Fi Q 18" enclosure

[thebiggmann12 14]

hey guys, this is probably a noob question, but i have an 18" fi q in my car with the bp option. i have it hooked up to a hifonics 1610 1600watts rms, i am running about 1200 (i set it with a dmm). its in a 6.3 cubic foot box after displacement, and its tuned to 31 hz. my question is, why is it that when my doors are open it sounds alot louder than with the door shut. opening the windows helps when im driving, but its getting cold and i wasnt sure if there was anything i could do to make it loud with the doors and windows shut. any help would b great. thanx.