Freq tuning for spl

i am desging a new box for my 18" hardkore MA Audio sub..i drive a 99 si civic sedan...i was wonder what port freq would be best for spl...i am going to be runing 8,800 RMS @ 1 ohm to the sub...i believe i need 5 cu ft net for the encolsure it'self.i would like to find the happy medium between spl and daily listening...i was wondering if u guys could help me out

There won't be any happy medium with that much power...are you kidding?  ;)  You either tune a box for SPL or you tune it to make the lowest Hz tones louder, which is a SQ function.  You compete?  If that's not your primary objective, tune for SQ by choosing a lower tuning frequency that results in a gradually sloping response curve without any gigantic peaks in it.  And by all means, make sure the port opening is large enough that you don't get extraneous noise at high SPL levels.

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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.

ok steve.. understandable...i sent a few emails back and forth to ma tech support on box size for my sub this is what they said... I would suggest 6ft3 total with a port that is 12 x 4 x 12 this tunes around
40Hz for the daily use with a extra peak around 50HZ for the SPL side... do u think this would be good?

I don't know.  I looked at the MA audio site to look for an 18" sub that would take 8,800 watts and all I found in 18's was the MA1800XL, 800 watts RMS.  What 18" sub are you using and where can I see the specs?  BTW, I noticed they recommend tuning to 40 Hz on all their subs...

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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.

i am runing the hardkore series..their max power is 10,000 qith quad 1.5 ohm voice coils...i never found any specs on the MA site.