Room modes occur when a sound wave's wavelength equals a room dimension (or simple fractions thereof). In a 12×10×8 ft room, the first axial mode (length) is at ~47 Hz. At this frequency, the room resonates — creating areas where bass is extremely loud (near walls) and areas where it nearly disappears (room center).
Every rectangular room has three sets of axial modes (length, width, height), plus tangential and oblique modes. In small rooms, modes are spaced far apart (widely separated frequencies), making individual modes audibly problematic. In large rooms, modes are densely packed and blend into a more even response.
Fix: bass traps in corners reduce mode strength. Multiple subwoofers placed asymmetrically can smooth mode response. Changing listening position by even 12 inches can move you out of a null.