A rule of thumb for selecting dimensions for a media room is to avoid numbers that have simple common denominators.
Example: a room with an 8’ ceiling and 16’ X 24’ walls would yield poor results, due to standing sound waves and/or bass that cancels itself out in the center of the room. A better choice would be 9' ceilings with a 17' X 24' floor plan.
The key is to avoid reflecting sound back into the center of the room, which generally creates poor sound performance in the area where people would be sitting.
Materials that absorb or diffract sound are good and things that reflect or amplify sound are bad.
So, excessive amounts of flat reflective materials like concrete, glass, or plaster are bad. You can offset this with rugs, furniture, and sound absorbing panels. Texturized surfaces or cabinetry can also help to break up the reflection of sound.
Placement of furniture and sound-dampening fixtures
Generally speaking, symmetrical placement of speakers and primary listening positions is good practice when laying out a media room. Most audio components can correct for askew positioning, but it's not ideal.
When placing sound-absorbing elements like furniture, rugs, or acoustic paneling, it's best to place them at points (along the wall, on the floor, and on the ceiling, respectively) halfway between the speakers and you. It's also a good idea to put other items in the corners of the room.
For more detail and explanation, here's an article from SoundAssured. There's also an article from Soundis on the difference between soundproofing, absorption, and diffusion.
With subwoofers, more is better. The ideal placement for a sub is usually not dead in the corner, nor in the middle of the wall.
If you have more than one subwoofer, don’t place them symmetrically. When the sound from two subs travels the same distance to the ear they cancel each other out, defeating the entire purpose of the device.
It's crucial to minimize outside noise from infiltrating a media room, of course. So is keeping the media room from overpowering adjacent rooms.
As such, placing noisy appliances in or adjacent to a media room should be avoided, whereas using low-noise air conditioning is very beneficial.
Isolating the sound from a media room can be achieved by placing anything that will limit the wall's ability to vibrate. There are also some specific construction techniques that can help, as well.
The ideal design scheme to use in a media room is neutral gray. Bright colors of wall paint or furniture will reflect and affect the performance of the screen. Bright white walls also reflect excessive light.
Placement of the Video Display
The video display should be placed where outside light cannot fall directly on the screen.
Rooms with a large amount of ambient light are never the best choice for media rooms. If necessary, you can install black-out shades to eliminate light.
For projectors and screens, there are many good resources to determine how far back your projector should sit (called throw distance). A good rule of thumb for a 100-inch screen is to have your projector between 122 and 133 inches (around 10.5 ft) away.