Re: two 15" alpine type r's some ported box questions
The best tuning range for subwoofers is dependent upon your goals, your box and the sub you're using. Sub bass is in the <80 Hz region and most enclosures will do well from 50-80 Hz and even higher, although you don't want your subs pulling midbass duty, which is why we cross them over at ~80 Hz.
As far as different sized subs playing different frequencies better, yes and no. A 6" (true) subwoofer will play a 25 Hz note and an 18" sub will play an 80 Hz note. But sound waves are peculiar, in that lower frequency notes are less audible to human ears and thus, must be played at higher sound pressure levels in order for us to hear them. And actually, once you get into the low 20Hz range, you really feel the note more than you hear it. Also, low notes are more difficult to produce in a vehicle because they are very long and a vehicle cabin isn't.
So... a 6" sub can produce 20 Hz notes but in order for those notes to be heard and felt, it needs to be in the right cabinet and it needs a lot of power. Even still, there isn't much cone area so while it is indeed playing the low note, it isn't moving a lot of air and thus, doesn't seem to be playing the low note. Contrast that with an 18" sub and you're moving much more air on the same power, so obviously, it would appear that the 18 is doing better. But technically, it isn't playing anything the small sub isn't... it's just playing the same notes louder.
As for mixing sub sizes, you can do that if you need upper sub bass reinforcement because of a narrow bandwidth box like a 4th or 6th order but on balance, it isn't necessary or very effectual for the space and expense.
And lastly, don't fall for the myth that big subs aren't musical. I'm running two monster 15" subs in a transmission line in my truck and the upper sub bass is tight, fast and accurate. And, I have no smaller subs in the system. Sound quality has to do with the box, not the amount of piston area each driver displaces.
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