PCRECORDING.COM - - Acoustics, pg. 2
Low frequency signals have very long waves and are omnidirectional. (That's why you can put your subwoofer anywhere in a room). As a result, depending on the size of your room, the wave may not be fully developed when it hits the first wall. The potential exists for a reflected wave to cancel out or boost a portion of a subsequent wave as pass each other.
As the frequency range goes up the way the sound waves act changes. Mid to high range frequencies have very short wavelengths. They act in direct lines much like a ray of light. As a result by the time the wave hits a reflecting surface it is fully developed. The problem is that these fully developed waves can reflect back to your ears and mix in with the sound coming from your speakers. This is known as "early reflections." Any early reflection that returns to your ear within about 20 milliseconds cannot be distinguished from the sound coming from the speaker. The problem is easy to understand - - it will affect your tonality as well as your stereo imaging perception. The early reflections may induce boosts and dips in particular areas of the frequency range in a process called comb-filtering.
What does this mean you ask? Here are the minimum things you should do to get the best performance out of your space.
Next, measure your room dimensions to figure out what what standing waves might occur. This will give you the chance to adjust for them during your mix.
Do not forget that sound travels in all directions. You will need to consider the vertical dimensions of your room as well. In addition, anything in your room will have an acoustic affect, be it a desk, file cabinet or the ubiquitous old college-days stuffed couch. Any temporary changes you make to the room will have an impact. For instance, mixing with your door open will have a different result than if you mixed with it closed.
For some real world, inexpensive fixes, I suggest you visit the page at Insync.Sweetwater for ideas other musicians have used to improve the acoustics of their rooms.
For a more technical page with graphics (and sorry, a bit of a commercial message) check out Listen to the Music.