Port questions, I'm confused?

[bkrieger]

Ok so I just got done building my box for 2 level 3 dc audio 12s, and I don't think its gonna sound good from what other people's designs are. So screw that box I'm gonna start fresh . My max dims are 43w x 15h x 17d. , so if my calculations are correct I have got a gross of 5.025 cubes. Now here is where I get confused. I don't know what port area to go with. Some say 12-16 in2 and some say up to 20 in2. I would like this enclosure to be musical but have nice bass response. So let's say I go with 16 in2 per cube on my port volume, that would be 80.4 in2 on my port volume. So my question is do you subtract that port volume from the gross to get your net cubic feet or what? I'm just not getting it all my enclosures I've built in the past sound decent . But I'm seeing people now with 2 12s that got just these massive ports. When I did my calculations for my first box my port was only 13.5 tall by 3in wide and almost 16 deep, but my net was only 3 cubes. I'm just wondering if there's an advantage of going with more airspace then what the manufacture says cause that's what I built my first box to was spec. By the way power is dc 2k with good electrical. What is the proper way to go about this process, I don't wanna scrap another sheet of mdf. Thanks a lot in advance

Here's my first box,

[philrab]

80-85 square inches of port sounds appropriate for a pair of those 12's. The formula I always go by takes into account tuning freq and Xmax, depending on whether you tune for 32-35 hz it'll change a little but you're definitely in the ball park.

The volume of the port (the volume inside the port, and volume taken up by the wood making the port) is all subtracted from the gross to give you the net.

More net airspace "to a point" will increase efficiency of the enclosure, but there's definitely a point of diminishing returns. Personally, I run these kinds of things through WinISD to have a look at the freq response and excursion/frequency any time I'm deviating from recommended airspace, but a little bigger/smaller isn't usually cause for concern. Baby steps man, just don't dive right off the deep end and double something unless you're prepared to possibly damage the sub.

Personally, I'd download Torres box calculator (there's a link for it around here somewhere). That'll definitely lend you a hand doing the math and design work more quickly than crunching numbers manually, especially because every time the port length changes you now have to recalculate the net airspace again, and the change in net effects the tuning freq. Torres just simplifies that whole puzzle.

My one concern (and it may not be an issue) is the port being offset like that MAY lead to one sub unloading slightly differently from the other. May not be an issue, hard to guess at it. Lot of twin sub enclosures people stick the port in the middle (T shaped) to keep the whole enclosure symetrical.

[Tarball]

port in the middle maybe

[bkrieger]

80-85 square inches of port sounds appropriate for a pair of those 12's. The formula I always go by takes into account tuning freq and Xmax, depending on whether you tune for 32-35 hz it'll change a little but you're definitely in the ball park.

The volume of the port (the volume inside the port, and volume taken up by the wood making the port) is all subtracted from the gross to give you the net.

More net airspace "to a point" will increase efficiency of the enclosure, but there's definitely a point of diminishing returns. Personally, I run these kinds of things through WinISD to have a look at the freq response and excursion/frequency any time I'm deviating from recommended airspace, but a little bigger/smaller isn't usually cause for concern. Baby steps man, just don't dive right off the deep end and double something unless you're prepared to possibly damage the sub.

Personally, I'd download Torres box calculator (there's a link for it around here somewhere). That'll definitely lend you a hand doing the math and design work more quickly than crunching numbers manually, especially because every time the port length changes you now have to recalculate the net airspace again, and the change in net effects the tuning freq. Torres just simplifies that whole puzzle.

My one concern (and it may not be an issue) is the port being offset like that MAY lead to one sub unloading slightly differently from the other. May not be an issue, hard to guess at it. Lot of twin sub enclosures people stick the port in the middle (T shaped) to keep the whole enclosure symetrical.

Thanks for the quick response philrab, preciate it. Man I've been been banging my head off the wall trying to figure this out. I got winsd and Torres and what I got from dc website on the enclosure specs is what I came up with on my first box in the pic. But it only has 37 in2 of port area . I just did all my calculations according to dc website. What I'm trying to understand is why I'm seeing people with the same subs having a port that's twice the size? I just wanna make the next box right . Here's what I got from Torres for the first box. My max dims weren't as big cause I was trying to keep it around dc specs. So philrab, are you saying to keep the box around 3 cf net with a port area of 80-85 in2? Or are you saying to go bigger with the enclosure and up my port area? Thanks again for the help

[philrab]

http://www.carstereo.com/help/Articles.cfm?id=21

That link is directly to the match I trust for figuring minimum port diameter (there's also a calculator to save the brain cells, I'm a math nerd). That'll give you the min port diameter for a round port for a single sub. The formula spit out 8" (which I think may be a little bit conservative, could probably go smaller) and doubling the port area comes to 100 square inches of port, which I think is a bit excessive. 80-85" and I think you'll be fine.

Dc calls for 1.25 cubes per sub ported, so I think 2.5 - 3 cubes should be a good starting point. I think part of what's driving that large port number is the fact that 22.5 mm of Xmax means these subs have the ability to move a LOT of air, which means the port has to be larger or you're guaranteed port noise. Other guys may use smaller ports/less area because they are either

A. Fighting space constraints. Larger ports by nature have to be longer to maintain the same tuning freq. Longer and larger ports do not play nice in small areas. Compromise won the day.

B. Using aeroports which allow (debates on this for days) the use of less port area to avoid port noise. Personally, I do slot ports and hit both ends of the port with a 1/4" or 1/2" roundover router bit to smooth the airflow a little.

If you can make a design that's what you're talking about, 3 cubes 80 inches of port, I think that'll be a good starting point though I'd welcome someone else taking a look at it. Never hurts to have a second opinion.

[thefourth]

What amp are you using on the dcs?

[philrab]

What amp are you using on the dcs?

Original post said a DC 2K

Even after running the math twice, 40 square inches of port per sub and 1.25 cubes each just feels weird. That's a LOT of port for DC only recommending a 1.25 cube enclosure.

I hate not having access to all my software when someone asks a question like this.

[bkrieger]

What amp are you using on the dcs?

Original post said a DC 2K

Even after running the math twice, 40 square inches of port per sub and 1.25 cubes each just feels weird. That's a LOT of port for DC only recommending a 1.25 cube enclosure.

I hate not having access to all my software when someone asks a question like this.

I was thinking the exact oppisite, that it wasn't enough port area. I just wanna make sure I got this all figured out before I fire up the table saw again. Space restriction reall isn't an issue, like I said my max dimensions to fit this box are 43w x 20h x 20d. If I need a big enclosure with a lot of port area then that's what I'll do. I'll rerun the numbers again and see what I get in Torres when I get home. Once again thanks for the input and keep the sugestions coming.

[philrab]

Well, look at it from this perspective: I'm mid build on a ported enclosure for a Sundown SA12. 2.15 net, tuned to 33 hz, 3"x8" port (right at 24 square inches, Sundown recommended 28" for 2 cubic feet, I'm cheating and putting a fat roundover inside and outside the port). Now, that's a 19mm Xmax sub rated for 600 rms, vs. your 22.5 mm Xmax and rated to 900 rms, but needing 40 square inches per sub in that kind of apples to oranges comparison kinda makes sense.

Still wish a DC guru would step in and offer their experience.