I own a 2005 dodge quad cab. i had a fiberglass box built for 1 sub(only .55 cubic feet). my memphis mcd500 and polk 10" svc sub are only using 250 rms at 4 ohms (it sounds very nice). i would like it louder so i purchased a L5 10"(450rms @ 2ohm) in a sealed box it sounds a bit louder but my wife doesn't like the fact that i does not fit under the rear seat. I am thinking about buying a box that holds 2 10"subs under the rear seat and 2 memphis pr10s. i will either buy a wood box(1.4 cubic ft.) or a MTX thunderform(1.5 cubic ft.). Has any one heard either of these two setups. also i have read in other posts that going from one sub to two subs will only give 3 decibels more. if this is true is it really worth the extra expense?
It will yield more than +3 db for a few reasons. The +3 db is just for the doubling of cone area. You also have to weigh in with the fact that you would be giving the second sub power; if each sub is given the amount of power the original lone sub had, there is another +3 db due to the doubling of power. Then there is the cabin gain increase (unknown quantity) that will take place due to factors 1 and 2.
Then, of course, there is the added decibels due to voice volume when everybody has to shout to be heard over the music... ;)
I'll leave your first question open as I've not experienced the Thunderform. But from what I've heard on this forum from respected people it is a good enclosure.
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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.