[youngho]

Discussions have a tendency to go off at tangents. Madfloyd would probably benefit from judicious placement of broadband diffusion elements, but these tend to be pricey. It would probably be helpful to see a diagram of his room setup, including loudspeaker and listener placement, as well locations of existing room treatments. Also, the details of the screen would be relevant. In terms of the carpet, reflections from the floor are generally undesirable. Adding quadtric residue sequence-derived diffusion panels to the sidewalls and/or sides of the back wall could contribute significantly to the sense of envelopment/spaciousness.

[Ethan Winer]

Treating the bass in a small room is not only for dealing with frequency response issues, but also to bring relative decay times into line.

Geddes calls for "a large amount of sound absorption at low frequencies" ... Floyd Toole ... also suggests that "damping of room modes is advised" in his book

Exactly. Good discussion.

--Ethan

[JackD201]

Discussions have a tendency to go off at tangents.

No kidding.

Madfloyd would probably benefit from judicious placement of broadband diffusion elements, but these tend to be pricey.

True but not necessarily pricey. Green glued airtight convex diffusors made of ribbed quarter ply or almute sheeting won't break the bank.

It would probably be helpful to see a diagram of his room setup, including loudspeaker and listener placement, as well locations of existing room treatments.

True again. We weren't given much to go on.

Also, the details of the screen would be relevant.

Only if he has an acoustically transparent screen with one heck of an absorbtive wall behind it.

In terms of the carpet, reflections from the floor are generally undesirable.

There are two choices for flooring that depend on the desired acoustic goals. This is exemplified in any recording studio. The recording space is generally fitted with hard floors wood being the material of choice and control rooms are carpeted. So if you want a "live" reverberant soundfied go wood and if you want an EFZ soundfield you kill the floor and everything else practically so all you hear is what's in the recording. Note that in the latter, nearfield configurations are the practice. Madfloyed is listening in the midfield.

Adding quadtric residue sequence-derived diffusion panels to the sidewalls and/or sides of the back wall could contribute significantly to the sense of envelopment/spaciousness.

The quadratic residue derived cavity depth and spacing is likewise frequency dependent. He could very well end up using the wrong quad diffusers.


Enough quotes though. Here's what I gather from what madfloyd described.

1. He has ASC viscous damping on his walls and ceilings. These absorb bass and reflect pretty much everything else. It's not the culprit.

2. He has realtraps for bass. Again these don't suck up midrange energy or HF energy in amounts that could deaden his room. Not the culprit either.

3. None of his speakers can be considered laid back. These are all fairly neutral speakers with good harmonic capabilities. Definitely not the culprit.

4. His walls are untreated so that leaves bass absorption and his carpet as effectively the only treatments. The bass isn't doing it so that leaves the carpet.

If you do the basic math excluding areas covered by furniture and discounting the materials of the furniture, 20% of his surface area is covered by a material that absorbs upwards of 75% of 4kHz energy. He could be down (worst case scenario) 15% amplitude at 4k. Wire a resistor to your tweeters that reduce output by this much and I'll bet the sound will really be very noticeable "deadend". Kill this region and vocals will become recessed and harmonies will indeed tend to blur.

I of course given the lack of data could be totally off on my postulations so may I suggest madfloyd try a couple of experiments that won't cost him a single cent.

Experiment 1 - lets get the carpet out of play

Take a chair and sit no more than six feet away from your speaker plane and set your speakers six feet apart with a slight toe in. See if the sound is still dead and lifeless. At this distance you should be getting the direct radiation. If it's still lifeless from this position it's time to check upstream components.

Experiment 2 - lets play with your floor

If you still have those ASC wall panels you removed, lay them on the floor lengthwise between you and your existing listening position. If you don't have them anymore go snag some plywood and use that instead. Make sure the path of first floor reflection points to your ears are covered with a panel. Note the difference. You might find that floor reflections aren't always a bad thing ESPECIALLY since loudspeaker designers always factor this in when voicing their loudspeakers.

Once acoustical energy is absorbed it's gone for good. You can subjectively bring it up by absorbing everything else more but again achieving proper tonal balance under such a scenario will be extremely difficult to do passively. An equalizer might suddenly seem like a viable option under such circumstances.

madfloyd, I feel your pain. I've been there myself. At any rate I hope you find a solution that works for you.

If it does turn out that your carpet is the culprit and it still has to stay I guess it's time to go and audition a good pair of line source loudspeakers in your room..

[James Romeyn]

Quoting from Geddes' book may be misleading because it was published in '02; his thoughts re. bass absorption may have evolved since then.  I have no knowledge of his promoting bass absorption except for postings here; IIRC I recall reading posts within the past couple months wherein he recommends against it & rather very much favors as much reflectivity in the bass region as possible, further, stating it is a prerequisite toward the goal of replicating large room bass in small rooms. 

I regretfully admit much if not most of the scientific discussion goes over my head, but this remains: I spent lots of hard earned money & effort to build a proper soffit & other treatment designed by a scientist who studied the room.  After following Geddes' & LeJeune's advice I would never consider for a moment returning to EQ & bass absorption to solve bass modes.  If Geddes/LeJeune's methods were not employed & if serious modal problems existed I would strongly consider forgoing deep & powerful bass rather then employ EQ &/or bass absorption.    

[woodsyi]

I use bass traps (both Real and GIK) as well as high frequency absorbers and diffusers in more or less a "traditional" way.  I also set up Duke's swarm in a bigger room with no treatment to augment full 'stats.  Both work great. You have to asses your space and requirements and go with the best option.  In my cave, using a large quadratic diffusers on the back wall and smaller ones in the early reflection points on the side walls in lieu of absorbers added sparkles in the upper frequencies.   

[James Romeyn]

I use bass traps (both Real and GIK) as well as high frequency absorbers and diffusers in more or less a "traditional" way.  I also set up Duke's swarm in a bigger room with no treatment to augment full 'stats.  Both work great. You have to asses your space and requirements and go with the best option.  In my cave, using a large quadratic diffusers on the back wall and smaller ones in the early reflection points on the side walls in lieu of absorbers added sparkles in the upper frequencies.   

SWARM + stats: S-E-X-Y! 

[youngho]

Jimbo, here is a recent quote from Earl Geddes on DIYaudio on 12/22/08 in thread titled "Multiple Small Subs": "In your model, how do you handle absorption? This is a very difficult thing to do right. In the most general room with damping, the modes are not orthogonal and the simple series solution which you use is not valid as it assumes orthogonality in its derivation. For light damping the modes are nearly orthogonal and the series is OK with an assumed complex kn to handle the damping and you can even use this complex kn in the eigenmode cosine term, but this is only an approximation that is only valid for light damping. In a very highly damped room, like I suggest, a solution does not actually exist in general." Sounds like he still advocates bass absorption to me, but I won't presume to speak for him. Maybe "very highly damped" actually means highly reflective.

Jack, "Green glued airtight convex diffusors made of ribbed quarter ply or almute sheeting won't break the bank" will create a diffraction grate, rather than a true diffusor. Toole discusses this in his book when showing "diffusion" data for repeating polycylindrical elements. I didn't realize that Madfloyd was recording in his room. I was thinking of a true deep QRS-derived diffusor like the RPG Diffractal, which is broadband down into the modal region. I was actually more curious to know whether the video screen is indeed highly reflective as Madfloyd imagined. I don't know how many Realtraps Madfloyd has, nor where they're placed, nor even what types of "Realtraps" they are. Obviously, I cannot make any educated guesses: 40 Microtraps or HF Minitraps would be very different from 2 Mondotraps, and the effects of the various traps depends on the details of their placement (Mondotrap on wall versus corner), but the absorption data on Ethan's website shows significant absorption outside of the bass region for all of his products. However, since you're clearly an expert on these matters, I will defer to your greater wisdom. Madfloyd, forget about quadratic diffusors, my apologies for wasting your time.

And I'm out! Cheers, and happy listening!

[JackD201]

Hi Youngho,

Sorry for the confusion. By ribbed, I was referring to the inner frame to reinforce the convex diffusor's structure. This diffusor's smooth arc linearly diffuses through redirection instead of the more "random" diffusion pattern found in quadratics which depending on depth and width also attenuate certain frequencies to the extent that the effect is akin to absorption.

Yes I am US certified in Design and Construction for Critical Listening Environments but personally I don't consider myself an expert, rather I consider myself a student of acoustical sciences with a long way to go. At least a longer way to go than many who have already posted on this thread. I also apologize for using a tone that makes it appear that I know more than I feel I do. As I mentioned these are just my postulations and I did say I could be totally wrong. My suggestions however cost nothing but a bit of elbow grease so I hope madfloyd gives them a shot.

You have made very good points so I wonder why any apology to the starter of the thread is needed.

cheers!

Jack

[James Romeyn]

Jimbo, here is a recent quote from Earl Geddes on DIYaudio on 12/22/08 in thread titled "Multiple Small Subs": "In your model, how do you handle absorption? This is a very difficult thing to do right. In the most general room with damping, the modes are not orthogonal and the simple series solution which you use is not valid as it assumes orthogonality in its derivation. For light damping the modes are nearly orthogonal and the series is OK with an assumed complex kn to handle the damping and you can even use this complex kn in the eigenmode cosine term, but this is only an approximation that is only valid for light damping. In a very highly damped room, like I suggest, a solution does not actually exist in general." Sounds like he still advocates bass absorption to me, but I won't presume to speak for him. Maybe "very highly damped" actually means highly reflective....

Youngho
Thanks for the input.  The following is Earl's words (per your post) w/ my accent demonstrating my interpretation.  Of course I could be completely wrong.  EG: "...In a very highly damped room, like I suggest, a solution does not actually exist in general."  In other words, like Earl suggests, there is no general solution to achieve the desired reproduction quality (or solve whatever problem is being addressed) in a highly damped room.  The comma separating "room" & "like" is consistent w/ my interpretation; parenthesis around "like I suggest" would be consistent w/ Earl suggesting high room damping (the opposite of my interpretation).  But like you say, I won't speak for him.  It's only my understanding & If I paraphrased incorrectly earlier, profuse apologies to all for any confusion caused.  I intend to get more clear quotes later.

   

[youngho]

Jimbo, I hope this makes things perfectly clear for you. I tried to frame the question as unambiguously as possible. If you want to clarify your interpretation further, you may wish to contact Dr Geddes yourself.
http://www.diyaudio.com/forums/showthread.php?postid=1734361#post1734361

Jack, yes, I was a little hurt by the tone, so I accept your apology, but it turns out that I don't have much else to say, anyway! Rather than a student of the acoustical sciences, I'm more like a truant or dropout. I have a casual interest but no formal training or education in acoustics, so I will defer to those with such or practical experience. Although I still suspect that Madfloyd would benefit from modest investment (compared to the four pairs of speakers) in QRS-derived diffusors, I can't predict it with a high degree of confidence. Anyway, like I said, I don't have much else to say for now. Cheers, and happy listening.

[BobM]

Why not just remove your speaker grills 

Sorry, couldn't resist. I think everyone here is recommending a simiar approach. Some trial and error is warranted. Judiciously removing some of your room treatments and seeing what the effect is will probably lead you to the culprit, whether it be the carpet or the traps or the other items.

Just a thought, which I hope is not the culpret, but have you had your hearing checked recently? Any excess wax in there?

Bob

[JackD201]

youngho,

Thank you for your magnanimity. Cheers and Happy Listening as well!

Jack

[youngho]

Jack, I did think of something. Floyd Toole discusses a paper by Erwin Meyer from the '50s where diffusivity is calculated for a bare room (68%?), the same room with a perfectly absorbent carpet (much lower, but higher than 30%), the same room with a perfectly absorbent carpet but diffraction grating placed on all the other surfaces (64%?), and the same room with a total amount of absorption equal in area to the carpet but now divided into six pieces and placed at the first reflection points on all six surfaces (23%? 26%?). I'm sorry, I don't remember the exact numbers, and I don't have Toole's book handy, but one of the major points for me was that diffraction/diffusion can restore much of the diffusivity to a room, even with one boundary being perfectly absorbent. Interestingly, I took a look at Everest's Handbook, and he discusses a case exactly like Madfloyd's where the unevenly absorbent carpet causes an undesirable acoustic environment. The good news for Madfloyd would be that the carpet is not perfectly absorbent and that its most significant absorption is only down into the lower midrange, meaning that shallower and consequently less expensive diffusion elements could be used. Anyway, if the carpet is immovable, and if multiple rigid objects (like the panels you suggest, or even coffee tables or unupholstered chairs) cannot occupy a large part of the floorplan, then diffusion could still help restore a significant amount of "air" or "life" to the room. The amount of absorption compared to diffusion seems to be low for the RPG products I glanced at, and the most absorption for many of these seems to occur outside of the carpet's main effects. The nice thing about the RPG products is that they give some absorption and diffusion data that can be used to compare the various possibilities. Unfortunately, they are a little expensive but not compared to the cost of the loudspeakers. 2D diffusers like the Skyline or Hemiffusor could be placed on the ceiling, perhaps mostly in the center but including the first reflection points so that this energy is not reflected mostly back towards the floor and lost; 1D diffusers like the Diffractal could be tried on the rear and side walls (and, if the screen is acoustically translucent, even on the front wall) and moved around as desired. I don't know how easily a convex diffuser could be moved once Green Glued down.

[JackD201]

Youngho, not only does RPG give the values, in the past they even provided detailed construction plans of their quadratics!    My guess is that madfloyd's room problem (if indeed he doesn't have a synergy problem upstream) is frequency specific. You correctly pointed out that carpet absorbs very little into the midrange and absorbs mainly high frequencies, the very same high frequencies from 4kHz and up called the presence region (4khz to 6kHz) that recordists via microphone positioning pay very close attention to when trying to acheive that "live", "you are there", or "they are here" feeling. So again you are very right that covering the surface area of the floor with leather furniture of even the verbotten coffee table would help. Hence my freeby suggested experiment to put those old ASC wall panels or even some plywood on the floor "just to see". 

It is important to note that since all of his speakers crossover at around 2kHz we're dealing with his tweeter's outputs and their on and off axis response patterns. Diffusors on the ceiling would be theoretically more effective than side wall diffusion if indeed the problem is the floor. The energy reaching the floor would be diminished greatly since what used to be almost full force on axis 1st reflections would mostly be scattered into free space. Side wall diffusion at the first reflection point would be second in effectivity because this would be diffracting off axis output. In a room sans diffraction 2nd ,3rd, and 4th reflections would end up hitting the floor. Even if loudspeaker spl diminishes by roughly half for every doubling of distance and a loss of energy at every boundary collision the sum effects are still very audible. So come to think of it you are more right than I initially thought actually I should have taken this into account earlier. Thanks for pointing this out. So again you are correct as others who have said in this thread that diffusion could very well bring some life back into the room. The caveat is that it would IF the problem is indeed room and not upstream component related. As we know having "good equipment upstream" as madfloyd said he has is no guarantee that there are no mismatch gremlins. We have to remove one of the variables then. Since this is the acoustics circle I guess that's why we are dealing with the acoustic variable and not the electronic one

The order of the day then is still is to determine whether the floor is the culprit or not. Without that determination potentially expensive and more back breaking work on the walls and ceilings could be for not.

To clear up any confusion on the green glue. It along with the ribbing is applied on the inside surface of the ply or metal airtight convex diffusor to stop the surface from ringing at high frequencies and make the surface more inert to keep it from acting as a bass pressure trap. The structure is airtight using ordinary silicone sealant so it doesn't act as a helmholtz resonator either. The panels are movable. In practice we see this type of diffusion when vocal intelligability is the concern. I brought it up as a possible tool since madfloyd found voices recessed and had a hard time picking out harmonies. This is essentially what John Storyk of WSDG, my teacher uses on the front walls and also the what Rives does as well. They don't look like diffusors but that's what they are.

Cheers!

[sendler]

 Nobody wants to mention an eq? You can make your sound any way you want it in 30 minutes with a digital eq. Highly absorptive  rooms can offer the best see through transparency but most speakers that measure flat on axis will sound rolled off above 8k in treated rooms, especially if you position them for the least baffle diffraction, ie. not toed in directly at the listener. You can try a DEQ2496 for a fraction of the cost of changing your room treatments again. Running it digital in/out to your dac will be very transparent, or mod it and gain a great sounding dac as well. There must be other surround processors that offer eq as well.
http://www.behringer.com/EN/Products/DEQ2496.aspx

[youngho]

Jack, the WSDG website has lots of supercool installations in their portfolio. I looked primarily at the home theater ones, some of which do seem to use diffusion elements on the back wall, also sometimes on the ceiling in the carpeted rooms. Are these ceiling elements the convex diffusers you're talking about? http://www.wsdg.com/scalephoto.asp?img=data/edithtml/portfolio/showcase/gallery/Images/ROWLEY/ROWLEY_right_1_large.jpg&scaled=r650,464 and http://www.wsdg.com/scalephoto.asp?img=data/edithtml/portfolio/showcase/gallery/Images/ROWLEY/rearviewL.jpg&scaled=r650,464

I had imagined their construction to be rather like the ones shown in Figures 9-26 and 9-27 here: http://books.google.com/books?id=sgwg1Vwm9VUC&pg=PA209&lpg=PA209&dq=everest+handbook+polycylindrical&source=bl&ots=JGvFPw6IMB&sig=PrDslqTRIi3b_pohf_sum8AVh3I&hl=en&ei=WSuMSbqHGKKBtwfc0IWbCw&sa=X&oi=book_result&resnum=1&ct=result#PPA212,M1. However, these will have some bass absorption, and I believe that even airtight structures would still have some absorption because of the sealed cavity acting as a "spring", so I'm curious to hear a little more about the convex diffusers you describe. For example, the Rives ones I've seen online tend to look rather like this (http://www.positive-feedback.com/Issue16/lavigneroom.htm), which I have trouble imagining as movable.

If the construction details are not proprietary, would you mind consolidating and sharing them with a level of detail appropriate for someone like me with extremely little DIY experience? I wonder if I could hire someone local to construct a few for me.

Madfloyd, would you mind letting us know what you do and how it works?

[JackD201]

Youngho, the ones I was reffering to are more like this:

http://www.wsdg.com/scalephoto.asp?img=data/edithtml/portfolio/showcase/gallery/Images/MERCURY/Mercury%20-%20TableL.jpg&scaled=r650,464

The ones on the ceilings in the HT set-up do double duty since they are open. Here's a pic of my HT with a metal perforated version that does double duty as well.



Here however in my 2 channel system I use twin sealed diffusors with quarter ply. I should have done just one big one but these are sliding doors that hide windows the view of which was the neighboring building. I wasn't allowed to seal of the wall by my building administrator despite my protestations that the opposite resident who was shall we say past her prime always changed without closing her blinds much to my consternation    Anyhow this twin configuration caused some comb filtering and the ASC panels pretty much had to go on them to fix the problem. The curvature is slight but if you look closely you can see it.



If you follow the plans of the polycylindrical diffusors and want the bass absorption out of the picture and wish only to have the diffraction, the trick is to multiply the ribs so the surface becomes very rigid much the same way cross bracing inside speaker cabinets makes the baffle more rigid. It negates the spring effect that you mentioned. Plastering the rear with green glue also serves to dampen it. If you REALLY want to make it rigid then plaster of paris or epoxy impregnated with marble dust can be used. There is a less labor intensive way though. skip the ply or standard metal sheets and use the smooth version of this.

http://www.almute.com

This sucker doesn't ring. It's made of compressed aluminum shaving and is slightly less dense than a solid sheet. When sealed off and curved it acts as a diffusor but when opened can act as absorbers. (Airspace determines frequency just like any helmholtz) I'm constructing a new listening room with prime number dimensions 3m H x 7m W x 11m length and it looks like I'll be using a lot of this for the ceiling. It is soft enough that any powertools used for wood will work with them without much more wear on the blades as one would suspect. Surprisingly, it also isn't as expensive as it looks

For a medium horizontal diffusion patterns I normally raise the center 4" for every 4' of width. Just try and make sure the surface arc is constant throughout

[madfloyd]

Thanks to all for the responses.

I thought I'd add some pics of the room.   I am considering removing the carpet - or perhaps the front 1/3 of the room (and replace with hardwood).  As an experiment I laid down pieces of plywood in the front of the room (up to where the speakers are placed) and it helped the high frequencies, but not necessarily the upper mids.   I also tried removing my screen and I think it helped some (I only had a limited window of time to do audition this, so I'm not certain).

Here are some pics to give a sense of the room.

From the listening position:


Further back:


The rear of the room:


Close up of the funky alcove where my equipment rack is:


Here's the experiment with plywood etc on floor and screen removed and a friend's diffusor sitting in between speakers:


Note that in these pics most of the ASC wall planks have been removed.   I'm thinking of adding diffusors to give a bigger sense of space and liven it up.   Where are the best places for diffusion?

Thanks again,
Ian

[madfloyd]

Hmm, apparently I don't know how to link to pics very well.  I'm using photobucket and used the image button (inserting the URL between the two img codes).   What am I missing?

[woodsyi]

You have to right click on the picture and copy the image location.  Then paste it between the image brackets.