MORE questions about "edge effect" absorption cont

[sharward]

I sense more hurt feelings and another locked thread coming........... ...........Please........... Not again.

[Ethan Winer]

David,

> do you remember how you first learned about corner absorption? <

Very clearly! In 1975 two friends and I had a production company and commercial studio in Fairfield, CT where we produced jingles and "industrial" sound tracks for a variety of local and national ad agencies. In 1976, when we outgrew that space, we decided to build a newer, much larger pro studio a few towns away in East Norwalk. We hired a local archictect to formalize the plans I drew up, and also hired the acoustic consulting firm of Klepper, Marshall & King from New York to approve my design and show us about sound isolation and acoustic treatment.

They were the ones who explained bass traps and other acoustic treatment to me, and that's where I learned to build the wood panel bass traps I later wrote about in THIS 1995 article for Electronic Musician magazine. They also explained using an air gap with rigid fiberglass and why it helps, and why treating corners is important for bass trapping.

I wrote that article in 1994, long before I knew any of these acoustics guys from the 'net. If you read that article you'll see mention of the importance of corners. Although those traps are membrane absorbers, the same principles apply as with rigid fiberglass only, and I understood all about that then.

You can read more about that period of my studio building career on this page of my personal site:

www.ethanwiner.com/music.html

You'll see photos there of the suspended 705 tiles we placed in a checkerboard pattern, a few inches below the double-sheet rock ceiling in the live room (air gap principle). You can't see the suspended (air gap) grid ceiling made using 3 inch thick 705-based ceiling tiles, but we had that too.

Man, I've been doing this stuff for so long it's almost embarrassing!

--Ethan

[David French]

Thanks for sharing.

It seems like the stuff you're talking about from back then is about pressure-based traps that work well in corners because every mode has a pressure maxima there. Then, there's the velocity-based idea where you put a porous panel across a corner and it consequently absorbs a lot of bass. These are different things, and I'm interested in studying the history of the latter.

[Ethan Winer]

David,

> It seems like the stuff you're talking about from back then is about pressure-based traps that work well in corners because every mode has a pressure maxima there. <

Yes, though the very same bass build-up makes porous traps work well in corners too. I can't say when I first saw someone straddle a corner with rigid fiberglass, but even back in the mid-1970s acousticians knew the usefulness of an air gap. So it doesn't seem like much of a leap to go from two rigid fiberglass panels spaced off a wall in a corner to one panel straddling the corner. I wonder if that patent you posted could even be defended, since it doesn't seem to offer much of an advance over existing practice.

BTW, I too am very interested in the history of this stuff. I love reading that story about ancient clay urns used as Helmholtz traps, though I have to wonder if that's really what they had in mind. Maybe they were just fancy candle holders?

--Ethan

[David French]

To my best knowledge, it wasn't until the 90's at earliest when straddling corner absorption was first tested scientifically. Hopefully someone will come along and help us out with this.

About the clay urns, I would not put it past them. There are some acoustic miracles in the ancient world. Check out these articles on the Temple of Kukulcan in teh ancient Mayan city of Chichen Itza.

http://news.nationalgeographic.com/news ... emple.html
http://www.museumofconceptualart.com/na ... etzal.html
http://www.ocasa.org/MayanPyramid.htm

Sorry to hijack the thread, BTW.

Ok, so how 'bout that edge effect, eh?

[John Sayers]

The first studio I worked in in 1967 had acoustic tiles on the wall - these were 2' x 2' soft fiberboard panels with mutiple holes drilled in them.
The other acoustic treatment was cork panels which were 3/4" thick. we used them everywhere.

Finally in 74' we hired two acoustic consultants from Sydney Uni who treated our studio by building a slot resonator wall for the low mids, another wall was panel traps, ie. a 2" x 4" frame that created 4' x 4' spaces that had various thicknesses of plywood panels mounted with insulation behind to create a panel resonator. Rigid fiberglass was not available then so we used rockwool for another wall where the rockwool was mounted 6" off the wall and across the corners.

In 75 we rebuilt the control room and I was able to access 4" Sonex which was the first of the foam products with the patterned forms you see in auralex products.

My point is that all the techniques have been around for years. Look at the old BBC studios from the 50s and you will see boxed in helmholtz resonators like I use and panel absorbers like Ethan uses.

I don't think anyone here can claim to be the first in anything acoustic.

cheers
john

[lovecow]

OK. So I would like to get back to the edge effect thing. I put in a link to an SOS thread earlier. Here is the most important thing to understand from that (too long) thread:

The reasons for absorption coefficients calculated using the reverb room method being greater than 1.0 include, but are not limited to:

A. Specimen Shape: Odd shaped specimens may...create additional absorption due to interactions between them. For example, absorption coefficients become meaningless when testing theater seats. Thus results on theater seats are given in Sabins per unit rather than Sabins per area.

B. Specimen Size: If a specimen is large in relationship to the test room, increased absorption may result. If a specimen takes up too much space, the sound diffusion in the room is decreased and, depending upon the location or the closeness of the microphone to the absorptive material, higher absorption could (result).

C. Diffraction/Bending Wave Effects: Essentially the bending wave effect, also known as diffraction effects, theory of absorption is that certain sized or shaped
specimens can cause the sound waves to bend around them, turn, or bounce back and forth. The energy used up by the sound wave bending, turning or bouncing around a specimen shows up as increased absorption caused by the specimen.

D. Edge Effects: The edges of some samples are not included in the area used to calculate absorption coefficients. For example, the area used to calculate absorption coefficients on office panels is the width and height of both sides (face arras) only. Although the edges of the panels are exposed to the sound and can provide additional absorption, the edge arras is not included in the calculations. Thus, the thickness of an office panel can be a major contributor to the coefficients exceeding 1.00.

All of the above are, IMO, of equal importance. None of them can be singled out as the "main" cause.

To them, I would add an item:

E. Calculation method: In a basic (stripped-down) form, the calculation (from ASTM C423, or equivalent) is a difference of decay rates divided by an area. Mathematically:

Change in rate of decay/Area

does not constitute a percentage of anything. The selection of a number for Area is arbitrary. The change in rate of decay is the absolute.

Changes to any of the above, A-E, will constitute changes in measured and calculated values per the standard. Thus, it is not going to be sufficient to talk about the main cause. Personally, I think a main cause is E (I'd probably reorder the list to put it #1), but I am not fool enough to disregard A-D.

I hope this helps. Sorry for getting slightly off-topic earlier.

[lovecow]

My point is that all the techniques have been around for years. Look at the old BBC studios from the 50s and you will see boxed in helmholtz resonators like I use and panel absorbers like Ethan uses.

I don't think anyone here can claim to be the first in anything acoustic.

I second that.

[Eric_Desart]

To all,

Absorption is no insulation.

One can imagine the rays in the picture below from whatever angle.
At straight incidence it's clear that the edges are longitudinal with those rays. At any other angle the behavior of the incident and reflected rays are shown by the picture.
The dotted line are the reflected waves. It was too difficult for me now to let them represent the sound energy.
A standard absorption panel as 703 doesn't absorp at the back because the sound crawls in-between allowing some additional incidence, but because that's the logical path of the mirorred sound. As such there is no main difference in principle between a panel with and without cavity.

(this is all a bit stylized).

[lovecow]

Doing a little bit of (more) research on this, I found a reference to a paper by a W. Kuhl ["Der Einfluss der Kanten auf die Schallabsorption pöroser Materialen", Acustica, 10, pp. 264.276 (Roughly, "The influence of edges on the sound absorption of porous materials")] Kuhl's findings are summarized well by C.L.S. Gilford in his book, Acoustics for radio and television studios. For a sample of rockwool (thickness not given), the midband absorption increased roughly 20-30% for each doubling of exposed edge area. Higher band differences were less linear and lower band changes appeared to be bordering on insignificant. I hesitate to define the cutoffs for these ranges since a thickness for the rockwool material was not given. FWIW, Figure 9.8 in the above Gilford reference shows obvious increases in mid and high band absorption for a 10 m² sample with 14 m, 30 m, and 60 m of edge exposed.

I would argue that there's more going on, including diffraction effects. This might help (partially) explan the non-linear changes in absorption at higher frequencies. This sort of diffractive behavior would make sense in light of Eric's diagram above. While I agree with the conclusion that despite the arrangement, rays pass through the same area of material, I would point out that the panels spread out creates impedence mismatches along newly exposed edges. When edges are abutted, rays pass into fuzz and stay in fuzz. When edges are exposed, rays are given the opportunity to pass into (and possibly back out of) the same edge from air - not from fuzz.

So, to improvise a little:

- Relatively insignificant increases in low band absorption come about by spreading panels out.

- As is well-known, airspaces behind panels cause increases in absorption, mostly in the low bands, further reinforcing the points Eric made above with his diagrams and accompanying text.

- Increases in midband absorption are directly related to edge exposure and - over a limited bandwidth - could be a simple function of the ratio of exposed perimeter to area of materal, as suggested by Kuhl and Gilford.

- Increases in high band absorption are also evident when more edges are exposed. But the shorter wavelengths could lead to more diffraction effects, which in turn could (help) explain the non-linearity of high band absorption increases.

- To expand on Eric's diagrams (and perhaps dispute some of the conclusions in a minor way?), some edge effect absorption increase could be explained by increased impedence mismatch; rays pass through air/fuzz or air/fuzz/air with exposed edges, but mostly fuzz/fuzz when edges are abutted.

The above are NOT conclusions - just some possible generalities that could apply to porous absorbers. I welcome your thoughts on this, Eric!

[Eric_Desart]

Jeff,

Thanks for your thoughts.
I maybe expand on this later, or do it on a web page.

I do agree what I drafted is a stylized approach (which is what I wrote), more to bring the "numbers game" article in its context.

What I don't understand is, and which is why I don't feel tempted to discuss a mathematical approach and model, that I feel that I have to fight my lungs out, just to help people protecting themselves against misleading info.

The author of the "numbers game" article just publishes things, has no background or data to substantiate any of it, didn't even bother to check any literature, but still (mis)uses it to question measurement data of competing products (even based on a nonsense mathematical approach) and to enhance their own products and pseudo-objectivity (!?!).
This has nothing to do with summarizing things in order to make a concept more accessible for a broader public.
Just by being handy, people just swallow it, don't see the disrespect this represents for themselves, even fight the guys fighting for them.
Maybe I'm just too naive ......

Hence I either stop this (self-protection) or have to find more efficient manners by writing more official, less stylized stuff, where I really will irreversibly expose such authors.
This instead of repeating the same things over and over, without hardly any effect.

Jeff I do respect you a lot, you know that.

Eric

PS: this is frustration speaking

[Eric_Desart]

why did you post then messages again and again? i can't understand you, maybe it's a language problem... however, you're posts are also waisting my time, because each time i'm looking for a good advice, but i rarely found one

Maybe glasses can help?
But I can give you good advice here: if you find my posts a waist of your time, simply don't read them, rather than spending time to write that you waist your time reading them.

[lovecow]

Eric,

No worries, bro. I have a pet interest in this topic.

Of course, I could provide another - perhaps a fresher? - perspective on THE NUMBERS GAME:

From http://www.ethanwiner.com/acoustics.html and http://www.realtraps.com/art_measure.htm


THE NUMBERS GAME
To understand why MiniTraps outperform other acoustic absorbers it helps to know how such devices are measured at a testing lab. As you read in the previous section, acoustic products are commonly specified by their absorption coefficient and this number ranges from zero (no absorption) to 1.0 which means 100 percent of the sound is absorbed.

Theoretically, this is true. But in the contexts of acoustical treatments and the "commonly" used ASTM C423 test, the absorption coefficient can never be 0.00 and can exceed 1.00...which does not mean 100% of the sound is absorbed.

For example, an absorption coefficient of 0.5 means that half the sound is absorbed and the other half either passes through the material or is reflected.

For normal incidence absorption coefficients, this would certainly be the case. But, again, in the contexts of acoustical treatments - almost always tested for random incidence absorption - it is not the case.

Since no material absorbs all frequencies by the same amount, absorption coefficients are usually given for different frequency ranges.

Although 1.0 is the largest legitimate value possible, you may have seen higher numbers claimed for some products. Needless to say, this causes confusion, and makes it difficult to compare published data.

While there is potential for confusion, numbers greater than 1.00 do not make comparisons difficult. All other factors being equal, an absorber with a 1.10 absorption coefficient at 500 Hz is better than an absorber with a 1.00 absorption coefficient at 500 Hz. This is not difficult. And the values larger than 1.00 are completely legitimate.

Once you understanding how absorption is measured, and how the data can be manipulated - both fairly and unfairly - you'll be able to assess room treatment products and materials more wisely.

Data manipulation is not part of the laboratory method. Of course, in the context of THE NUMBERS GAME, "manipulated fairly" would be an oxymoron.

Acoustic absorbers are tested using methods defined by the ASTM, an international organization that establishes standards and practices used by acousticians and testing labs.

The ASTM is a US-based organization. Absorbers in other countries will be tested in accordance with the standards those countries have adopted. Just so there's no confusion, the ASTM sets testing standards for many industries besides acoustics.

By requiring its members to follow the same rules, materials tested to ASTM standards in different facilities can be compared with confidence. However, a flaw in the test method does not take into account the edges of the material.

Edges are discussed in ASTM C423, Section 9. To summarize, test specimens are to be mounted "in a way that simulates actual installation." A subsequent Section states that edges of materials intended for ceiling installation (i.e., a T-bar grid) should be sealed. A good lab usually asks a client whether the edges of the test specimen should be sealed. Since the edges of wall panels are typically exposed, this is not often done for foam and fibrous panels. Since, to the best of my knowledge, the edges are rarely - if ever - sealed for wall panel tests, it is usually safe to compare the results of absorption tests, provided all other factors are equal to one's satisfaction. If there are any doubts, the manufacturer should be contacted. Section 12 of the C423 test method requires a full test report, which must include a detailed description of the exact conditions of the test specimen in the lab.

Although the edges are exposed when the material is tested, the calculation for absorption coefficient considers only the size of the front surface, and ignores the edges completely. For a panel that is two by four feet and four inches thick, the edges add 50 percent to the absorbing surface during testing, yet they are ignored in the coefficient calculation. This is further complicated because there is no standard sample size. Since a small sample has proportionally more edge than a large one, a sample that's 8 by 8 feet will measure better than one that's 10 by 12 feet, even if they're the same thickness and made of the exact same material.

In practice, multiple panels are placed adjacent to each other during testing, to minimize the contribution of the edges. So when 2 by 4 foot panels are tested, typically eight of them are arranged into a larger square.

A minimum sample size is provided in Section 9. Again, a full test report will show and/or describe the size and layout of the test specimen. (Most labs recommend at 72 ft² sample size; smaller than 64 ft² is typically not recommended).

The test does "ignore" the area of the exposed edges. But, as mentioned above, most every absorber on the market reports absorption coefficients that omit the edge area. Thus, it is largely moot with respect to fair comparisons.

Since the subject at hand is edge effects and how they're related to absorption coefficients greater than 1.00, it might be worth discussing a product mentioned above: the MiniTrap. If memory serves, 8 MiniTraps were tested in the "A" configuration (specimen directly on test floor; no airspace). This yields 64 ft² of face area. Add the edges (of the perimeter - assumes panels are abutted in a square pattern) and you get about 74 ft² of area. The absorption for the array of MiniTraps at, say, 500 Hz is roughly 112 sabins (data all extrapolated from here). So, if the standard method is used, the absorption coefficient at 500 Hz would be

112/64 = 1.75

If the additional area is included - which is not in accordance with the standard method, btw - the absorption coefficient at 500 Hz would be

112/74 = 1.51

which is still greater than 1.00. Thus, it is once again shown that the added area of exposed edges cannot fully account for absorption coefficients greater than 1.00.

Even if all four edges of the panels were exposed (panels spread out), the absorption coefficient comes out to about 1.17. Which is still greater than 1.00.

But even when placed to form a single surface area of 8 by 8 feet, four-inch thick edges still inflate the measurements by more than 16 percent.

This final sentence seems to be insinuating that there is some unfairness going on. If virtually every absorption coefficient is calculated using the method in question, then all the numbers are equally inflated. So, again, the subject is largely moot. The method in question is not perfect, but it is the standard method. If it is not followed, then the data are meaningless outside of their own context.

This panel (figure could not be linked - see above URLs for graphics) is 2 by 4 feet and 4 inches thick. During testing the four edges add 50 percent to the total surface area, yet they're excluded from the absorption calculations. And when many panels are mounted adjacent on a wall, the edges are not absorbing even though they contributed to the published specs.

During the test, all four edges would not be exposed. The edges of the perimeter of the test specimen would be, and would be omitted from calculations. If the edge area were to be included, the results would no longer be in accordance with the standard method.

When many panels are mounted adjacent to each other on a wall, the configuration is closer to the test method and thus the published numbers (not "specs," btw) will be more applicable. The edges did not contribute to the published "specs." They were omitted from the calculation. Since most - virtually all - wall panels are tested this way, the point is largely moot.

Further, most acoustic panels are meant to be installed adjacent on the wall in a cluster.

This is a generalization. The needs of the space dictate the method of installation. How an acoustical panel is "meant to be installed" is largely up to the designer and installer (and owner...and spouse...). The manufacturer may have its own suggestions, but the needs of the space will dictate how most acoustical panels get installed.

In this case the edges are not available to absorb even though they were when the material was tested.

Most likely, they were not "available" during testing. As mentioned in THE NUMBERS GAME ( ), panels are typically placed edge-to-edge for the ASTM test in order to minimize edge effects.

When an entire wall is covered with four-inch thick panels none of the edges are exposed, so the real absorption is only 2/3 what the published numbers indicate - and those numbers were already inflated!

The area of the "entire wall" is not given, so the statement about the absorption being "2/3 what the published numbers indicate" cannot be verified. If the area of the entire wall is roughly equivalent to the entire exposed area of the test specimen, the absorption could be very close to 1x the published numbers...

Of course, these last few snippets seem to be hinting at the impact the test method has on the reliable use of absorption coefficients in predictive architectural acoustics calculations. Most acoustical professionals are well aware of the limitations of using lab test data in their models and will typically take those limitations into account when selecting acoustical treatments for a room design.

For readers of THE NUMBERS GAME, it is unlikely that there will be much of a need to calculate, e.g., reverberation time. This leads back to the point that the absorption coefficients are fine for comparisons, all factors being equal to one's satisfaction.

The same thing occurs with corner absorbers, as shown in the figure below. (Again, figure can be found in above links.) Unless the vendor describes how these triangle shaped samples were grouped during testing, there is no way to determine how much of the stated absorption is due to the edge effect and how much is due to its effectiveness as an absorber.

For the devices in question, total absorption, or absorption per unit, could provide the necessary information for comparison, but would require modification to the standard test methods to accommodate. Otherwise, comparisons of devices such as these using data obtained from the reverb room method are difficult. I have mentioned (opined) in other threads that manufacturers should follow the lead of RPG and perform low frequency specific measurements on their low frequency devices.

Foam blocks like this are meant to be mounted in a corner, stacked one above the other from floor to ceiling. When measured for absorption four of the five surfaces are exposed, but when installed as intended only the front surface absorbs. So in practice, a two-foot corner wedge like this provides only 65 percent of the absorption claimed. The shorter the wedge, the larger the disparity between the published and actual absorption.

While the intent of this paragraph is understandable, the "65 percent" comment has no basis. The devices would have to be retested with the edges in question sealed in order to determine what the actual difference in absorption would be.

For some products, like a tube trap, it is not practical to specify an absorption coefficient because there is no front surface. In that case the correct way to specify absorption is in Sabins, named for acoustics pioneer W.C. Sabine (1868-1919). The Sabin is an absolute measure of absorption, independent of surface area, and it can be used to compare any two absorbing devices directly and on equal terms.

First, the unit is the sabin (small "s"). As is standard with units derived from a person's name, the unit - spelled out - is not capitalized, but the abbreviation - in this case, "Sab" - is capitalized. (Sorry - pet peeve... )

Second, the last sentence here would seem to contradict claims made elsewhere in THE NUMBERS GAME. If such comparisons can be made (and they can be made), then this statement with some supporting instruction to the reader as to how to find information on the absolute absorption of a panel or product would suffice. One short paragraph on this worthwhile concept could replace a lot of the misleading content that preceded.

************

The above is for clarification purposes only - constructive criticism. The author of THE NUMBERS GAME is asked to kindly consider it as such.

All the best!

[Ethan Winer]

Jeff,

> [long winded point by point dissertation]
> The above is for clarification purposes only - constructive criticism.

I read all of that - twice - and I still have no idea what your point is. The purpose of my article is to explain how absorption coefficients can be larger than 1.0. It also explains how what's measured in a lab can be very different from what someone will actually get in use, and explains why. As far as I can tell my article does that very successfully.

Thanks for the clarification on Sabin versus sabin though.

--Ethan

[Ethan Winer]

Jeff,

> I found a reference to a paper by a W. Kuhl <

Excellent.

> For a sample of rockwool (thickness not given), the midband absorption increased roughly 20-30% for each doubling of exposed edge area. <

Apparently the surface area wasn't given either? That 20-30% figure is totally meaningless without knowing the surface area!

> Increases in midband absorption are directly related to edge exposure and - over a limited bandwidth - could be a simple function of the ratio of exposed perimeter to area of materal, as suggested by Kuhl and Gilford. <

And please don't forget: As suggested by Ethan Winer.

> To expand on Eric's diagrams (and perhaps dispute some of the conclusions in a minor way?) <

I'll let you dispute Eric. Whenever I do that it just makes him even more irate.

I will say that Eric should go back and do some more testing. That's the only way he can truly establish how much of the increased absorption is due to each of the four possibilities John Kopec mentioned. It could be that edge surface as I propose accounts for 99 percent of the increase, or maybe it's only 1 percent. Likewise for diffraction and the other possibilities. For any of us to argue the other guy is wrong, but with no hard proof, is intellectually dishonest.

I agree it makes sense that different factors might affect different frequency ranges. This too could be much better understood with a few new more carefully devised tests.

--Ethan