box for fosgate 12 subs

A ported enclosure does not handle more power than a sealed enclosure. A ported enclosure uses the rear wave of the subwoofer's movement to further increase the output of the enclosure (by a theoretical 3dB). Technically both enclosures will handle the same power if designed correctly but a ported enclosure will not handle nearly as much power below the tuning frequency of the port. Below the tuning frequency the woofer will oscillate violently (unload) and become unstable. Low bass drops off sharply below the port tuning frequency. Basically the woofer is just flapping around. This is why ported enclosures are more complicated to design and less tolerate of fabrication errors. The woofer, enclosure and port all need to work in harmony together or you'll have problems. And no, ported enclosures do not see higher pressures. Free air subwoofers generally take less power because they are not supported by a true enclosure. There's much less air spring in free air so you can't feed the woofer as much power. That's why they take less power. Not because they're more efficient or a better design. They just can't handle the higher power. And while I'm shooting down these theories let me go for the aperiodic. They do not provide lower bass. Just the opposite is true which is why you have to use an equalizer to restore the low bass. That's why Autosound 2000 supplied woofer specific active equalization modules with their MemBrain mats. The lack of low bass in an un-equalized aperiodic system is due to the low Q (highly damped) nature of the design.

i appreciate all of the advice and opinions that i have got from everybody, but i still don't have an answer to my question. does everyone else except me build box's bigger than recommended? i have never done it before, and i have built box's for several years for several different people for several different type subs. this one type (fosgate) is the only ones that have not sounded like i thought they should.  '85 Toy

I doubt that many installers build enclosures larger than the recommended size. That's why it's the recommended size. Making them larger "just because" is kinda goofy. That's not to say you can't but just doing it as a matter of habit doesn't make much sense. Have you tried the enclosure in a different vehicle? What about only hooking up one woofer and see how that sounds?

i haven't tried either of these yet. the amp he's using is an old zeus by hifonics. he had it hooked up in the same car running 2 mtx 8000 15's, and it sounded great. only problem was he kept blowing the subs. that is why he went to a "better" sub that is supposed to handle more power. i tried to tell him that 2 12's probably won't sound like 2 15's, even if the 12's are a "better" sub, simply due to the pressure difference that the 15's are capable of giving. or that has been my experience through the years. maybe he is just expecting too much out of the 12's?'85 Toy

If you think that based on your experience that there is a problem then there probably is. Those are good woofers. Speaking of Rockford woofers I just got off the phone with their technical support. I was asking for specs on an older woofer. I asked for the recommended enclosure size and then the Theile/Small parameters. The tech told me the recommended enclosure size and then said I wouldn't need the T/S parameters because this woofer is for sealed only. He said that you only need T/S parameters for designing vented boxes. Wow, and this guy is in tech for RF! That's enclosure design 101.

I don't believe rockford ever made a dual 4 ohm POWER HX2 unless you mean the new T1's or T2'S but maybe yet again i'm wrong I do that often... I had my 2 12" power hx2's in a sealed box about 1-1.25 facing forward with a bd1500 on each one at a 4 ohm load and rocked a whopping 135 db (VERY EXTREAMLY HEAVY SARCASIM) I was totally dissappointed in the enclosure... so i'm selling the subs and upgrading to this years T1'S and going ported!

i am starting to believe that this sub is very overpriced and overrated based on what i have heard so far. i'm just glad that i didn't buy them myself. i would not be happy with the sound that they put out.'85 Toy