Bass traps target room modes — resonant frequencies caused by a room's dimensions. In small rooms, modes create 10–20 dB peaks and nulls in the bass region, making accurate mixing impossible. Bass traps (4+ inch thick rigid fiberglass or rockwool panels) placed in corners reduce mode strength by converting acoustic energy to heat.
Foam 'bass traps' are mostly ineffective — they lack the mass and thickness to absorb low frequencies. Proper bass traps are made of Owens Corning 703, Rockwool, or similar dense materials. Two traps in the front corners of a small room dramatically improve bass accuracy.