Basically. Every driver(speaker of any kind) has a certain set of parameters called T/S specs. These are made up of numbers based on the type of motor, the power of the motor, the total displacement of the cone, and lots of the other things. There are lots of drivers out there with all kinds of specs. Some are designed with hard suspensions so they can run Infinite baffle(think trunk or doors), Some are designed to go into small enclosures. For the most part. MIDBASS in kicks doesnt usually work because space makes bass. Imagine you take a 6.5 speaker and put it into a super small box, say like .2cuft and then do a freq sweep on it. Itll probably go down to 500hz and then drop DRASTICALLY. now if you were to keep adding space and keep doing frequency sweeps. You'd see your low end get more and more output. .3cuft. prob go down to 400hz, .5 cuft, go down to 200hz. And so on and so forth.
There is basically no space in a kick unless heavy modification has been done and it has been vented to the outside and then its considered Infinite baffle(like cutting big holes through your cars metal). If a kickpanel has been vented to the outside then you can get output but other problems tend to arise like resonances of the metal and what have you. The Big drivers you usually see in a kick arent actually playing low frequencies, they are simply big midranges. A bigger driver has a lower beaming but gets louder on the same power. For instance if you put 100 watts on a 4in speaker and itll be loud. Put 100 watts on a 6.5 and itll be really loud. So its not always about how big your enclosure is if you dont need your speaker to play low. In fact a good example is tweeters. Tweeters need such a small enclosure(tiny tiny tiny) that their enclosure is actually built into the tweeter itself.
You do tend to get a lower qtc in a sealed box(this is good) but it takes more power to get louder so there is a trade off.
As for doing speakers in doors. The spl and the Sq guys usually do it completely different. The spl guys for the most part reinforce their door card(the part you take off) and then deaden it to the nines and then fiberglass the FRONT of the enclosure onto them. Then they put the door card back on and their drivers fire directly into the actual door.
Like that. This is a great and cheap way to get alot of drivers on a door and get loud easily. however, These drivers are nowhere near optimized or properly dampened but they do get loud and will keep up with soundstages.
The sq guys either do one of two things. They mount their drivers directly to the door and then seal/deaden the door, then they modify their door cards to look good around the speaker mounted to the actual door. like so.
OR they do some crazy in door sealed enclosure like i plan too. like so. Not mine btw VVV
