Originally Posted by Maniac
I have 2 sundown x18s could i just use the ported box design sundown provided an just build an sealed part on top to make it 4th order (i think the ported was tuned at 35 i want to be at 28 on ported part) really hoping to learn more about 4ths from you guys
no.. One thing about 4th orders is they roll off very different than ported.
ported boxes have a 4th order roll off below tuning. So below tuning they roll off at 24 db/octave. So if you tune at 38hz, it'll be 24db down at 19hz, not including your cabin gain. Because of that, if you want lows, you tune near the lowest frequency you want to be able to play, or at least in the ballpark since cabin gain can help you. On the top end above tuning, they act more or less like a sealed box, minus a bit of help within 1/2 octave of the ports tuning.
4th orders also roll off at 24db/octave total, but it's 12db per octave below tuning and 12db per octave above. So if you tune at 28hz, you'd be able to play VERY low, since it rolls off half as slow as ported.. However you'd have almost no range on the upper bass notes, since it's rolling off up top and you generally have less cabin gain as you go higher in frequency.
Most 4th orders are tuned closer to 45-50hz for this reason. The slow roll off on the bottom end means they can still play 20-25hz at respectable levels, which the higher tuning still allows 50-60hz bass.
How much space do you have, in inches and what frequency range are you trying to get the subs to play?
That may have been a bit long, so long story short you'll need to get a new box built realistically.