What type of sub woofers should i get in order to sound "outside"?

i mean i want to park my car, open the trunk and sound really loud; i do not want to be loud inside my car! ... i know that i need few drivers and mid bass range but the sub woofers are my concern, because i have a pair of Alpine type R 15" and sounds loud inside but once you open the trunk it basically turns in mute... box is tuned @40hz