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I been debating lately on what size my next subwoofers will be. How much louder would a 15" sub compare to a 12" sub (same subwoofers)...

 
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You also have to take into account the amount of power each sub will receive...but, if each sub receives it's maximum nominal power, the 15" should be able to move quite a bit more air, and therefore be quite a bit louder.

 
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yea same wattage..both in eclosures made to specs. I've heard people say 2 12's of the same kind would sound close to 1 15". is this true?

 
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I'd say 2 12's would be louder than a 15. I mean, the total amount of air moved by two 12's seems like it would be more than 1 15. I would also say that, if you can either have one 12 or one 15, I'd probably go with the 12, because you'll make more efficient use of your power. If the RMS wattage from your amp is closer to the max RMS wattage for the 12, then it will probably sound better than the 15.

 
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the 15" will hit the lower notes and have a larger coil/magnet than the 12". but if were talking 2 12" and 1 15", the 12's will be much louder overall and have a better balance thruout the entire car system.
conclusion:
-2 12's hit more smoother
-1 15" hits lower notes with better thumping bass

 
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agreed with above post, the 15 will hit some of those lower notes easier than the 12. I was faced with this delima as well and opted for the single 15. I like the way it hits those low notes the ones u can feel not as much hear but also the ones u can hear too