Why do many Acousticians recommend absorption for non-bass freq: Valid OR prefer dead room & Dogma?

caesar

Well-Known Member
May 31, 2010
3,149
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48
#1
Why do so many acousticians recommend absorption for everything in the room, vs. just using absorption for the bass portion of the space?

Are guys just unquestioningly going back to some research paper someone wrote a century ago? Have acousticians become accustomed to dead sounding spaces and everything else sounds wrong? Or are there valid reasons?
 
Feb 5, 2014
163
3
18
60
Long Island, NY
#2
there are a ton of theories on how to set up acoustic treatments, from live end - dead end, to dead room, to deflect everything, etc. You just have to do some more research and try things until something works for you.

In my experience, with typical cone speakers, I found dead end at the speaker side and live (reflective/deflective) end at the listening side works best, with some absorption at the first reflection point.

With panel speakers I've found the exact opposite to sound better.

With horns you might want more dead than reflective.

Good luck and enjoy the ride.
 

Al M.

VIP/Donor
Sep 10, 2013
4,513
353
83
Greater Boston
#3
In my experience, with typical cone speakers, I found dead end at the speaker side and live (reflective/deflective) end at the listening side works best, with some absorption at the first reflection point.
+ 1

I had to deaden the substantial space behind the speakers up to the front wall quite a bit (my speakers are 7 feet out into the room), and liven up the space around and behind the listening position. The former was needed to bring too recessed images more forward, the latter to liven up in particular the treble. Now I have the, for my tastes, perfect balance for imaging: lots of spatial depth for orchestral and other music that benefits from it, but also very intimate, more forward imaging for a lot of small scale stuff which, however, still allows for appropriate spatial layering towards the back of the stage. Treble is not quite as extended as it could be if I'd remove the carpet stretching from the speakers to the front wall, in the dead end of the room, but this is a minor sacrifice to be made for getting the imaging optimal, which translates into more listener involvement.

An image of the front end (dead end) of my room can be seen on this page:

http://www.whatsbestforum.com/showthread.php?17334-My-minimonitor-subwoofer-system/page9

Not quite up to date in terms of system, but the acoustic configuration is the current one.
 
May 30, 2010
15,313
596
113
Portugal
#4
Why do so many acousticians recommend absorption for everything in the room, vs. just using absorption for the bass portion of the space?
(...)
Most probably because it is much easier to do, absorption in the bass needs a lot of work, space and more money. Curiously it seems that absorption in the bass is what is more needed in most rooms.
 

JackD201

[WBF Founding Member]
Apr 21, 2010
11,290
260
83
Manila, Philippines
#5
Why do so many acousticians recommend absorption for everything in the room, vs. just using absorption for the bass portion of the space?

Are guys just unquestioningly going back to some research paper someone wrote a century ago? Have acousticians become accustomed to dead sounding spaces and everything else sounds wrong? Or are there valid reasons?
Good acousticians don't make dead rooms unless that is what is needed. The only uses I can think of are isolation booths and anechoic chambers. Design should be done to suit purpose and in our case as non-professional listeners, preference. To get that preferred sound the basics are to achieve a desired reverberation time curve across the spectrum while dealing with damaging peaks and troughs whether at one spot or across a larger area. It then becomes a game of surface coverage area vs frequency and geometry. A bare space will have its own baseline parameters based on volume, shape and boundary construction which can be altered in many ways. It should be noted that objects in the room including the people in it, not just the walls have absorptive coefficients. Many forget that diffusers absorb as well. The math is straightforward but it can be very tedious because as I said, everything in the room needs to be accounted for. Then there is the proper construction and placement of the treatments ASIDE from the normal stuff, furniture, artwork, shelves, drapes, rugs, what have you.

IMO if an "acoustician" does a dead room where that is not what is specified, he hasn't done the math and you need a refund. While I suppose many clients just say "build me a room that sounds good", a real acoustician would at least make some efforts to communicate with the client and somehow figure out what "good" means to that client. Otherwise, beware.
 
Jul 25, 2012
2,556
5
38
NY
#6
there are a ton of theories on how to set up acoustic treatments, from live end - dead end, to dead room, to deflect everything, etc. You just have to do some more research and try things until something works for you.

In my experience, with typical cone speakers, I found dead end at the speaker side and live (reflective/deflective) end at the listening side works best, with some absorption at the first reflection point.

With panel speakers I've found the exact opposite to sound better.

With horns you might want more dead than reflective.

Good luck and enjoy the ride.
I found basically the same thing.
 

Ron Resnick

Site Co-Owner, Administrator
Jan 25, 2015
6,550
976
113
Beverly Hills, CA
#7
. . .

With panel speakers I've found [live at the speaker side, absorption at the listener side] to sound better.

. . .
An oversimplification but I largely agree with this.
 
Nov 3, 2014
394
0
0
#8
Most probably because it is much easier to do, absorption in the bass needs a lot of work, space and more money. Curiously it seems that absorption in the bass is what is more needed in most rooms.
Agreed. That is why many acousticians prefer multiple subs and DSP EQ in the bass, with precise center frequency and Q, rather than absorption. That, and most off-the-shelf bass absorbers are not very effective below 100Hz, which is where the biggest room modal issues in dB typically occur. Check their measurements and, most importantly, measure the room!

Floyd Toole once said he did not like absorbers because they are an inefficient, broad frequency band solution to what is typically a narrow band problem.

Above the bass, absorption may be effective and the problems of a more broadband nature. But, even there, you gotta know what you are doing. You can come to prefer and get used to even an inferior solution. Me? I use none. I just use a thick padded rug, careful speaker placement, dipole speakers with limited vertical and horizontal dispersion, a high ceiling and full range EQ. I am very happy with that.
 
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Mark Seaton

WBF Technical Expert (Speaker & Acoustics)
May 21, 2010
329
15
18
43
Chicago, IL
www.seatonsound.net
#9
Why do so many acousticians recommend absorption for everything in the room, vs. just using absorption for the bass portion of the space?

Are guys just unquestioningly going back to some research paper someone wrote a century ago? Have acousticians become accustomed to dead sounding spaces and everything else sounds wrong? Or are there valid reasons?
Over the years most have progressed away from an oversimplification of the horrible sounding, thin absorption all over the room. The real hurdle is not a question of the gross amount or area of how much absorption vs diffusion vs reflective. In domestic listening rooms it's all about addressing the bandwidth and spectral balance of the energy in the room over time. Extensive absorption gets a harsher critique than is warranted from so many putting 1-2" thick panels everywhere in days fortunately long past. While this makes a big change to the acoustics, it does so only above 1-4kHz, leaving the rest to roll around the room, and hugely shifting the balance of energy to sound tubby and dull. If the absorption is adjusted to have a more broad band effect and less aggressive in the upper frequency range, the results are much more pleasing, with very noticeable improvements in clarity, intelligibility, image specificity, and tonal balance.

A more reflective, diffusive solution which attacks strong reflections so they don't stand out from the crowd as much will result in an improvement in spaciousness, soundstage width, and tonal balance while making for a subjectively larger image vs the precise and pinpoint image of a less reflective room. Again this is all about addressing the full bandwidth.

Absorption is much easier to conceptually understand in terms of wide bandwidth, and apart from the devices being necessarily deeper and bulkier, they are easier to design and deploy in room. I like to recall a tip and realization Keith Yates gave to me over a frosty beverage. Paraphrased: "Well placed, broad band absorption is the poor man's diffusion." The "poor man's" qualifier here was not as literal as it was a colloquial term, and really meaning those not investing in extensive modeling and complex, and then expensive, diffusion devices which benefit from more detailed knowledge when selecting and deploying. The point to take away and being made was that treating the right areas with broad band absorption and leaving reflective areas between can achieve the desired effect through a more empirical and straight forward approach with affordable or easily constructed devices. The key here is the broad band qualifier, where this basically means anything protruding less than 3" from the wall/surface should be scrapped. Occasionally such panels are useful in larger rooms to attack a few surfaces after other extensive treatments are applied, but the treatments that have significant, and really desirable absorption effects will be thick.

There are products which combine some diaphragmatic and unique, resistive elements to achieve absorption below 500Hz in reasonable depths, and this is where data can be useful to qualify products. RPG's Abfusor and newer designs from Quest Acoustics combine higher frequency diffusion with mid-band absorption to better balance the high frequency and lower frequency treatment in the room, and are a very useful step beyond the 2-3" absorption panels. Another more recent trend in products more readily available (previously more so custom solutions) are those like GIK's Alpha and Impression series of thick absorbing panels where a hard face with differing area and patterns of openings maintain some reflectivity and diffusion effects above 1-4kHz, while the reduced open area on the front often enhances the lower frequency absorption. In simple terms, the idea is to limit the higher frequency absorption to better balance the 125-500Hz region.

The lower region ~100-500Hz is typically has long decay times and significant ringing in all but the largest and most open rooms or those with many openings and lossy surfaces. Ironically the easiest absorption panels to construct and deploy are most effective above 500-2kHz, and hence it requires some thought and creativity in selecting devices and approaches to control the energy in the room, be it through absorption and/or diffusion, in a balanced manner.
 

caesar

Well-Known Member
May 31, 2010
3,149
60
48
#10
Gentlemen, thank you very much for great responses!

What I am still a bit unclear about is that every time I have heard diffusion, it adds spaciousness, while absorption - when used in spots where diffusion could be used - kills the room. There is even a video somewhere of Ethan Winer playing a guitar near diffusion panels and you can tell - even on youtube - that diffusion allows one to hear a more spacious sound. But then, as a seeming contradiction, when one looks at Ethan Winer's room, it has so many absorption panels, it is probablly deader than disco...

So if diffusion positively adds to the experience in terms of hearing the spaciousness of the venue, while absorption takes away from that experience (other than in lower regions), why don't most people use it?

Is there a room measurement that can help explain this? Or is it just preference? Thank you!
 

Mark Seaton

WBF Technical Expert (Speaker & Acoustics)
May 21, 2010
329
15
18
43
Chicago, IL
www.seatonsound.net
#11
Gentlemen, thank you very much for great responses!

What I am still a bit unclear about is that every time I have heard diffusion, it adds spaciousness, while absorption - when used in spots where diffusion could be used - kills the room. There is even a video somewhere of Ethan Winer playing a guitar near diffusion panels and you can tell - even on youtube - that diffusion allows one to hear a more spacious sound. But then, as a seeming contradiction, when one looks at Ethan Winer's room, it has so many absorption panels, it is probablly deader than disco...

So if diffusion positively adds to the experience in terms of hearing the spaciousness of the venue, while absorption takes away from that experience (other than in lower regions), why don't most people use it?

Is there a room measurement that can help explain this? Or is it just preference? Thank you!
The short answer is to not confuse sound creation with sound reproduction. If you have too much contribution of the room, every recording sounds like it's in that space and gives up the ability to convey the characteristics of recorded space.

There are measurements to indicate what effect different reflections will have. Unfortunately there are many qualities and we will never have one single metric or graph to see all acoustic qualities of the room. The thread you started about speaker distance from walls comes into play as well, just as the simplified graphic I linked showing the general effect of early reflections with respect to time of arrival and relative level vs the direct sound.

So far as preference for using diffusion vs absorption, it goes back to my last post about ease of deployment. Size of the room also matters. With larger dimensions, you greatly reduce the % of early reflections, where it becomes easier to attenuate the excessive early reflections with diffusion, and the higher order reflections (= reflections hitting multiple surfaces before arriving at the listener), quickly loose their strong directional qualities. As rooms get smaller, there are more reflections within a shorter time period, and you have to be more concerned with where the diffused energy goes and what happens at the next surface it hits. Some degree of absorption will often make it easier to get a desirable result in these smaller rooms.

Diffusion is also not a magic bullet, and there are some pitfalls. The shape and physical depth of a diffusor determines how low in frequency it has effect, below which it simply reflects sound. All diffusors are not equal in their scattering qualities, and many trade off different qualities in different dimensions or frequency ranges. Some diffusors also create patterns of scattering if multiples are lined up which can result in undesired hot and cold spots as you move across the diffusor in the dimension of the array. Simply put, it's best to have some understanding of the devices being used, else you might get very different effects from leaning back in your chair vs sitting forward. As is often the case, execution is everything.
 
Dec 20, 2014
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17
18
#12
Why do so many acousticians recommend absorption for everything in the room, vs. just using absorption for the bass portion of the space?

Are guys just unquestioningly going back to some research paper someone wrote a century ago? Have acousticians become accustomed to dead sounding spaces and everything else sounds wrong? Or are there valid reasons?
Interesting thread insofar as a you've had a few great answers thus far.

To answer your question as to absorption of 'the non-bass portion' the higher frequencies often require absorbition or alternatively diffusion in order to prevent getting a reflected signal.
Another way to look at it is that the mid-higher frequencies, particularly around 300hz is at a level the ear is veyr sensitive to - unlike bass - higher frequencies are more directional than bass. Now if this comes out of your mid and treble drivers there is a lot of it coming off in a relatively wide field - if this bounces off side walls and ceiling it gets to you after that coming directly from the tweeter and mid - this means that there will be 'time- smears' of the reflected sound. This means the image becomes more diffuse and loses focus. Likewise timing becomes smeared as well (think of echoes and how the reverberant (repeated) sound of the echo remains in existence whilst a subsequent sound comes out)
Can I just add that tall bass absorption towers are likely to also damp higher frequencies too by reason of placement.

Have a read on the cards website about room acoustics as a good starting point. Good Audio Engineers don't always agree on a perfect room BTW - and that's among the really good ones out there - so try not beat yourself up too much so to speak.
 
Oct 12, 2010
140
0
16
Norway
#13
Many areas that come into play here. Especially the size of the room and distance from ears to the surfaces. Diffusion lower in frequency requires distance. Which in many cases aren't met in several small rooms. Using primarily high frequency diffusion will only treat part of the specular reflection, thus altering the spectral content. A diffuser is also less accurate than an absorber in the way that it doesn't treat the different frequencies as equal as an absorber.

While taste and acoustic principle will matter - generally speaking; using broadband absorption to treat early arriving reflections will be a more accurate way of treating a room. And using mainly diffusion for late arrival reflections.

If the room is large enough and built from scratch, it's possible to make it both very accurate without any absorption above the schröder frequency and with a lot of diffused energy. Psychoacoustically this might be the preferred way, but it has requirements that are not easily met.
 
#14
The answer to your dilemma is that absorption and diffusion are both needed, but in the optimum locations:

diffusion is generally needed between and behind the speakers

absorption is generally needed at the side-wall reflection points

Bass traps can be useful behind the speakers, but only if the placement results in bloated bass

Room resonances are another matter. I have not found treatments to be very effective in quelling these. I usually use S/W equalization.

Ocassionally, the backwall can benefit from treatment, depending on the situation, but I usually leave it reflective.

Steve N.
Empirical Audio
 

Ron Resnick

Site Co-Owner, Administrator
Jan 25, 2015
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#15
Mark, Do you have any experience with the SMT acoustic treatment products?
 
May 30, 2010
15,313
596
113
Portugal
#16
(...) Simply put, it's best to have some understanding of the devices being used, else you might get very different effects from leaning back in your chair vs sitting forward. As is often the case, execution is everything.
Applause!
 
May 30, 2010
15,313
596
113
Portugal
#17
Ron,

Sigmund Linkwitz has extensively studied dipole interaction of the room - see http://www.linkwitzlab.com/listening_room.htm to find if your envisaged speaker position meets the greater than 6 ms delay for the first order reflections.
 

Ron Resnick

Site Co-Owner, Administrator
Jan 25, 2015
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Beverly Hills, CA
#18
Francisco, I read this to advocate full absorption on the rear wall. That is what I have done in two other rooms and what I plan to do in the new set-up as well. I intend to use ASC Tube Traps in all four corners, and Bonnie's Lumitex-lined drapes on the back wall. I could even position a row of ASC Tower Slims in front of the drape on the back wall.

Note that these acoustic treatment subjects resist consensus even among the experts because Keith Yates advocated diffusion on the back wall, not absorption as advocated by Sigmund in this article.

The only delay I worry about is the delay between the direct wave hitting my ears and the reflected wave (off the front wall) hitting my ears. I adjust that delay with the distance between the speakers and the front wall.
 
#19
Francisco, I read this to advocate full absorption on the rear wall. That is what I have done in two other rooms and what I plan to do in the new set-up as well. I intend to use ASC Tube Traps in all four corners, and Bonnie's Lumitex-lined drapes on the back wall. I could even position a row of ASC Tower Slims in front of the drape on the back wall.

Note that these acoustic treatment subjects resist consensus even among the experts because Keith Yates advocated diffusion on the back wall, not absorption as advocated by Sigmund in this article.

The only delay I worry about is the delay between the direct wave hitting my ears and the reflected wave (off the front wall) hitting my ears. I adjust that delay with the distance between the speakers and the front wall.
Speaking of ACS, the 1/4 rounds make great diffusors for a HT screen. Eliminates the screen acoustically:

Vapor Nimbus1.jpg
 

caesar

Well-Known Member
May 31, 2010
3,149
60
48
#20
Francisco, I read this to advocate full absorption on the rear wall. That is what I have done in two other rooms and what I plan to do in the new set-up as well. I intend to use ASC Tube Traps in all four corners, and Bonnie's Lumitex-lined drapes on the back wall. I could even position a row of ASC Tower Slims in front of the drape on the back wall.

Note that these acoustic treatment subjects resist consensus even among the experts because Keith Yates advocated diffusion on the back wall, not absorption as advocated by Sigmund in this article.

The only delay I worry about is the delay between the direct wave hitting my ears and the reflected wave (off the front wall) hitting my ears. I adjust that delay with the distance between the speakers and the front wall.
Hi Ron,

Experts disagreeing. Forum members disagreeing. I prefer diffusion, like Yates, and think the diffusion products on the back wall- alone- are as big as any gear or speaker upgrade you can make. Yet Steve prefers the magic curtain and I have no doubt it works phenomenally for his room and his goals.

What's new? All good in the end- as long as guys are enjoying their systems.

Ultimately, only real life experience will help you make the decision. What's the incremental cost of trying both the curtain and Smt/ rpg products? Maybe you can get a sample of the curtain to try before you buy? I know you can fairly easily resell the acrylic smt wings on the used market.

Best of luck!
 

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