Standing waves

Each frequency put out by your speaker has a different wavelength determined by the speed of sound in air. A wave reflecting back and forth between two parallel walls will reinforce itself if the nodes of the wave coincide with the room boundaries. Thus your room selects a set of frequencies coming from your speaker to reinforce. If you sit at the PEAK of a reinforced standing wave (also called a mode) that frequency will appear louder. If you sit at a null of the standing wave, the frequency will appear to be sucked out.

So EVERYONE listening inside rooms with opposing parallel walls has a standing wave problem

Amwarwick: Check a thread entitled "Near field listening and speaker placement" In the various posts of this thread are references to several useful websites which address exactly the question you have asked