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i got 2x12" premiers 400 watts rms each
currently in a sealed box.......i heard a slot vented
box would make them sound a lot louder.......
which box would be better??

 
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Ported boxes CAN increase the volume above the port frequency, but at the same time reduces the power handling abilities of the woofer.

In order for this to work, you need a box specifically designed for your exact sub. No off the shelf box is going to work right.

 
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Properly ported would make them louder as a rule of thumb. And sealed generaly sounds better.

 
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^^^ ditto

 
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It is all preference and enclosure...IMO My Subs ported BLOW how they sounded sealed away!!! They are isobaric (clamshell) and it still gets slighty louder then four subs sealed. So basically it some cases ported can sound better and be louder (but a good crossover helps too). I have my subs ported, and I could not imagine subs sounding better sealed.

 
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I think it's also a matter of what kind of sound you're going for. If you like the booming sound of rap tracks you'll probably be better off with a ported or bandpass box. If you listen to rock I think you can get good sound out of a ported box, but you'll loose some of the definition and tight sounding bass that a sealed enclosure would give. As for me, I've got a bandpass and I hate it - waaaay to much boom for most of the stuff I listen to.