- Needs to be a decent size - at least 3m x 4m, say.
- Needs to have good proportions. Square is bad, a rectangle is better unless it is twice as long as is wide. All shaped rooms have "nodes" - certain frequencies which are emphasised or minimised by reflections between the walls. The dimensions of the room determine which frequencies will be emphasised, and the goal is to spread these nodes out as much as possible, to give the most even response. There is a great free utility called ModeCalc which allows you to calculate the room nodes for a given room, and the same page has a great introduction to the subject. Ideally, you could build "splayed" plasterboard walls to give a wedge shape which reduces the nodes even further.
- Needs acoustic treatment. Every room will benefit from some acoustic panels - you can make these quite simply yourself using high-density fibreglass ("rockwool") mounted in wooden frames, or buy them from a company like RealTraps, who run the site that the ModeCalc utility is hosted on. These "bass traps", or more accurately "broadband absorbers", will further even out the frequency response of the room, enabling you to make better mixing and mastering decisions. Foam panels may also be helpful but are far less effective, especially in the bass end, where rooms have the most problems. Egg-boxes are useless !
- Needs the speakers in the right place. They should be on a shorter wall, not too tight into the corners, equally spaced from opposite walls. I've seen several very theoretical methods of determining this, but perhaps the best advice is to get a recording you know well, play it at your chosen reference level, and move them around till they sound "right".
- Needs a good listening position - equally distanced from both speakers, obviously, but not in the middle of the room. Whatever the shape, some bass frequencies will cancel most strongly in the middle of the room, so it's almost always a bad mix position. If you are one third of the room length away from the speakers, you'll be off to a good start.
- Needs the right materials. For example, Pro studios use very expensive acoustic panels to control the sound by reducing unwanted reflection from smooth plastered walls. A cheaper alternative is to put carpet on the walls, with the idea of "deadening" the sound. However this isn't always a good idea in a mastering room - since the idea is to get the best possible result for a "normal" listening environment, a certain amount of reflection in the room is important, otherwise you'll be tempted to add too much high frequency to everything. Heavy curtains are probably a better idea for controlling the "liveness" of the room, not least because you can easily change their arrangement and effect on the sound in the room.
All of the factors above affect the way your room sounds, hopefully for the better ! In a later DIY post I'll describe a few simple tests to help you check if your room sounds good.