The acoustic characteristics of your room and the placement of the speakers within it have a far greater effect on the quality of the sound you will hear by changing any single piece of electronic equipment. While acoustics can be a confusing subject, and extensive room treatments and alterations can cost thousands of dollars, there are a few simple, inexpensive things you can do to maximize the performance of your room and setup.
There are three important phenomena of room acoustics: room modes, reflections and wall resonance.
Room Modes (also known as standing waves) are a function of the dimensions of your room and create uneven bass response throughout a room. To hear the effect of standing waves, play a CD with deep, sustained bass. Unless you are one of a lucky minority, you will hear the bass loudness go up and down as you walk around the room. Where there is more bass and where there is less bass is a function of the dimensions of the room and your speakers' placement. The closer your room is to being a cube (equal length, width and height), the more uneven the bass response will be. If you live in a cube-shaped room-move. For the rest of you, keep reading for advice that will help you minimize room mode problems.
Reflections are just that, reflections of the sound off walls and other objects. Reflections are not bad per se. You just want to avoid strong, coherent reflections. Strong, coherent reflections will color the sound, make the system sound "bright," and ruin any chance of getting lifelike imaging. If you have ever heard a sound system in an empty, hard-walled room, you know what we mean. The reflections you have to deal with the most are those that happen within a few feet of the speakers.
Wall construction will also effect the sound of your system. Walls can act as big resonating panels that can add "boomy" coloration to the sound. Bang in between studs on the center of a long wall in your listening room. The odds are good that the wall will vibrate and make a booming noise. Sound waves from your speakers will excite the natural resonance of the wall in much the same way. If your room walls are either 1/2" drywall or wood paneling the effect is even more pronounced.
You can minimize bad reflections by placing your speakers far away from walls and by absorbing or diffusing the reflections.
The first thing to do to improve the sound of your room is fill it with "stuff." Any kind of stuff: books, furniture, drapes, knick-knacks, paintings. The stuff can be soft, hard, big, little; it hardly matters as long as the stuff isn't flat like the walls. The more irregular the wall surfaces, the greater the diffusion of reflections and the better the sound. Do whatever you can to break up large expanses of bare, flat walls and windows, especially the areas near the front speakers.
The most critical areas to treat are the walls behind and alongside the front speakers and at the first reflection points along the side walls.
To find the first reflection spot, sit in the main (center) listening position and have a friend slide a hand mirror along the side wall at tweeter height. When you can see the nearest speaker in the mirror, you have located the first reflection spot. Treat that area, and the wall opposite, with either sound absorbing materials (drapes, tapestry, or professional materials - see resource list below), or sound diffusing materials (irregular surfaced furniture, broad-blade wood blinds, or better yet, RPG diffusers). If you want to eke out a little more performance, treat the second reflection spot. Have your helper slide the mirror farther down the side wall until you can see the opposite speaker in the mirror. That's the second reflection point.
Most rooms/systems don't need ceiling treatment, but if your speakers are within three feet of the ceiling, treat the first reflection spot on the ceiling (in front of each speaker) with absorbing material. If your floor is not carpeted, place a thick area rug (with pad) in between the speakers and your listening position. If you have any large expanses of glass in the room, use heavy drapes to absorb, or shutters to diffuse, the reflections.
If the wall behind you is a wide, flat, empty wall, treat it with diffusing material. Generally, you want to have absorbing materials in the front half (the end your TV and front speakers are in) and diffusing materials in the back half of the room. Resist the temptation to over-damp your room with too much absorbing material. An overly "dead" room will sound dull and lifeless with little sense of spaciousness.
To minimize the effects of room modes (standing waves) you can add bass traps in the corners of the room. Start with the corners behind the front speakers and add traps to the rear corners if needed. Homemade bass traps are difficult to build correctly. The commercial traps are effective and look nice. Stop being a cheapskate and get the real deal.
Treating Boomy Walls
If the knuckle test tells you that your walls are resonating (we'll bet they are) you have a couple of options. The first is to replace the original wall material with heavier material like 5/8" or 3/4" drywall. That involves a lot of hard work and/or money. This clever device also doubles as a reflection-absorbing panel.
"Absorptive material was applied to the walls from floor to ceiling beside and behind the speakers and along one side wall. I used multiple 2x4-foot by 1-inch thick Owens #705-rigid compressed fiberglass panels glued directly to the wall surfaces between a stiffening frame made from 2x2-inch pine studs. The 2x2s were attached to the wall with drywall screws through the sheetrock and into the interior studs. Each of these 2x2 frames included two vertical and three horizontal studs to prevent the drywall from flexing. I covered these framed panels of compressed fiberglass with designer fabric and finished the edges using mahogany molding. This damping method serves a dual purpose: it absorbs reflected energy at frequencies above 200Hz and prevents the storage and release of low frequency energy by the flexing of the drywall panels."