Acoustic treatment for my living room - Page 3

4" absorber w/ 4" air-gap for OC703 (3pcf) or 4pcf MineralWool. yes - broadband absorption such that the entire reflection is attenuated, versus thin absorption which will fully attenuate the mid-HF band of the specular reflection, but allow the lower band to persist (which will still superpose at the listening position with the direct signal as a colored/filtered/EQ'd reflection).

Would you define the frequency ranges for the terms "mid-HF band" and "lower band" you used? Thanks.

~300hz to 20khz. carve it as you wish. it's not relevant -

as toole states, if you are applying treatment (eg, if you choose to attenuate a high-gain indirect reflection) - then the treatment needs to be broadband and NOT modify the spectral content like a thin porous absorber that is NOT effective to the lower cut-off frequency would.

Why is there so much beating of this horse regarding porous absorption already?

Porous absorption has fundamental limitations that unfortunately balance its ready availability and cost benefiting from piggybacking on its economies of scale used as a common building material. One of which is thickness, and another of which is spacing from boundaries. Deal with it.

If one wants more information, about its behavior, how such behavior is properly evaluated and also regarding alternative configurations and designs useful for improving performance over simply, but in some cases, prohibitive characteristics, please refer to Acoustic Absorbers and Diffusers by D'Antonio and Cox. Yes, its a bit pricey (~$140), but it can be obtained through almost any public library via the interlibrary loan program.

If one wants to 'improve' on these limitations, we have already suggested several alternatives that one might want to explore.

They are not simple cookie cutter DIY projects, although some are not prohibitively complex. But they do require a bit of research and learning HOW such concepts work!

I know, I know... that should have more than a few folks running for the exits - or at least searching for new marketing brochures promising new methods to overcome physics.

Either that or one pays a company for their time and effort to do what one chooses not to do. I would only recommend that one not simply grasp for the first product with a marketing brochure claiming to have overcome the limitations imposed by physics, but one with solid design foundations and REAL test results. Thus, if you DO look for commercial products, I would seriously suggest caveat emptor.

And variously cited 'NRC/STC' values of ~.1-~.5 in various tables and databases are not very encouraging, especially when considered in context with the nature of the test results and the recently identified problematic nature of the test methodology.

There are online tables of this kind of information.

Google

Sound Absorbtion Coefficients

Here's just one of many:

http://www.bobgolds.com/AbsorptionCoefficients.htm

LOL, and not a bass trap in sight.

--Ethan

There are two issues: Absorbers for bass trapping, and absorbers for reflection points and general ambience control. This short article explains the basics:

Acoustic Basics

--Ethan

reverberation chamber mesurments are not directly translatable into Small Acoustical Spaces - due to lack of a reverberant sound-field (as stated above).


this is where impedance tube measurements are beneficial...

Seems to me that the discussion is going in the direction of complexifying, not simplifying.

Can you agree or disagree with the following:

So-called absorbers that have low (< 0.1) absorbtion coefficients at say 500 Hz in these tables, don't do a lot for bass boom (around 100-200 Hz) in small rooms. The lower the cutoff frequency in these tables, the more effective they are at controlling bass.

Much of any actual "confusion" is the result of erroneous interpretations of the test results.

Absorbers that have low (< 0.1) absorption coefficients at say 500 Hz in these tables, not only don't do a lot for bass boom (around 100-200 Hz) in small rooms, they ar almost worthless at 500 Hz!

The fact is, about all you can use the cited database listings for are relative comparisons of one product compared with another. They are not some indication of 'absolute' performance as many assume.

Must of the problem is that folks commonly make casual assumptions about just what the numbers actually mean, which is further complicated not only by the historically flawed testing procedures (that are undergoing a revision 'as we speak') as well as results based upon testing behaviors which are not directly applicable to the common uses for which we use the material in a small acoustical space.

Add to that that much of the absorbent material is anisotropic, meaning that the orientation of the internal structure - the stranding, etc. - is oriented in such a way that the material behaves in one manner oriented a certain way, and behaves in quite another another manner when the material is oriented (say rotated 90 degrees) in another orientation. Additionally it will behave differently in an environment featuring less than normal (perpendicular) oblique energy incidence, further skewing the behavior from what one may assume.

Therefore the proper way to make things more understandable is not to simply "simplify" things by ignoring the real variables, but rather to enlarge our awareness so that one does not make errors in interpretation based on an ignore-ance of the test and behavioral factors.

This is really a key point. Industry measurements for 2" OC703 (for example) from Bob's web site show an absorption coefficient of .86 at 250Hz . You are saying that the true performance of this material in a small room for specular absorption is far less - to the point of being essentially non-absorptive at this frequency (if we are considering somewhere around 250Hz to be the target for "Broadband" treatment). Is that accurate?

I would be interested to know what more relevant modeling (in absence of more relevant measurements) predicts typical 2" treatment to be effective to - 500Hz? 1000Hz?

when you see a value of .86, what does that mean to you?

To me, it means "substantial". Not 86% of all energy at that frequency, but closer to full absorption than no absorption.

For me, it means that 2" piece of 703 rated at .86 at 250Hz is a better absorber than the 1" version which is rated at .28. Now, I'd be tempted to say that 2" piece will absorb 86% of whatever 250Hz stuff you can throw at it based on the first few lines on that page, but we all know that's not true right?

Why is this even on there?

Fiberglass, Rockwool, Polyester, Cotton, and Sheep Absorption Coefficients
0.00 = no absorbtion.
0.50 = 50% absorbtion.
1.00 = 100% absorbtion.

playing devil's advocate...
why would bass traps have to be visible???

how do you know the visible walls/boundaries in that room are not LPF's of which reflect specular energy but allow LF to pass through to be addressed on the other side?

this is fundamental to a particular room design from 1980+.

eg, have an outer (rigid) shell for modal control, and then an inner shell/room placed asymmetrically within the outer shell - of which manages the specular energies.

thus, by having the listening position in the middle (symmetry maintained) of the inner-shell for the specular region, the listening position is actually off-set from the outer shell - which moves the listening position away from being situated in the null for that axial mode.

again, likely not the case in that room but just playing devil's advocate that there is usually a lot going on "behind the scenes".

Good answer!


Oops! One hand giveth, the other taketh away! How about some data?

Let's look at this graphics by Dr. Toole. This is a speaker placed in different living rooms and measured:



As the captions gives away, it is apparent where the room is most powerful. Clearly it is to the left of the graph. Yet, thread after thread like this one, focus on the right hand. Since even thick acoustic absorbers lose effectiveness around transition frequency, we are clearly barking up the wrong tree based on this analysis (for the typical living room -- bare rooms are different).

What is neat about the right hand side is that it can be predicted fairly well. Here is another graph from Dr. Toole showing that:



The graph shows the computed response based on acoustic chamber measurements of the speaker vs actual in-room performance. As noted on the graph, the predicted response is right on the money from 125 to 8 KHz. Above that, it is actually a gentle roll off due to too much high frequency absorption in this room. In other cases, that gap is closer.

Now look below 125 Hz and you see that the variations get quite large and unpredictable based on measurements. So clearly the room is in control of what you get.

Given this, a good loudspeaker designer can optimize the in room performance of the speaker for the typical listening room. In other words, instead of you trying to solve what happens there with stuff pasted on the wall and with zero measurements of the speaker, you can demand that he does it and does it right. You don't need to "take away the room" as he has done that for you for the right side. Indeed, if you try to take away the room again, you might be interfering with what he has dialed into his design.

Once there, then you can focus your attention on the left hand side. There, a number of solutions exists, some of which have nothing to do with acoustic material. This is good news in that you don't have to go and buy that $140 book. One of them for example, the right placement of speakers and listener, is actually free! Yet it can be quite powerful.

I agree with most things you have said here. Indeed from my own "experimenting" I have found my 4" fibreglass panels have little effect on the big nulls and peaks of the lower frequencies. I have had to use speaker and listening position placement to help manage those issues.

Apart from measured frequency response, we all also know about reflected signals from walls and ceilings with varying delays relative to the direct sound from the speaker because of varying distances to said walls and ceilings. I have found that my 4" fibreglass panels in certain places like behind my listening position (that is somewhat close to the back wall as I have a relatively small room set up in a long wall arrangement) and behind my speakers and a couple on the ceiling above my LP has improved localisation/imaging/soundstage and tonal qualities. Indeed I did reach the point that adding more panels started to make things sound too focused and lost atmosphere. I found my own balance to what I like in my own particular room.

so do we still have anyone here disagreeing with toole - that if you are absorbing a reflection then it needs to be effective throughout the entire specular region?
is this issue cleared up or is there still contention or confusion?





or is anyone debating this statement?

4" of rigid fiberglass is NOT a bass trap. and they are placed in other locations than for specular reflection absorbers.
and for appropriately sized porous LF absorbers, it's actually more effective if you utilize a material with lower gas flow resistivity such as cheap pink fluffy attic insulation (loosely filled) - which has a GFR of approx. 5000rays/m.

and ideally you wouldn't use porous, velocity-based absorbers anyways...but pressure-based.

and ideally, you would soffit mount the speaker to minimize/eliminate further LF issues (eg, SBIR).


exactly.
but by blindly placing broadband absorbers in the room without actually identifying whether a problem exists (eg, high-gain indirect reflection is even incident off that boundary), you will quickly create a highly damped/dead space of which you did not prefer it. this is why the ETC is used to measure to identify boundaries of ACTUAL high-gain destructive reflection paths, vs simply using a mirror and placing broadband absorption at any and all reflection points. surgically LIMITING the amount of broadband absorption you are applying in the room. and ideally, you wouldn't use absorption at all, but splayed walls (geometry) to redirect the early arriving high-gain specular reflection AWAY from the listening position and towards the rear wall - retaining ALL of the sonic specular energy within the room and managing it appropriately (eg, returning to the listening position as a laterally arriving, exponentially decaying diffused sound-field for spaciousness).

Not from me. I did ask you a question related to it though that I don't recall you answering (my apology if you did and I missed it). Namely, since the high frequency absorption remains in even thinner absorption material and hence elimination of comb-filtering there, are you now only worried about spectral issues here as opposed to time domain?

Anyway, back to what you quoted form him, this is the abstract of the paper. Would you kindly state if you agree with it? https://secure.aes.org/forum/pubs/jo...86&rndx=207119

"The physical measures by which acousticians evaluate the performance of rooms have evolved in large performance spaces—concert halls. They rely on assumptions that become progressively less valid as spaces get smaller and more acoustically absorptive. In listening rooms the loudspeakers and the rooms interact differently below and above a transition region around 300 Hz, similar to the Schroeder frequency in large rooms. Above this transition we need to understand our reactions to reflected sounds; below it the modal behavior of the space is the dominant factor. A review of the scientific literature reveals that natural reflections in small rooms are at levels where they are perceptible, and their subjectively judged effects range from neutral to positive. At low frequencies the long-standing problem of room resonances can be alleviated substantially through the use of multiple subwoofers, thereby providing similarly good bass to several listeners in a room. A provocative observation has to do with human adaptation to the complexities of reflective rooms, and the extent to which it allows us to localize sounds correctly in direction and distance, and to hear much of the true timbral nature of sound sources. In the case of loudspeakers, an analysis of comprehensive anechoic data is found to be sufficient to provide a good prediction of sound quality, above the low-bass frequencies, as subjectively judged in a normal room. Although the interactions of loudspeakers and listeners in small rooms are becoming clearer, there are still gaps in our understanding. A number of these are identified and are good opportunities for future research. "

And in general, I like to understand if you agree with Dr. Toole's teachings as I am getting confused by you simultaneously quoting him as you are doing here and disagreeing. For example, I thought you said in the other thread that we don't need listening tests to understand room acoustics -- just ETC -- yet this section you quoted, 7.1, is based on listening tests:

"7.1.1 Correlations between Subjective and
Objective Domains
For over 20 years the author and his colleagues have
conducted comprehensive anechoic measurements on
many loudspeakers, and have examined the results of
double-blind listening tests performed on these products"

Do you now believe in usefulness of double-blind testing in acoustics?


Same back at you partner .

different aspects entirely. if the user's choice is to attenuate (absorb) early arriving high-gain indirect specular reflections, then one would identify the BOUNDARY that is incident of such a high-gain early arriving reflection in the time-domain with the ETC. the frequency-domain and FR does not detail you ANY information on whether a boundary is incident of a HIGH-GAIN reflection or a low-gain reflection. one would NOT blindly apply broadband absorption at "all reflection points" - but instead surgically place the broadband absorption only at boundaries that are incident of HIGH-GAIN indirect reflections. if the reflect is low-gain, then no need to be concerned. but the boundaries are identified within the time-domain. the frequency/spectra content is important and referenced, because if one is absorbing the reflection, then it needs to be fully absorbed across the entire specular region such that the spectral content is NOT being modified (just as toole states; that the treatment must be BROADBAND) - which requires sufficiently effective (thick) broadband porous absorption. or ideally, use a large/flat reflective surface to redirect the high-gain reflection away from the listening position such that one is not applying ANY broadband absorption within the room.






"thought i said" ?? use the quote feature along with context...

Certainly it is highly desirable for that treatment to be effective throughout the entire specular region - But what exactly do you mean by "need"?

If by need you mean "in order to conform to Toole" then I agree.

If by need you mean "if you don't you have created new problems at the same time you have solved others" then okay.

If by need you mean "those new problems have resulted in no better sound than when you started" then I would dispute that - both in terms of my own subjective experience, and statistically by the number of satisfied HT owners on the design forum who have reported significant improvement in sound quality with 2" (and in some cases 1") treatments.

Do you agree or disagree that imperfect treatments are often/usually an improvement over no treatments at all?

you can dispute all you want based on your own personal subjective tests --- i merely provided what Toole has to say on the subject, and he says to NOT alter the spectral content of the reflection via thin/non-uniform absorption. you'll have to direct those questions to him.

Thank you for the detailed response. Alas, there is nothing in this explanation that talks about comb filtering which was the question I was asking about. So in that regard, it is again not answering the question I asked. Is there no way I can get you to address the issue of comb filtering relative to absorption? They are related, are they not?


Hard to quote a locked thread but here it goes:
[these are full sentences but capitalized/missing punctuation as such]

It does seem that you have quite a negative view of listening tests if one is "hiding" behind them when they quote them, especially since I was quoting research done by Dr. Floyd's team.

If ETC speaks for itself, can you point us to Dr. Toole teachings that says that measurement is useful? As you know, I have quoted him saying it is not but I may have missed something.

In addition to above, that bit about "speaker manufacturer" as it relates to Dr. Toole seems to be a put down, not an endorsement of him. If he is a speaker manufacturer and hence not be trusted, why quote him at all?

And on this bit:
Originally Posted by Toole
If reflected sounds are absorbed, the listener is placed in a predominantly direct sound field, making the experience more intimate, and the imaging tighter and more precise. If the reflections are allowed to add their complexity, the overall illusion is altogether more spacious and open, to many listeners, more realistic. In part, this is a matter of taste.

This wasn't from the journal. I would appreciate if you could provide full attribution on these references . Here is the full quote, with the parts missing in your quote in red:


Since you quoted this article (hoping and assuming you had read all of it rather just the part quoted in another forum ), I assume that you agree with its entirety and preface of a good speaker design as I just explained to Kiwi2. Yes?

Thanks again.

Well then like I said, I agree - to conform to Toole's recommendations you need broadband treatment.

To me, the far more interesting discussion at this point is what happens if you don't use broadband treatment, because my observation here has been that this is true for most people (for a myriad of reasons) - likely including the OP

This amorphous ill-defined question is fatally flawed by self-imposed limitations.

Let's see, if I apply a treatment sufficient to address non-existent flutter echo that EQs the high frequencies but does not address the problem of a late arriving high gain reflections...is doing something that exacerbates an issue without mitigating the real issue "better than nothing"? No.

If I apply sidewall treatment the strategically intercepts a destructive early arriving high gain sparse reflection that effectively EQs the high frequencies of the reflection, leaving the mids and low mids to continue and as a result STILL cause imaging, localization and intelligibility issues as well as the addition of treatment induced coloration "better than nothing"?

No, it is NOT better than nothing. It is simply a new problem. And whether you think it is a problem or not is objectively verifiable. Whether you FEEL that it is better or not, I have neither control nor an opinion.

Personally I think the choice a rather absurd one akin to the concept of determining whether a person is a witch by immersing them in water, where if they float they are deemed a witch and are killed, or if they sink and drown they are deemed innocent.

So, if you are not a witch, is that a fair and acceptable verdict and subsequent outcome?

And since you have asked others for an opinion, instead of utilizing a fundamentally flawed course of reasoning and subsequent course of action, I would research alternative treatments methods that satisfy your particular constraints and which also address the nature of the actual problem in full, and utilize that treatment properly applied.

I know that concept seems to escape a few who continue advocating the viability of using improper treatment methods that fail to adequately address a problem condition, but simply asking the question another 100 times will not change the outcome.

The REAL problem here, contrary to what a few seem to maintain, is NOT the treatment that may, in a given circumstance, be inadequate to the task. It is the limited mind set that fails to investigate alternative types of treatment that can indeed address such issues completely and adequately, but which just might entail one having to enlarge their awareness and understanding sufficient to identify, understand, source and apply the said new alternative solution.

I find it rather amazing that a few are so stuck on one or two forms of treatment that one 'medicine' continues to be imagined as a universal cure-all for whatever the ailment, and instead of recognizing the fundamental limitations of such treatment and instead continue to fault others for recognizing the fact as they themselves fail to pursue learning about other more effective alternatives better suited to the particular circumstance.

re-read your question. it was answered. mean what you say and say what you mean.



yep - notice the context of the signal being "accurate". no listening or subjective preferences change the fact of whether the direct signal as measured at the listening position via the ETC is "accurate" or not. accuracy is measured - not a "preference". i say what i mean and mean what i say. no where did i state whether people prefer an accurate signal or not. that's not my place to decide.





thank you! - it looks like i had left out the most important sentence in that paragraph:

"This can be described as a loudspeaker with a flattish, smooth, axial frequency response, with constant directivity (which together result in flattish, smooth, sound power). Then it becomes an option, whether the room is acoustically damped, or not". "In part, this is a matter of taste. ". yep. a matter of taste whether the user wishes to do it or not - exactly!. thanks for posting the rest of the paragraph to solidify what i have been stating.

I am happy to repeat it:

See emphasis. As I noted, your response was devoid of comb filtering.


I am sorry but there is no answer here to my question in what you just wrote. I am simply asking if you believe in subjective listening tests since what you quoted from Dr. Toole had this in its intro:

"7.1.1 Correlations between Subjective and
Objective Domains
For over 20 years the author and his colleagues have
conducted comprehensive anechoic measurements on
many loudspeakers, and have examined the results of
double-blind listening tests performed on these products"

Do you believe in subjective listening tests as a basis here as he does?

I am also asking if you believe in everything Dr. Toole teaches. Or whether he is biased in what he says as a "speaker manufacturer."


I am glad I made you happy. Can you make me happy by answering any of my questions? In above, I was asking if you agreed with the *precondition* of a good speaker design relative to what you followed. I bolded that section. Since you quoted it all, you believe it all?

Full disclosure dragonfyr - my own personal treatment strategy has been prescribed by Erskine group, using primarily the Quest Perf-Sorber 2" product for early reflection treatment. After several lengthy discussions with Dennis and Shawn at Erskine Group, I am very comfortable with this treatment strategy.

Note that 2" Perf-Sorber likely does not meet the definition of broadband as you have defined it.

That being said, I personally see the discussion of 1" or 2" or more treatment in the same good/better/best lens the same way any other system decision would be made (for example how a Wilson speaker may be more ideal than a Paradigm speaker, or how a 3 chip DLP is better than an LCD projector or perhaps a plasma TV)

It's interesting that you believe imperfect treatments are not better than nothing - but I personally disagree with that opinion, and we can't objectively carry that conversation any further.

toole is the expert, right? he says it's a matter of "taste" and "opinion" . what is it that you don't agree with Toole on here? im really curious the contention for how a user decides what he or she prefers.


"If reflected sounds are absorbed, the listener is placed in a predominantly direct sound field, making the experience more intimate, and the imaging tighter and more precise."

a loudspeaker with a flattish, smooth, axial frequency response, with constant directivity (which together result in flattish, smooth, sound power) and high-gain early reflection absorbed such that the listener is placed in a predominantly direct sound field, making the experience more intimate, and the imaging tighter and more precise. sounds good to me! Toole sure does has a way with words!