MORE questions about "edge effect" absorption cont

Concerning the topic found at this thread, http://www.johnlsayers.com/phpBB2/viewt ... c&start=15 which is locked now, I have a legitimate questions regarding the much discussed '"edge effect"s contribution to absorption coefficients over "1" Mr. Desarts explanation of how this effect contributes to absorption by diffraction/diffusion, and "proof" via covered edges of a test sample is interesting. In that regard, I have TWO questions. Please remember, I am a laymen only interested in this phenomena, and NOT in continueing to animosity between members. 1. Has any test been conducted whereby a set of specimens were tested exactly the same as normal specimens(as per standard-on the floor), but reverse the circumstances, whereby the FACE of an absorber is covered, while only the EDGES were exposed? And if there were, was any absorption attributed to actual absorption via the material, or would this again be attributed to "diffaction/diffusion effects". Or would this be completely redundant? 2. Understanding that this is a complex and not fully understood phenomena(I think), would it be completely worthless to stand specimens ON EDGE, where BOTH FACES are covered, the same as in Erics test, leaving 3 edges exposed, to see what happens?:lol: Of course, I also understand that to meet certain standards, the square footage of edge would have to meet the standard "spec", which now would probably result in an absorber MUCH THICKER than any normal panel.....hmmmm, maybe this is simply REDICULOUS :oops: :lol: I guess my "common sense" tells me an edge of rigid fiberglass is no different than the face, and therefore should contribute SOMEWHAT to the absorption in sabines REGARDLESS of "diffraction / diffusion effects". Especially if I get totally pissed, about it. :D Heres what I mean. If I take a stack of SIX 2'x4' 4" thick rigid fiberglass panels, and INSERT them into a cavity in a wall, where ONLY THE EDGES are exposed, forming a 2'x4' FACE, are you telling me these EDGES will NOT ABSORB??????????????????? :twisted: :roll: How bout this too. Same STACK but now stood on edge, with one edge against the wall, one edge on the floor. This leaves TWO faces exposed, BOTH being 2'x4', and TWO edges exposed, ONE 2'x2', and ONE 2'x 4'. Which SURFACE will absorb via RESISTANCE absorption, and WHICH surface WON"T, but will contribute to absorption via "the edge effect? :wink: fitZ

Fitz, I answer, not to give the impression to avoid this. :) But this looks a bit like a quiz. This calls for a lot of energy, for things which for me feels as discussing for the discussion. Hence I don't intent to go deep in this. Maybe in the future I'll go more in dept in how one can TRY to calculate such stuff (but then for normal absorbers, not any possibility you can think off, as diamond shaped and whatever you thought of before). But you went into this already before at Studiotips. This phenomenon is complicated.

1. Has any test been conducted whereby a set of specimens were tested exactly the same as normal specimens(as per standard-on the floor), but reverse the circumstances, whereby the FACE of an absorber is covered, while only the EDGES were exposed?

No idea, who measured what, but what you describe is a panel damper. And no a panel damper doesn't need closed edges, and no that does not prove that those edges in my example were absorptive, and yes those edges will absorb which is wavelength related.

2.

No comment, but everything you bring into a room will absorp, and absorptive edges will absorp. If you put a table in the room it will absorp. Sorry, this goes too far for me .......

Fitz, That's exactly why I suggested that the test would be much more compelling if it also showed the same panels not enclosed in steel cases. That would show precisely how much of the added absorption is due to the edges and how much is due to diffraction or some other effects. --Ethan

8) 8) 8) 8) 8) 8) 8) 8) Thanks A LOT gents. Not only for the replys, but for putting up with my tongue in cheek as well as mycuriosity You see, if you didn't happen to notice, there was a "wink" at the end. :lol: Guys, even though this was a genuine attempt by a layman to understand what must be a frustrating ?????? even for experts, let alone people like me, I must admit, my motives were two fold. The only way I have to learn what it is I want to know, is asking questions, EVEN if they are submitted with a grin. :lol: Which is what I hoped you would see. I was trying to ...ahem..."needle" you into a snicker by idotic if even related, quetions that kinda pokes fun at the heart of ....well, lets just say..this "exchange of viewpoints" as well as hoping for answers, however dumb the questions. Although, I must admit, there is a side of me which doesn't handle ambiguity very well, and therefore place truth and understanding at the top of my motive hiarchy. I know this was like comparing apples and oranges, which I assumed you would ignor because of the "wink", so I appreciate you even acknowlegeing my post.

I answer, not to give the impression to avoid this.

Wow Eric, I'm delighted you responded with sincerity. I wouldn't have blamed you if you had rolled your eyes with contempt. :lol: Or maybe you did here...

Sorry, this goes too far for me .......

:? :oops: I'm glad you came to your senses though. I wish I could.:P

That would show precisely how much of the added absorption is due to the edges and how much is due to diffraction or some other effects.

Gadzooks, you say in ten words what takes me a hundred. Well, at least I'm the ballpark...somewhat?:P maybe? no? Ok, so I wasn't even in the same town...what can I say? :lol: :lol: Thanks again guys. fitZ

HOLY MOLY!!!!!!!! :shock: :shock: :shock: :shock: :shock: I seemed to have missed this :roll:

and yes those edges will absorb which is wavelength related.

Holy johassaphat. Isn't that what this difference in viewpoints was about??????????? :? :? I thought that is what Ethan was saying all along??? Or am I wrong? Hmmmm. If I'm reading this right, why couldn't you have confirmed it from the beginning Eric? If I'm NOT reading this correctly, then what 's the difference? Damn, I'm confused now. fitZ

cadesignr wrote:

HOLY MOLY!!!!!!!! :shock: :shock: :shock: :shock: :shock: I seemed to have missed this :roll:

and yes those edges will absorb which is wavelength related.

I thought that is what Ethan was saying all along??? Or am I wrong? Hmmmm. If I'm reading this right, why couldn't you have confirmed it from the beginning Eric?

Fitz, yes you are wrong. The numbers game article, states that the edge effect/absorption is linear edge surface related, which is a geometrical, frequency independent, constant based on the exclusive assumption of pure ray acoustics (and even then should be the projected area). My measurements and ALL related literature + acoustics in itself proves this wrong. The fact that absorption material absorps is a phenomenon in itself not related to to the edge effect as discussed in the article which is what this is about. Not one single soul on the net can claim that I ever said that absorption doesn't absorp. You design an arbitrary example listening to its own physics, and then conclude: "He Eric admits absorption absorps. Why didn't he do that in the first place?" Fitz, you have followed, more than 1 thread (I think) at Studiotips about this. I give you another example: Take the LENRDs or the Studiotips Super Chunks. Put them in a vertical corner from bottom to top. As per the "Numbers game" article there shouldn't be ANY edge effect. And indeed there are geometrically no free edges whatsoever! That triangle fits perfectly in the corner between floor and ceiling. Well the typical corner absorption forcing those low frequencies to become so high (not the resonant piek) is mainly or very important defined by the edge effect, with a free exposed edge surface of 0, nada, nothing. I can't count the number of times anymore I've written this example, which is also significant part of the typical corner behavior, no matter what you put there, with or without geometric edges. If I remove 50% ceiling tiles from a suspending ceiling, the absorbtion expressed in absorption coefficient of the remaining 50% will enormously increase by the edge effect. No matter those tiles are so thin that they are completely covered at the edges by the profile system. Let's say it in other words: The numbers game article, referred to direct and indirect, via net, articles, reviews and as such read by ten thousands already, is not or hardly related to acoustics, and abused for commercial and ego purposes. Why do you make the author of the referred article not responsible for what he writes? The author must be able to substanciate his writings. He's the one responsible in function of his readers. The author does not need to make others responsible for what he writes as suggested expert.

Hello Eric. Man, I don't mean to ruffle your feathers here but you confused me with that quoted sentence. I wasn't sure if you were refering to YOUR specimen, as if the edges had the metal removed, or my stupid example.of oranges/apples.

Fitz, yes you are wrong.

Cool, it wouldn'd be the first time. At least it proves I'm still kickin. :wink:

The numbers game article, states that the edge effect/absorption is linear edge surface related, which is a geometrical, frequency independent, constant based on the exclusive assumption of pure ray acoustics (and even then should be the projected area). My measurements and ALL related literature + acoustics in itself proves this wrong.

Really? How? It would seem glaringly obvious that with no COMPARISON TEST with the edges exposed as Ethan suggested that it doesn't prove anything other than diffraction/diffusion ADDS absorption. But just to be on the safe side, I'll re-read your first reply. But by your own admission, these edges WILL ABSORB, which tells me that there should be even MORE absorption if the edges UNCOVERED, no? :? Maybe not in the way Ethan suggests, as you state..the numbers game. But now that I think about it...AND by your various explanations on other threads.. isn't ALL of this a ....NUMBERS GAME.? :wink: Afterall, IF diffraction significantly alters ALL absorption in a room...then how can one REALLY predict absorption in a room? From what you've suggested here...you CAN''t....hence..its still a numbers game untill you MEASURE THE ROOM IN QUESTION...after the fact, no? Furthermore, I've NEVER seen a caveat in ANY advertized acoustic product specs, that state..."DEPENDING ON USER CONFIGURATION OF THE PRODUCT IN QUESTION, DIFFRACTION EFFECTS MAY SIGNIFICANTLY ALTER ADVERTISED ABSORPTION COEFFICIENTS, HENCE...THEY DON'T MEAN A THING OTHER THAN AS TESTED IN A LAB" :lol:

ou design an arbitrary example listening to its own physics, and then conclude: "He Eric admits absorption absorps. Why didn't he do that in the first place?"

I already admitted to inventing my own set of physics :wink: Man, I ate an orange this morning which tasted exactly like an apple too. :roll: Comon Eric, I said you confused me. So what else is new?

If I remove 50% ceiling tiles from a suspending ceiling, the absorbtion expressed in absorption coefficient of the remaining 50% will enormously increase by the edge effect. No matter those tiles are so thin that they are completely covered at the edges by the profile system.

Thankyou. If THATS not a NUMBERS GAME...I'll eat my hat. No wonder the manufacturers don't tell you this. They would loose 50% of their sales. :P

Let's say it in other words: The numbers game article, referred to direct and indirect, via net, articles, reviews and as such read by ten thousands already, is not or hardly related to acoustics, and abused for commercial and ego purposes.

After what you just told me, I don't think Real Traps owns exclusive rights to the "numbers game" !! On the contrary...it looks like it comes with the territory. :shock: :roll: Well, I'd like to continue, but its time to go to work. You do know what that is don't you Eric? :lol: fitZ

Fitz, For now this cost me all too much energy. I search for more efficient methods and intent to write about this stuff. And you can believe and interpret what I write in groups as you want. Just trying to help people sucks al the energy out of me. Eric

Fitz, I diagonally read your message again and I decided to give up. Your provoking tone, I don't like and you know what? I'm not the least interested anymore what you think or believe is right or wrong. And I don't have to prove anything to you. Nothing, Nadda. You get presents, and you demand more. This all cost me a lot of energy. I've almost as much trouble with ........ I don't won't trouble here so I swallow most of my words. Whatever, I can spend my energy better.

Fitz wrote:

But just to be on the safe side, I'll re-read your first reply. But by your own admission, these edges WILL ABSORB,

If you should read then you should see that that's already integrated in the first thread. How dumb do you formulate this. I do science, I analyse stuff, I don't admit stuff. I don't admit that 2+2=4, I constat that 2+2=4. But if you like: I also admit I eat, I admit that the sun comes up. I also admit that the moon is in the sky. I even admit that my door can be closed and opened.

Eric Desart wrote:

The numbers game article, states that the edge effect/absorption is linear edge surface related, which is a geometrical, frequency independent, constant based on the exclusive assumption of pure ray acoustics (and even then should be the projected area).

Any idea what ray acoustics stand for, or what I'm saying here? No need to answer ..... This tone has nothing to do with wanting to learn, and I won't be part of that. Science and acoustics is not about loosing and winning, but analysing, studying. And if you can't recognize that in me then don't bother addressing me anymore. And this has all nothing to do with the discussion. WHERE IS YOUR FIRST TO THE POINT RELATED QUESTION TO ETHAN? HE PUBLISHED ABOUT IT. HE'S THE EXPERT.

Man, this is some funny poop.

Ethan Winer; from locked thread wrote:

The main reason you can get absorption coefficients larger than 1.0 is when the edges of a panel are exposed - the total absorbing surface is increased but not accounted for when converting Sabins to absorption coefficients.

You don't "convert" - you "calculate." And this is not the main reason the numbers are larger than 1.0. They're larger than 1.0 because they are being calculated in the first place. It's all been discussed before.

If diffraction occurs at the edges of a panel that is a totally separate issue, and is certainly unrelated to simply having more total surface area. If diffraction alone really could account for a big increase in measured absortion, we wouldn't have to use fiberglass at all! We'd just make panels from sheet steel.

I am of the opinion that diffraction has less to do with it that the fact that the denominator in the equation is arbitrary - divide by area to get a percentage??? Can't happen, unless the numerator is a relative area...which it's not, in this case. See David French's post above.

Another way to get more than 1.0 is by spacing a panel off the wall or hung a few inches below the ceiling. This lets the rear absorb as well as the sides, and this type of mounting is a big feature of RealTraps products. The air gap also increases bass absorption, so it's a win-win all around.

If you place a panel a few inches from the test surface, you aren't technically supposed to be calculating absorption coefficients anymore. It is true that an air space tends to improve the low frequency performance of passive acoustical devices. It's also true that RealTraps are not unique in their ability to be mounted away from the surface. That sort of mounting can be accomplished with just about any panel absorber on the market...including those offered by Ready Acoustics, LLC, it would seem... 8)

I diagonally read your message again and I decided to give up. Your provoking tone, I don't like and you know what? I'm not the least interested anymore what you think or believe is right or wrong. And I don't have to prove anything to you. Nothing, Nadda. You get presents, and you demand more. This all cost me a lot of energy.

eric, 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 :-( don't take it personal, but i think you should concentrate on giving answers based on the question or let it be... regards roman ps.: time to close this thread? :-D

Jeff, > Man, this is some funny poop. < Yes, and you ain't helping matters much. :) > I am of the opinion that diffraction has less to do with it than the fact that the denominator in the equation is arbitrary < Right, diffraction has less to do with it. As I stated. Your point is good too, though of course it's exactly the same point I made! The denominator considers the front surface only, and unless a panel is infinitely thin, some additional amount of edge surface is not accounted for. As I have explained at least, oh, 100 times now! This is the whole point of my Numbers Game article, to explain how the edge surface unfairly increases the absorption coefficients that are calculated. Now that you have "come out" saying you don't agree diffraction is the main cause of absorption coefficients greater than 1.0, I'll be most interested to see if the "Acoustics Policeman" gives you a pass, or if he comes down on you harder than a safe thrown off a 20 story building. As he would do to me. > It is true that an air space tends to improve the low frequency performance of passive acoustical devices. It's also true that RealTraps are not unique in their ability to be mounted away from the surface. < Agreed, though to the best of my knowledge I was the first to design a rigid fiberglass bass trap product meant specifically to mount across a corner or off the wall. All the others since then are imitators. Not just Joel either - Joel is only one of several RealTraps imitators. Even Auralex is an imitator with TruTraps. I recall when you first came out with TruTraps (that name is a rip-off of us too BTW) and I made the comment that you guys would become known as the "Behringer" of acoustic treatment companies. I also recall that comment went over your head. You get it now though, yes? :roll: --Ethan

Ethan Winer wrote:

> Man, this is some funny poop. < Yes, and you ain't helping matters much. :)

Neither are you.

> I am of the opinion that diffraction has less to do with it than the fact that the denominator in the equation is arbitrary < Right, diffraction has less to do with it. As I stated. Your point is good too, though of course it's exactly the same point I made! The denominator considers the front surface only, and unless a panel is infinitely thin, some additional amount of edge surface is not accounted for. As I have explained at least, oh, 100 times now! This is the whole point of my Numbers Game article, to explain how the edge surface unfairly increases the absorption coefficients that are calculated.

If you're going to account for the edges, you better start counting the interstices between absorbent fibers for your panels, too. That was my point. Diffraction: Yes. Edge effect: Yes. Equation to calculate absorption coefficients (using the chamber method) does not yield a percentage: Yes. All these factors (and more) affect the outcome of the tests. If there's going to be a discussion of these factors, we should not focus only on the facts that serve ulterior motives, whatever and whomever's they may be...

Now that you have "come out" saying you don't agree diffraction is the main cause of absorption coefficients greater than 1.0, I'll be most interested to see if the "Acoustics Policeman" gives you a pass, or if he comes down on you harder than a safe thrown off a 20 story building. As he would do to me.

Diffraction is not the "main" cause - nothing is the "main" cause. What's so hard about understanding that there are quite a few variables to this absorption calculation business? I believe Eric Desart understands this quite well. (I prefer not to call him names - though I do find that a bit hypocritical on your part - assuming Eric Desart is the person towards whom the above mud was slung.) My intent here is not to argue, but to share my knowledge of these concepts with others; help clear up the decades-long misunderstandings. 8)

> It is true that an air space tends to improve the low frequency performance of passive acoustical devices. It's also true that RealTraps are not unique in their ability to be mounted away from the surface. <

Agreed, though to the best of my knowledge I was the first to design a rigid fiberglass bass trap product meant specifically to mount across a corner or off the wall. All the others since then are imitators. Not just Joel either - Joel is only one of several RealTraps imitators. Even Auralex is an imitator with TruTraps. I recall when (Auralex) first came out with TruTraps (that name is a rip-off of us too BTW)

(Blue text is mine - I did not come out with the TruTraps - Auralex did, whilst I was in their employ.) This is still perplexing. I admit the name was quite clever. But how can a foam product be an imitator of a bunch of mineral fiber in a metal frame? That you were "the first to design a rigid fiberglass bass trap product meant specifically to mount across a corner or off the wall" is specific enough to not be arguable. But you did not invent the concept of putting an absorbent panel over a corner. Others had been doing it long before Real or Mini or Tru or Ready or WhateverTraps hit the market, including me. Having a product specifically designed for it - where the incidental (and incendiary) packaging in the box can be conveniently used for a spacer - was a good idea on your part. Why throw those polypropylene cubes into the landfill? - Glue 'em to your wall instead! :) That you "designed" that particular approach, i.e., the MiniTrap - no question. (Duh.) That you "designed" the concept in general (in case you're trying to imply that) - not so much.

and I made the comment that you guys would become known as the "Behringer" of acoustic treatment companies. I also recall that comment went over your head. You get it now though, yes? :roll:

I do recall that comment. Both companies make fine products, IMHO. Is this some sort of attempt at an insult? To Auralex, or Behringer? Perhaps you should take your beefs up with one or both of them instead of all the readers here at John's site that probably don't care (including me). Of course, if you're trying to imply that somehow a TruTrap is a MiniTrap knock-off, then I guess I'd go back to: TruTrap = Foam MiniTrap = Not Foam What's so hard to figure out? :roll: :roll: :roll:

Jeff, I don't know why you're using such an argumentative tone. :roll: It doesn't help anyone here who wants to read and learn, and it just adds to the animosity which I'm sure you agree there's already too much of. > If there's going to be a discussion of these factors, we should not focus only on the facts that serve ulterior motives, whatever and whomever's they may be... < This is a perfect example of what I mean. I have no ulterior motive but to learn, and in turn to explain, what matters and what does not. As I have said repeatedly, as soon as someone shows valid data that contradicts my opinion, I will change my opinion in a heartbeat. > Diffraction is not the "main" cause - nothing is the "main" cause. < If you have any evidence to back that up, this would be the time to show it. Common sense tells me that edge surface is a big factor. Now, I do appreciate that everything in physics doesn't follow what Ethan considers common sense. But so far all I've seen is people (you, Eric, others) claim various things but without offering any evidence. The problem with Eric's test data is it's incomplete, as I already explained. > My intent here is not to argue, but to share my knowledge of these concepts with others; help clear up the decades-long misunderstandings. 8) < Ditto, my friend, ditto. So let's both keep that in mind, okay? > I did not come out with the TruTraps - Auralex did, whilst I was in their employ.) < If this was not your idea, who else at Auralex would have thought to copy the idea of our name? (A "real" bass trap.) Nevermind, I'm sure you won't answer that, and its irrelevant anyway. > I admit the name was quite clever. < Copying someone else's idea is hardly clever. RealTraps was clever, TruTraps was not. > you did not invent the concept of putting an absorbent panel over a corner < No kidding, and I never claimed I did. Though I've known about the importance of having bass traps in the corners long before you, and probably anyone else here too. I knew about corners 30+ years ago. But that too is irrelevant. Again, let's skip the pissing contest and get back to the science of acoustics. Okay? Thanks! --Ethan

I knew about corners 30+ years ago.

I'm interested in studying the history of corner absortion. Ethan, do you remember how you first learned about corner absorption? I didn't know it was quite that old. The earliest thing I know about is this patent which was filed for in 1981.

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

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

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.

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

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? 8)

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

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:

Savant wrote:

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

John Kopec, former Director of Riverbank Acoustical Labs wrote:

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. 8)

John Sayers wrote:

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.

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).

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! 8)