Diffusion versus absorption behind speakers?

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.

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.

Dracule1,

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:

http://www.kineticshometheater.com/index.html

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:

http://www.rpginc.com/

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