General Sub Firing ? Technical

  OK so I've been into car audio for a couple years and of course know that sub firing can make a huge impact on your sound.  I know that in a car you want the box firing towards the back of the trunk usually, or in the back corners, and I know it sounds louder that way, but I don't know why.  I've heard and read a thousand people saying its better that way, but why?  I really want to understand why this is. 

I only have one theory why... I've heard that a bass not takes something like 20 feet to fully unfold, so is the extra distance before the soundwave gets to you what does it? 

In a very basic nutshell, you are correct. 

A 50Hz soundwave is ~22.5 feet long (at 20 degrees C or 72 F.  Sound wavelength in air changes with ambient temperature.)  This means there are 22.5 feet of physical distance between the peaks of any sound wave at this frequency.  So in order to "hear" the peak of the wave, and thus experience the max SPL, you must be exactly 22.5 feet from the radiating surface.  By "bouncing" the lower wavelengths off the back of your trunk before they can get to your ears you increase the effective distance between yourself and the woofer cone. 

Thanks, that helps a lot.  If anyone would like to go into further detail, I really do enjoy getting as technical as I can when learning things, thats just the way I learn.  Thanks for the help.

While were on this subject. Im in the process of building an enclosure for my personal car that has the woofer firing into the rear seat and ported through the rear deck, I havent tried this approach before and was wondering what if any affect this would have on sound. Woofer is MTX 9500 12" recieving 2000 watt's rms. The box is 1.97 cu ft  tuned at 44Hz the peak SPL on the vehicle is 55-56 Hz. Previous box was a monster from hell 5.6 cu ft with a 12x4x10 slot port rear firing at the trunk. Best score new term-lab miced at 145. Any thougts or comments would be greatly appreciated. Yes I am going to  upload pic's when the project is completed. Almost step by step photo's.