4th order port size?????

[themachinist]

Hey guys Im building a 4th order box with two SA-12s and giving them 600w rms each. Im a little confused on the proper port size to use because I thought port size depended on the x-max of the subwoofer. I have seen other 4th order box builds and the port seemed a little small. Can someone tell me what the port size should be because im getting 85 inches min port area for 2 SA-12S. I was thinking of going with a 2:1 ratio and tuning it at 41hz. Let me know what you think, I greatly appreciate it.....thanks

[CleanSierra]

That is a super low tuning for a 4th order. How did you come up with that amount of port area? Is the 85" for both subs, or just one?

85" is for a regular ported box( which is a 4th order btw). A bass reflex box IS a 4th order. I think you're probably referring to a single reflex enclosure though, a 4th order bandpass right? The port area for a bandpass with those subs is around 180". That's a huge number and less can be given in a vehicle cabin with no port noise I'm sure.

[themachinist]

I came up with that port area because I'm trying to run 2 12's in the enclosure. I was thinking of having 2ft3 behind the subs and 4ft3 in front of the subs and tuning that some where in the 40hz range.

[CleanSierra]

Some guys get away with low tuning like that but you could possibly have a real narrow pass band. Have you ever heard 4th order bandpass tuned at 40? Do you know why you wanted 41Hz?

I'd say you can probably get away with 130" and above. You're getting the low end response for the subs from the sealed section, and your high end rolloff comes from your ported section. In order to run such a low tuning, you may fall off to quickly into the higher frequencies. If tuning that low, I'd be sure to have a STRONG midrange to pick up the slack.

[CleanSierra]

And you didn't really answer me how you came up with the port area, you just said "it's because I want to run 2 12s in the enclosure". Have you done a lot of other enclosures, building and designing wise?

[themachinist]

I got my info from the smd forum and people were saying to tune a 4th order banpass boxes from 40-50hz and to use a 2:1 ratio. I also have a few formulas to calculate Dmin like this is one. Im just trying something new because I always build L shape ported boxes or quarter wave t line boxes for all my friends.

I understand now how you got your numbers now...I guess i just am not to sure on what frequency to tune this box at because what I have seen has been real low. Should I go with 50-60hz instead???