Nulls occur at specific positions in a room where direct and reflected sound arrive 180° out of phase at a particular frequency, causing cancellation. In small rooms, bass nulls at the listening position can create 20–30 dB dips at specific frequencies — making those notes essentially inaudible.
DSP cannot fully fix nulls: boosting a frequency at a null position just creates more cancellation (the boosted direct sound reflects and cancels itself even more). The solution is physical: change listening position, reposition speakers, or add bass traps to reduce the reflected energy. Multiple subwoofers also help by distributing bass sources.