Room modes are standing waves where sound pressure varies dramatically with position. At a mode frequency, one spot in the room might be 15 dB louder than another spot just 3 feet away. In small rooms, modes are spaced widely apart (e.g., 47 Hz, 63 Hz, 85 Hz), creating an uneven bass response that makes mixing impossible without treatment.
The axial modes (between parallel walls) are the strongest. Tangential modes (involving four surfaces) and oblique modes (all six surfaces) are progressively weaker. Bass traps in corners, multiple subwoofers, and careful listening position placement are the primary solutions.