit in a diffuse pattern. But its not. Instead we observe a MUCH higher degree of energy loss (which I will incorrectly refer to as 'absorptive' purely out of convenience) than what is expected.

And until 'now' there had been a variety of vague , and quite frankly rather confusing explanations that referred to energy jumping from one well to another, etc. that most finally just squinted and accepted for lack of a better more coherent explanation.

As it turns out, and the research verifies, edge diffraction results in energy 'cancellation'. And the framed panel exhibits the same edge diffraction off the frame that contributes to a small degree of additional 'absorptive' energy loss that, at least at the scale we are dealing, is very close to the additional losses supplied by the additional absorptive surface area of an unframed panel.

So, the practical takeaway is use whichever method you prefer provided they are made of an appropriate material that is sufficiently thick - but I might suggest that the framed panels, sufficiently deep made of the appropriate materials, are MUCH easier to make and cover 'neatly', handle and hang, and this have many practical advantages over the frame-less panels. So, use whichever method you prefer...just insure that the material choice and the thickness is sufficient to actually absorb the lowest extents of the energy that is being addressed.

(The more fascinating implications of this is how this discovery has invalidated many of the traditional "official" ASTM/ISO/EBU measurement standards requiring a significant modification to methodologies. And beyond that, how many prominent labs have also been using completely invalid methods whereby scale models were used and the results extrapolated - which is NOT a valid method! Quite the little scandal that few have hear of...and of which many are not interested in having emerge n a public scale! Fun times!:rofl:)
 
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