Diffuse is best.
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I have heard cone speakers and horns sound best when there is some absorption between them, meaning the image solidifies in the center and the depth increases. However, with panel speakers diffusion seems to work best.
Unfortunately, I think the real answer is - try an inexpensive method to test both and wee shat works for your situation before you buy something more permanent. Every room is different.
I am all for experimentation first. In my own setup, I went with a panel that uses absorption on the sides and a small hard diffuser in the center. This worked well for me.
A friend has a room that was designed by Rives. It has a very large, convex, curved, hardwood diffuser centered between the speakers.
I also have a fireplace (fake) with a glass front between my speakers (Silverline Preludes). The speakers are about 8 inches further into the room than the (still fake) fireplace, and 7 or so inches from the edge of the aforementioned (fake) fireplace, so reflections are gonna go sideways and it doesn't seem to hurt soundstaging, imaging, musical content, phase, or lyrical sensibilities (maybe that one a little). I've pulled them into the middle of the room and otherwise tried various degrees of tow-in, height adjustment, spikes, etc., and they sound best pointed straight forward raised up on butcher blocks with vibrapods under them. Also, the little corner the fireplace creates behind the speakers is perfect for a little REL Q150e sub. This information is useful only to me or those exactly like me with identical gear in a room exactly like mine.
>>Absorb or Diffuse in between speakers?
I would say that it depends on the speakers and on your room. As a rule of thumb, I usually absorb between box speakers and diffuse between dipoles and bipoles. But again, it depends on your room.
Rather than blind testing, I would suggest that you spend a little time and a very little money and perform an analysis on your room. I know that this seems like less fun than just blindly buying a bunch of treatments, but it will almost certainly result in better sound.
Both GIK Acoustics and RealTraps have posted a lot of free advise on their web pages. Start here: RealTraps - Room Measurement Series
and here: RealTraps - Acoustic Articles
It's really not at all hard to do, once you understand the basics, and will definitely result in the best result.
I agree with the other posters that it depends on your room. I also agree that you should experiment.
Personally, I've tried both absorption and diffusion between my speakers. I prefer diffusion by a considerable margin. In my case, absorption resulted in a significant increase in clarity, but at the expense of decreasing both the size of images and the depth of the soundstage. Diffusion suffered from neither of those problems, and provided 90% of the gains in clarity that absorption provided.
IME, getting the balance of absorption to diffusion right is a critical, and often overlooked, determinant of sound quality. In addition to the expected effects, like resolution, coherence, imaging, etc., the ratio and location of absorption/diffusion is also a major factor in creating the illusion that "You are There."
>>Omnis diffuse naturally and save having to deal with directional issues for the most part.
Again, it depends on your speakers and your room. I have had some "omnis" that required nothing behind them. My Apogee panels and ribbons, on the other hand, were very hot off of the rear wall and sounded much better with a considerable amount of diffusion behind them.
I also preferred diffusion panels (between the speakers) by a considerable margin. I've written about my experience here
Hi Kiwi - Very interesting thread you linked. I agree with many of the things you said. Among them, that the choice between absorption and diffusion is often the choice between imaging and soundstaging. Absorption often improves imaging at the expense of soundstaging. And diffusion often improves soundstaging at the expense of imaging.
IME, the rate at which absorption detracts from soundstaging is *much faster* than the rate at which diffusion detracts from imaging. And of course well placed diffusion results in *much better* imaging than an untreated room, so together that gives diffusion a significant advantage over absorption for many applications.
Again, IMO, IME, etc.
Barfbag (interesting name),
As with most things there are so many variables at play and we don't know your desired end goals. You've heard that the following things matter:
* type of speaker
* current room's "aliveness" or "deadness" (ie. reverb time)
* imaging vs soundstage trade offs (although not mutually exclusive)
* effective bandwidth working range for a given absorber or diffuser
* what mix of absorption and diffusion provides the "right" sound
* what is the room's purpose - HT or stereo listening?
* and not least of which aesthetics may factor into things.
Personally, I am a fan of mixing it up. On my front wall I have the following sequence of treatments:
Diffusion. Absorption. Diffusion. Absorption. Diffusion.
All diffusion is of mid and high frequencies while doubling as bass traps. So bass trapping is used extensively while mixing the treatment effect for mid and high frequencies which aligns with my room strategy of absorbing below the transition zone and reflecting and diffusing above it. This is to preserve the aliveness to as close as possible match the natural tendencies of longer bass reverb times.
In the end treating the front wall is a necessary but insuffient condition for optimizing your listening pleasure. Be sure to treat the back wall as these reflections are equally 'damaging' meaning they are less advantageous than side wall and ceiling reflections. I would hazard to guess that if your listening chair is close to an untreated back wall that it likely doesn't matter much which kind of treatment you put on the front wall as what you're hearing is mostly indirect reflections from the back wall as front wall reflections will be attenuated by the time they reach you. So net net, be prepared to treat both front and back walls and not just one of them.