Diffusion versus absorption behind speakers?

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.

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.

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.

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.