interpretation of ineffective corner statement

[Bob]

In "Acoustic Absorbers and Diffusers" by Trevor Cox and Peter D'Antonio it says on page 13 paragraph 2

"In practice, many people place porous absorbtion in corners of rooms thinking this will absorb the modes since all modes have a 'contribution' in the corners. However, while the modes have a maximum pressure in the corners, the particle velocity is very low and so the absorption is ineffective."

[avare]

I don't understand your meaning. The pressure increases, and the velocity decreases. This is well known. The closer to a node, the greater the pressure build up. Move a little bit back and velocity increases, Partly why superchuncks are effective.. Absorption away from nodes of maximum pressure and minimum velocity.

[Bob]

Everything looked familiar to me too, until the last six words.

Nevermind. I re-read the paragraph with a hunt for a different interpretation.
I'm now pretty sure he's talking about thin material placed right on the wall. When I see the word 'corner' I think diagonals like Superchunks.
I don't know why he bothered with the 'corner' phrase.

[Eric.Desart]

Hello Bob,

I must say that after 25 years, a lots of related R&D I still have lots of questions about that.

You assume what the book means.
But if they speak about velocity they see this in function of wavelenght, so then it shouldn't matter very much if they mean this corner absorption or not.

If you see the measurements on my "playing with baffels" Word document, you also notice that materials the same panels FLAT on the ground also become better when shifted to the walls and corners.

I have a series of measurements of baffels (different size than the ones in my doc), where one started measuring on the standard spot.
They were positioned vertically on the floor.

This measurement then was repeated by increasing the number of baffels on the floor maintaing the same pattern and distance between them (so covered floor area increased).
This was continued until the whole lab floor was fully covered.

What happened was:
In mid and heigh the absorption per unit discreased (which is logical to be expected in function of decreased diffusity and the exponential behavior (Eyring).

HOWEVER:
In the low frequencies, the measured values per unit increased VERY systematically, as baffels were more and more positioned to the room boundaries.
And very strange: there was a very clear cross-over frequency, below which the values improved and above which the values diminished.

It is never really investigated to quantify this.
Clear was: the closer the floor coverage extended to the boundaries the flatter the overal absorption became (line angle turned around this cross over frequency).

[Bob]

In the low frequencies, the measured values per unit increased VERY systematically, as baffels were more and more positioned to the room boundaries

Terry M said something very similar once over at AVS. It was his explanation of why they used thin absorbtion on 4 walls to handle LF. I don't remember the quote off hand.

[Eric.Desart]

Bob,

It's just that:

1) It calls for huge amounts of time
2) Ditto for energy
3) I must check which data I'm allowed to publish, meaning contacting lots of people.

But I have lots of measurement data.
A whole lot of them with unsolved question marks.
I should like to put some on the net.

What I do know:
If something is special (with lacking or unsure known analogies), even profs, teaching related calculation models, will test material before using it.
If I asked the, in-between retired, technical head of the acoustic lab of the Leuven university who spend his professional live in this lab for decades, what he expected as outcome if I came with non-standard exotic applications (and this guy was good), he mostly responded with a smile, when I teased him to make some predictions.
His typical response was often laughing/smiling: Eric let's first measure, afterwards we come up with an explanation.

I NEVER in my live heard anyone predicting this corner absorption peak, before seeing any related measurement.
I never saw a book describing this phenomenon.

The first related measurement I did in a lab (long ago, can't remember), the Prof., following this study, never had seen this phenomenon before, and couldn't come up with an explanation either.
And this guy is Prof. Dr. Ir. specialized in Room and Building acoustics.
http://perswww.kuleuven.ac.be/~u0030736/
,and published in all leading journals.

E.g. I wonder if Andrew's friend ever expected this result, from the boards he measured for Andrew (without knowing Andrew's descriptions, or seen related curves before).
He surely doesn't have a clear explanation, or Andrew shouldn't ask in the group here what could be done to improve it.
This guy also measures already a couple of decades and as such has a lot of theoretical and practical background.

I assume lots of things in function of low frequency absorption is not enough investigated because:
1) Labs are not build to do so. So one must first investigate the lab concept itself in relation to low frequencies and corner absorption.
2) For building purposes all standards range between 100 Hz and 5000 Hz, which suited their purpose in the time they were originally designed.
3) In large room acoustics corners are not that important (relative significance)
4) The market for studios is extremely small within general building and room acoustics.
5) Only the last 2 decades, low frequencies became more important. Previous music and HiFi (the elder radios) was hardly able to generate those low frequencies, or certainly wasn't that common in the daily live of John Do.
6) Following specifically studio related groups is hardly representative for the needs in acoustics in general.
7) Etc.

The above are just thoughts, not a thorough analysis.

[Eric.Desart]

Now I really sin, I'm sorry

While checking this Prof's page I saw a link to an article we wrote together in 1991.

Publication
VERMEIR G., DESART E., Akoestische technieken: toepassingen in het VTS-studio project, Bouwkroniek.
http://perswww.kuleuven.ac.be/~u0030736 ... %20VTS.pdf
Only few will be able to read it since it's Dutch, but my picture is in it. That's how I looked in 1990. It's much worse now :):)
This is really new to me. I did not know this was on the net.

This relates to a couple of TV studios I did back then (some of the larger in Europe, resp. 19530 m3 = 689695 cft and 18000 m3 = 635664 cft)
In this project the whole perimeter ceiling/walls is provided with corner absorption. Partly exactly the same as I show the measurements in the Playing with Baffles doc (Site Eric), but also diagonal absorption with much larger span width and a deviating design was used.
In Figure 7: last page you can see the reverberation time I started from (curve 1), and the end result AFTER treatment (curve 2), and the corrected result when the studio was furnished with the standard gear (averaged setting for a TV recording session) (curve 3).
Notice the > 10 sec. RT60 I started from, and how flat the RT60 became after treatment.
This is partly related to the corner absorption (which I called diagonal absorption), but not alone that.
The wall absorption was designed as self-spanning absorptive perforated steel elements, supported by the main vertical beams of the 15 m (49.2') high studio. The covered height was calculated in function of a sound field simulation in order to obtain the predefined room acoustic targets.
The resilient skin wall custom designed with light gauge steel elements to optimize to sound insulation between studio 1 and 2, with a total weight of 12.5 metric tons (skin weight, not basic wall) and a surface of 512 m2 (5511 sft), was designed for a resonance frequency of 30 Hz at plain wave straight incidence (higher for random incidence), and suspended by 300 silencers calculated for a horizontal resonance of 7 Hz and a vertical resonance of 15 Hz, acting as a huge damped membrane damper contributed as well to control the very low frequent reverberation time.

In the picture below the Top Row was the achieved result and the Bottom row the previous calculated target aimed at.
No corrections whatever in the original planned/designed treatment were made during the project to reach this target.

The second picture are some of the RT60 curves in the before/after stages.

I hope you forgive me for this reference to something I did before.

COPYRIGHT:
All data and picture in this message are copyright protected and can not leave this forum.
Any reference happens via this Forum OR the above mentioned link to the original article.

[bert stoltenborg]

Well well well, my neighbour......
Maybe you're not as scientific as good ol' Ethan, but you shure as hell look better :):):):)

Sorry, ik kan het niet laten zo iets te zeggen

[avare]

Well well well, my neighbour......
Maybe you're not as scientific as good ol' Ethan, but you shure as hell look better :):):):)

Do not compare Eric with trash.

[Scott R. Foster]

The guy in the upper picture looks like he sees a bug... but Eric looks like he is about to smash said bug.

Perfectly aligned with my sense of what Eric is all about.

ALL HAIL ERIC THE BUG-SMASHER!

[Eric.Desart]

Well,

I'm promoted to bug-smasher .....
and Ethan to Professor:
http://www.musicplayer.com/ubb/ultimate ... 6;t=000354

Note: I'm not sure I'm Belgian anymore ....

Andre that was Bert's Hollands humor (at least I hope ...)

[Andrew Steel]

Eric,
You are right, it has sparked interest in the lab and they want to investigate further now. I will have to pay for it though :-)

Bob,
Thanks for posting that, I read it and was just as confused. I found it on the salford university web page in some notes on room acoustics. I read at the same time a statement that sounded like 'tube traps' were good as they utilised gas flow in and out of the internal cavity as pressure around it changed. I will try to find the place where I read it but couldn't see it just now.

Andrew

[Eric.Desart]

Hi Andrew,

Eric,
You are right, it has sparked interest in the lab and they want to investigate further now. I will have to pay for it though :-)

Paying is good to check some effects (as private person), but is hardly related to investigating it.

The number of measurements needed to really investigate it is huge.
That's typical a job for somebody doing an endwork or thesis, having free access to the lab facilities.

Therefore, while certainly very interesting, your previous suggestion of doing it in a standard small rectangular room somewhere, can help you and probably others too, but doesn't tell a thing about lots of the influencing factors.
You suggested that that should be more representative for real-live circumstances. I suggest that that is true, but FOR YOUR ROOM.
As long as you don't know the influence of the room, there's nothing real live about it in relation to other rooms.

That's how labs and their related standards and measurement procedures are designed: TO EXCLUDE THE ROOM, as such resulting in neutral values only to be adjusted to the room acoustic circumstances the application is meant for.
With your suggestion, you start with a room, with unknown influence on the results, and this then should be meant for other rooms with another deviating influence (which can reenforce or neutralize effects).

It's like TL values: They are measured in rooms designed to be as neutral as possible AND the results are ADDITIONALLY corrected to exclude the remaining room acoustic circumstances caused by the measurement sample in the lab.
You can say: that's not real live. I can assure you that if you measure a separation wall next to a corridor with a small volume and compare it with the same separation wall between large rooms, certainly the low frequencies will differ a lot. So what are real live circumstances?

As I see it, as long either a company with enough money and human resources for R&D, or a thesis is involved lots of questionmarks will remain. And guys doing a (University) thesis can often get good co-orporation with the industry getting the necessary materials for free (it's a deal: free knowledge and facilities versus products).

As in your case:
You know the chunks work. What info can bring you more tests?
1) That you can do the same with less or cheaper material? Only to get an answer to that cost you a multiple of your room filled up with chunks.
2) Better results? Make them larger or put one or more boards in front.
3) A less explicite peak? Play with gasflow resistivity/density.

Investigation for me is:
1) Or to get answers and then doing it right.
2) Or to improve or cheapen an individual project were the necessary energy and financial resources must be weighted versus to be expected result (acoustically/budgetary).
In other words investigating is good when the estimated ratio input/output is good.

[Andrew Steel]

Hi Eric,

How do you find the time to make such great and detailed answers? :-) Thank you.

Thanks for your reply. It is a great reality check to keep me only doing things that yield good results proportional to investment.

To explain a bit, I work part time for a studio doing marketing (my background for the last few tears) and end up talking with numerous small/home studio owners who come in for mastering. Acoustics and audio electronics have been my passion for as long as I can remember and people always end up asking me questions about acoustics. In the end I end up trying to help or guide them as most of them have close to no money (if only I could charge them :-) ) This excites my interest even more and I do love trying to solve a problem. I will take your advice though and stick with what we already know works. My mind will keep ticking over on it but I do not want to reinvent any wheels.

Thanks again Eric,

Andrew

[Eric.Desart]

At RO a discussion started some time ago which, while originally not meant that way, turned to the traditional corner absorption thing.

Therefore, for the once interested, not following RO:

Heavy duty 701 in the corner
http://www.recording.org/postt20156.html

[Dan Nelson]

In "Acoustic Absorbers and Diffusers" by Trevor Cox and Peter D'Antonio it says on page 13 paragraph 2

"In practice, many people place porous absorbtion in corners of rooms thinking this will absorb the modes since all modes have a 'contribution' in the corners. However, while the modes have a maximum pressure in the corners, the particle velocity is very low and so the absorption is ineffective."

The part I don't understand is they state it is ineffective, yet RPG makes a few models of corner "traps"

http://www.rpginc.com/products/procorner/index.htm
http://www.rpginc.com/products/modexcorner/index.htm

Dan

[Eric.Desart]

Hi Dan

This second link, AFAIK which was the first on the market, is a membrane damper. As such mainly based on the pressure, not velocity principle, which is in line with that comment/quote.

Does anyone has an idea when the foam corner came on the market?
It should be nice to see the below 125 Hz behavior of this corner foam.

I think you never will now the significant impact your forum had on the studio market.
(which is a compliment, making your huge efforts worthwhile).

As I understood from another message somewhere, from a couple of months back, also ACS redid the measurements of their tube traps in corners now.
Direct and indirect your forum is monitored by people remaining in the background.

It only confirms the not so obvious behavior.

It should be nice to find the oldest related messages about corner absorption on the net.

Bob, is this quote the only thing the book mentions about corner absorption, or is that a selected sentence? What I mean is: does the book tell something more which can put your quote in a broader or more specific context?

If not, it goes close to the 1/4 wave, at the normal from the boundary, sickness, meant (in books and articles) as a good, clear and correct but stylized presentation of a plain wave at straight incidence, mostly shown with relative to very thin absorption on a large cavity, to explain the basics of pressure and velocity, which became (in forums) about the most, out of context, abused and misused principle ignoring real life sound fields.

The advantage however is that for corner absorption proof is present now and is read, while not many seem to care about the much higher availability of plain absorption measurements showing real live relations with the wavelength.

[Scott R. Foster]

This brings me to the question, does anyone know if the higher density stuff would be more effective below 100Hz, or would the 701 still have the lead all the way to the bottom?

Another question, how much more effective would a 4"+2" panel be over a single 4"? 6" over 4" total. This also considering there will be an air gap behind it, around a foot to the corner, maybe more.

Interesting thread over at RO - Eric is fine form.

But to explore, the original question [a smart question methinks], IMO the answer is meaningless if conveyed solely in the sense of acoustic properties. I don't want to parse the question in too brutal a fashion, but one must come to some agreement on what "effective" means in this context to get the "right" answer.

IMO proper resolution the question of what material is "more effective below 100Hz" has to encompass time and money... it also has to take into account how [what shape/arrangement the material will take upon installation] and where [what part of the room] the material is to be installed.

Hands down - the best how/where answer to arise thus far is a solid wedge of foam or mineral wool such as the MegaLENRD or Studiotips SuperChunk placed in a corner - [prevailing wisdom is to run the device floor to ceiling, a configuration not tested but bound to enhance results since in this configuration two tri-corners - where two, walls and the ceiling all meet, would be captured by the device's volume]

http://forum.studiotips.com/viewtopic.php?t=536

While the "100 Hz peak" is a fascinating artifact - I question whether it really matters much in reaching an answer to the original question. Supposing the "peak" arises from gas flow resistance of the substrate material, device geometry, and room geometry - can we not simply conclude that these factors will combine in any version [703, 701, Auralex foam] of a wedge, and that while we might move the peak around by choosing different materials, the bottom line is we get a peak somewhere sub 100 Hz?

If you'll allow me that much leeway then I think we can go on to say - based on the test data we have - that the question resolves to cost per linear foot of treatment. Given that 701 is what - half the cost of 703? - I think the answer to the original question is one Eric postulated [always as a guarded expression of his opinion]long before any of the data linked above was gathered.

Namely, for the "most effective sub 100 Hz" absorption, build:

1) wedges from floor to ceiling in every corner;
2) build em as large as is practical;
3) use a low density mineral wool such as 701/702 or equivalent.

For some discussion on costs see this thread:

http://forum.studiotips.com/viewtopic.php?t=537

If 701 is used, then the costs calculated at the above link get much lower [FWIW I am given to understand that 702 is a special order item and difficult to get compared to 701 - and that these materials do not have the intrinsic structural integrity of 703 and therefore a more elaborate framework will be required = more lumber / hardware / time cost].

Whadda Yall Think?

PS: Should we stop using the term 703 - any suggestions for an alternative?

[Bob]

Scott:

With 4" deep (on the wall, A mounting) 703 is great.
But with a 4' diagonal corner that's 24" deep to the center, which is 6 times the depth (24" / 4" = 6). So I've been thinking for quite a while that what we need is something with 1/6 the rayls. 701 is much more than that.
So perhaps 4" 701 as a rigid facing to keep the fluffy from falling on the floor, with fluffy pink in behind would give the largest LF absorbtion.
I don't know how to keep the fluffy from being more dense at the bottom than the top, without rattles or shelves.

[Bob]

Eric:

Bob, is this quote the only thing the book mentions about corner absorption, or is that a selected sentence? What I mean is: does the book tell something more which can put your quote in a broader or more specific context?

There's certainly not a lot about corner absorbtion in the first 100 pages. (I've only been reading it during lunch at clients sites when they don't want to talk over lunch, and I don't get out that much.)

Here's the entire paragraph, from "Acoustic Absorbers and Diffusers" by Trevor Cox and Peter D'Antonio on page 13 paragraph 2. The prior paragraph is about modes, and the next three paragraphs are about resonant absorbers (membrane, helmholtz).

Particularly prominent modes are usually treated with bass absorption, often referred to as bass traps or bins. (It is not usually possible to treat this problem with diffusion because the sizes of the diffusers become prohibitively large.) Porous absorbers are not usually used, as they would have to be extremely thick to provide significant bass absorbtion. Porous absorbtion is most effective when it is placed at a distance from a room boundary where the particle velocity is maximum. This is at the quarter wavelength position. For a 100hz tone this would be roughly 1m from the boundary. Placing porous absorbers directly on a room boundary, while the most practical, is not efficient because the particle velocity at a boundary is zero. In practice, many people place porous absorbtion in corners of rooms thinking this will absorb the modes because all modes have a 'contribution' in the corners. However while the modes have a maximum pressure in the corners, the particle velocity is very low and so the absorption is ineffective. For these reasons, resonant absorbers are preferred for low frequency modal treatments.

BTW, this following quote is from page 39.

... the design of modern critical listening rooms is heavily dependant on how one controls strong specular reflections between the sources and the listeners. The diffusers in RFZ spaces provided massive surround sound enhancing envelopment. Today, with surround sound reproduction formats such as 5.1 finding acceptance, the concepts are still valid but are employed differently. The rooms are not polarized between live and dead zones, but tend to be more uniform, with diffusers being used to enhance the envelopment and immersion of the surround speakers and to provide the desired degree of ambience. Absorbers and diffusers can still control strong specular reflections, which cause spectral and spatial distortion. One approach to designing a surround critical listening room, suggested by D'Antonio, utilizes broadband absorbtion down to the modal frequencies in the corners (using a combination of membrane and porous absorption). Absorbers or hybrid surfaces of 50-100mm deep are used on the walls between speakers and listener, to control first-order reflections, and diffusers are used in the middle of the four walls to enhance envelopment. Diffusing clouds, with broad bandwidth absoption down to the modal frequencies placed above the listeners, provide surround reflections and additional modal control.