Diffusion versus absorption behind speakers?

For forward firing cone speakers, should I use diffusion or absorption panels? I get the feeling most would agree on absorption, but for dipole panel speakers diffusion is better. Any opinions based on experience or science of acoustics?
I use absorption but I would think either could work if used correctly. It would depend on room characteristics and your own ears. Apogee told me to leave the wall behind the speakers bare, but I have heard planars that could have benefitted from room treatment.
Draculel: There is a plethora of information on this subject. The science of acostic engineering is as much a science as it is also an art. You must experiment with it as the manufacture's products are used in so many different inviorments that it is hard to determin how it will perform.You most likley will not go wrong with treating the rooms pressure zones with a product such as room tunes. treating your rooms upper corners is always in the right direction and also where parallel walls meet. The type of furniture one choose's can make a huge difference. Hard surfaces such as floors are best left uncarpeted with ceiling being treated will work best. Use plants and other objects as diffusors. Side walls should always be treated. And lastly make some bass traps which according to all is the single best thing you can do. Do as much reading and evaluate your room often and experiment much it's the single most important thing you can do for best sound.
It is really hard to generalize about what one should do to treat a particular room. I have heard a lot of rooms with major treatment, and as a rough rule, I would say go slow on room treatment. It is easy to go overboard and get a dull, sterile sounding room. I have heard rooms designed with extreme absorption in the half of the room where the speakers are located, and diffusion in the back half of the room (live end, dead end), and this setup sounds weird to me. Even a room custom designed by Rives (in a friend's home) sounded a touch too over dampened to me, but, it may be just the case of my not being use to its uncanny quietness. The Rives room used a mix of absorption and diffusion. Most of the treatment is diffusion, with bass traps located inside of the walls in the corners of the room.

If you can, get as big a set of bass traps for the corners of the room as you can manage. When it comes to bass absorption, size matters (thickness of panel traps, diameter for tube traps). Almost all rooms are helped by some form of bass trapping (even when a system seems bass shy, trapping actually helps), so I would start there.

Treatment involving higher frequencies can be as simple as a floor rug in front of the speakers, putting up a decorative tapestry on the walls, even using books, cds, records in a bookcase as diffusion. "Treatment" doesn't have to be ugly. I've seen acoustic panels that consist of small panels (both diffusors and absorption) that are covered with fabric that look pretty decent when decoratively arranged on the wall.

Again, I would suggest moving slowly on treatment. Start with bass traps. Then try some absorption in the form of rugs, tapestries or window treatment, etc. As for acoustic treatment products, I would try a mix of absorption and diffusion panels.

ASC tube traps offer both. The tubes have a partial reflective surface identified by a silver dot. You can change from full absorption to a combination absorption/reflection by rotating the orientation of the trap. I use 2 11x36 and 1 9x72 on the speaker wall. I use the two nearest the speakers with the reflective area towards the listening position and the traps at the back of the room in the corners in a more absorptive orientation. The changes wrought by orientation are readily discernable. My answer then would be both absorption and diffusion. On the down side, they are very pricey.
TubeTraps and ASC offer a free evulation of your room if you send them pictures and a diagram. I've use both as a starting point and it worked well, but as others have stated, it is a trial and error process.
I asked the same question for my soundlab m2's and was advised by rives audio to use diffusion over absorption;I started playing and found both seem to work very well and even hanging rug art behind the panels seemed effective;not really sure what the correct answer really is,but your ear will be the deciding factor.
I had a full "Live end - dead end " set up at one time using Sonnex. I liked it at the time. My present room is 35'x13'x7' and had serious problems. I would also start with bass traps in the corner and go from there. I eventually added 7 more, mostly on the back wall as well as some wool area rugs. the room is still quite live but I can now hear things I never heard before. The amount of detail lost to cancelation due to reflections will astound you. I got mine from Ready Acoustics, who were very helpful, they also will send you a suggested set up if you provide them a picture. They are reasonable and my wife likes the look of their products. No connection, just satisfied customer.
Depends on room, radation of loudspeaker, no right or wrong just whats right for your room system etc.
Why spend extra money? I placed tall (7 feet 6 inches tall, 24 inches wide, 10 inches deep) bookshelves in the far corner on both sides, filled with stuff (various books, DVDs cd). Behind the speakers, in the middle is a low leather chair Floor length curtains are opened on the back wall.
The room sounds boomy without anything in the corners. With the shelving it sounds more natural.
I am amazed that folks just do not use the normal home furnishings to 'fix' room problems. Does spending a few thousand on 'special' crap makes them feel better about what the stuff can or cannot do?
Thanks everyone for your responses. I am not a novice on this subject. I know treating corners of the room and back wall behind the listening seat with bass traps has beneficial effects to tame room modes. Also treating first reflection points help in imaging. But I have a specific question regarding the front wall. Some use absorption that helps with better pinpoint imaging. Diffusion, I heard, makes the sound stage bigger but images get larger also. I probably will end up with a box speaker and a panel speaker. So I'm trying to to see which front wall acoustic treatment will help both types of speakers. I get the feeling absorption behind dipolar planar speakers may not be a good idea.
I am more inclined to go with diffusion regardless of the type of speaker involved. As I stated before and Elizabeth has noted too, use bookcases, furnishings, wall hangings, etc, before looking at acoustic treatments. If one of the walls is a big glass window or door, that should be a priority (use blinds or curtains, etc.). If possible, avoid having a very big and flat reflective surface (like a coffee table) between the listening position and the speaker.

Another "free" fix is to listen in the near field by sitting fairly close to the speaker so that direct sound of the speaker dominates over the sound contributed by the room. My preference on that is to move the speaker further into the room. I almost always prefer the sound of speakers when they sit well away from the wall behind the speaker.

I consider extensive experimentation with placement of the speaker and/or listening chair to be the first, and most important step to getting good sound. Almost all difficult "room" problems can be substantially ameliorated with proper placement of the speaker and/or the listening chair. It is surprising how much even a small movement of the speaker, or change in toe-in or rake angle (how much the speaker is angled backwards) changes the sound. If random trial and error is way too exhausting and frustrating, you can try some of the more systematic approaches. A discussion of these approaches is a BIG subject area (google the "Sumiko" or the "Wilson" method of speaker placement).

If all of the non-acoustic treatment approaches fail, start first with tube traps or corner traps in the corners of one side of the room. Unless you build your own bass traps, these can be somewhat expensive. I am personally not inclined to go beyond bass trapping because the costs can get really high and the results are often inconsistent. Plenty of junk on the walls to diffuse sound usually works as well or better than specialized treatment.
I didn't really find it expensive and I am far from a "cost no object" audiophile. Your room is THE MOST IMPORTANT COMPONENT in your system, if it is bad nothing else matters. I sit in the same place now, which is close to the speakers, that I sat before I treated the room but the sound is very different. Improved? In every respect. If you haven't dealt with room reflections you have no idea of the way they blur the sound. I didn't. If you can try out some good headphones on your system to see what it sounds like without your room interfering. No, headphones and speakers do not sound the same, but you will still get some idea of whatever problem exists. I spent around $800 on my room, could have been less if I had used their DIY option, and there is nothing else I could have done for 4 or 5 times the money that would have given that improvement. My listening friends share the same opinion. Try the furniture by all means, as well as rugs. The wall to wall I inherited when I bought the house turned out to be useless in light of the improvement of the Costco wool area rugs that replaced them. I also had the walls lined with 2+ tons of books and my 3000 LPs and that did not cure the reflections.
I have tried rugs, curtains, furniture etc as "room treatments" but in general I did not find significant improvements. Professionally made acoustic treatments (ie, GIK, RealTraps, etc) are designed (with measurements done in certified acoustic labs) to cover appropriate frequencies and are very effective. In my experience, furnitures, books, rugs, etc are crap a shoot and are less than satisfactory unless you have glaring acoustic problems (eg, room with extensive glass windows) in which case some minor improvements can be heard. Yes, some professional stuff are very expensive, but there are some (eg, GIK) who sell reasonably priced treatments.

But let's get back to my original question regarding room treatments behind the speakers. I don't mind if you mention specific products as long as you are not selling them. Thanks.
Some say that 60% of what you hear from your system is your ROOM.
I was told to try some wool carpets or blankets behind my speakers to keep my downstairs neighbor happy. I tried some thick 2.5' X 3' cotton rugs behind my speakers and she can't hear a thing now. If it can keep the sound from running down the walls (and I do play it loud at times), then one can imagine how it 'tames' reflective sound.
I've tried both absorption and diffusion behind my speakers at the front wall and in the end removed them and now don't use anything behind them. Instead, the corners are treated with bass traps, the upper corners with Eighth Nerve triangles and the seams where the ceiling meets the wall with Eighth Nerve rectangles. Unfortunately Eighth Nerve is no more, but their concept on how to treat rooms is very effective for me.

As previously mentioned one can eliminate many room interactions by listening near field, which due to my room configuration is really my only option. You might also want to post this over on AA. There are some great resources over there, especially David Aiken, who have helped me address this issue, especially the best methods to use diffusion.
Thanks Clio. I think your approach is making more sense to me. Leave the front wall alone and treat the corners of the room and possibly side wall first reflection point.
I am using diffusors from Core Audio Designs behind my speakers. The speakers, which are a front radiating cone design, are just 28" from the front wall (measured from the tweeter), due to space limitations. Adding the diffusors behind my speakers increased perceived resolution, added depth to the soundstage, made the speakers less localizable, and enlarged the image sizes.

I also experimented briefly with absorption, but in my setup, the only improvement was increased perceived resolution. Soundstage depth, speaker localizability, and image size either stayed constant or got worse. I'm not sure what the results would have been if my speakers were farther away from the front wall, but I suspect it would have been similar in kind while less in degree.

The Core Audio Designs diffusors are also among the most attractive room treatments I have seen. I have no connection to the company.

Good luck.
One of the pieces of advice I received regarding diffusion is that the larger the room, the more effective it is. Makes sense considering that diffusion works by spreading the sound around.

These are the diffusers I used:


There is some good information on acoustics and room treatment from Decware too.

The Core Design diffusers are extremely nice. Wish I had the space to try those out.

Even if you actually provided information on the size, shape, construction, furnishings, etc. of your particular room, it would be quite hard to guess what would work for you. This is mostly a trial and error process. I've heard numerous rooms designed by acoustic experts and they all sound very different and vary greatly in the amount of improvement achieved.

As for treatment of the front of the room, I agree with those that, aside from corner bass traps, most other treatments give equivocal results. The best "treatment" tends to be moving the speakers as far out from the back of the room and the corners as is practical.

I use, and like, ASC tube traps in the corners of a room. I use double stacks of 16" diameter traps. The bigger the traps, the lower the frequency that they remain effective at controlling the bass response.

A friend of mine in the home theater/audio business has had good success treating the side walls of rooms with absorption/diffusion panels and with corner bass traps from the following company:


Their treatment looks really good too. Also, they don't take up a lot of room in terms of being really thick.

The following source also make a variety of diffusion and absorption products that work well for dedicated listening rooms and home theaters:


I hope some of these have something that might work for you.

I would consult with GIK on this. I think that wall should be third on your list behind bass traps on corners, ceiling etc, then high frequency absorption at the side wall reflections. But get some advice from the pros at GIK and Real Traps too.

Generally, if you don't have dipoles, then things like quadratric diffusers would go on the wall behind your head if your listening position is far enough away from that wall.
Using tube traps requires extreme patience. I have had them since 1988, and having the seams pointed towards the speakers will usually yield quite good results. HOWEVER, the seams will sometimes need to be slightly oblique and they must be moved in 1/64" increments (or less). Additionally, you must move them along the walls -- both sides and rear -- in miniscule increments, so small you can carely tell you moved them.
On side walls behind the speakers, Moncrieff suggested orienting the seams towards the speakers. Forward of the speakers, he suggested pointing the seams towards the speakers. This must be done over a period of weeks. One should move them one at a time. I have around 40 or so, stacked floor to ceiling. If you have jogs in the room, you MUST have traps in those jogs, or the upper bass/lower midrange will sound thickened. When I had an addition built onto the house, there was a jog, so the room narrowed from 13' to 12'8" and I heard the thickening, but it wasn't until a couple of years ago -- 5 years AFTER the addition -- that I looked at the jog and thought, "Hmmm...sound COULD be building up there, too." I put 2 traps in there (8' height) and the thickness disappeared.
It is crucial you move them exceedingly small increments in whatever direction you move them. Forget 1/8" as "exceedingly small." I mean small enough that you can barely see that you moved them. Tiresome? Yes. But the amount of spatial detail, low level detail, transient info, room size, image focus and an actual sense of how instruments sound in a real room will leave no doubt. You must also, when you think you have them moved in exactly the right spot, then move them BACKWARDS a tiny fraction. You will either find high frequency (i.e., bells, triangles, harp) transients better or worse. At that point, either move them back to where they were or leave them.
Seamson traps directly to the side of you are generally best (on cones) pointed at 45 degree angles INTO the room, but again, they require extremely small turns.
I also have the ASC's room damp setup with resilient channels. In this setup, no walls touch each other, not even on the floor, so there is no connection. An adhesive is used to seal cracks between foor and side walls and ceilings.
There's an article on ASC's site by Moncrieff back in 1989.

Hello ,
I prefer to absorb than to diffuse behind the speakers and to use diffusers behind the listening position.