help designing a ported subbox

[smokeyand thebandit]

I picked up a couple 15" kicker L7's in a sealed box that measures 13x16.5x34. it seems like this is the bare minimums for the speakers and I would like to build a nice ported box for it and tune it low. I know it's supposed to be between 3 and 6 cubic feet per speaker but I have no idea how to tune with the port length or size. Any help is appreciated

Thanks in advance.

[smokeyand thebandit]

I came up with these measurements... Length 42" x height 17" x depth 24". PORT size 3" wide x 16.25" high x 16.5" long. Looking for 4.5 - 5.0 ft3 per sub and tuning around 35-38 hz.
This is what i would do without any technical support. I emailed kicker the same thing but still waiting for a response.
I am running one rsd 1200.1 @2ohms but considering using 2 rsd 1200.1@ 4ohms.

[smokeyand thebandit]

This is what kicker had to say...John, at 5.0 cubic feet per sub (woofer and port displacement included) you will need a 50 square inch opening (5" X 10") at 15" deep into the box. If you have any further questions let me know. Can you take a short survey?

[LowandLoud]

Seems like a huge opening to me. I am not an installer or even built a ported box before (sealed guy), but cant remember ever seeing a slot port that big. Maybe 2 2.5"x10" slots?

I will be building my box this winter so i am very interested in topics like these.

[capitolj80]

I would personally recalculate the port length for tuning around 30hz. other than that it sounds like a really solid setup for those subs if you have the room for that massive box there's nothing wrong with that size of your port opening. it's virtually the same sized as mine. having a large port area like that simply means that the ports needs to be longer to achieve the same tuning as a more narrow one. the plus side is the reduction is chuffing (port noise). good luck!

[capitolj80]

power wise, i've heard those L7's are pretty hungry, so it'd probably be a good idea to add a secod rsd1200 if you can...not that they wouldn't be respectable on just one.

[ROBSCUSTOMSAPK]

I've built many boxes for these subs. Actually, as a rule of thumb, you normally want about 12-15"^2 of port opening per cube. They really come alive in ported boxes. I have some more specs for you, ill post them tomorrow.

[audiophyle_247]

Per the wonderful online tool at Linear Team (makers of WinISD)

This is the box specs recommended by Kicker

Look at that hump at 45hz, this box will be very loud around that range but wont hit like you'd expect a pair of 15's could. (20hz is 21db quieter than 40hz!!!)
It will also be VERY sloppy, because its accuracy from ~50hz & down will be next to nonexistent. (peak of hump typically represents where accuracy will be the worst)

This is what is considered an "optimum" box by WinISD (perfectly flat response curve)

Note that is 22cuft of box tuned to 20hz That box will be FOUR TIMES as loud at 20hz than the one above it (a 12db difference), enough said. lol

This is what Id consider a happy medium.

That is 10cuft tuned to 28hz. Personally Id tune a little lower, but Id never run this large of a system anyway.
As you tune lower that hump at 40hz drops and the roll off at 30hz gets less steep. Tuning at 28hz smooths out that hump, so the 40~50hz range wont be overbearing and the 20hz range comes up a whole 6db.



As for port area, 50sqin does not sound like nearly enough for two very capable subs. Kicker (as worthless as they are) no longer publishes complete woofer specs, so modeling port velocities and anything gets difficult.
According to this site http://www.carstereo.com/help/Articles.cfm?id=31 you would need almost 100sqin of port area for two subs tuned to 28hz, and up to 120sqin for a 35hz tuned port. I also had to guess the drivers have an Xmax of 16mm (thanks again Kicker ).
That site has never steered me wrong on port sizing, and sizing based on box volume is a no go. Ive had subs in .3cuft boxes that needed ports the same size as the subs themselves to perform their best. There are too many factors which dictate port area requirements to consider a flat average as acceptable.

If you take anything away from this post, it is to stay away from Kicker's recommendations. lol

[smokeyand thebandit]

thanks for the replies and sorry it's taken so long to get back to you guys. so I stuck with the original measurements for the inside of the box to keep it around 10 cubes ended opening 16 1/2 by 3 1/4 for the slot ports which gives me 53.625"^2 per sub. Now i just need to know how long to make the ports to achieve 30 hz. Thanks again

[audiophyle_247]

10cuft net internal air space, and 2 ports 16.5x3.25 tuned to 30hz, each port needs to be 24.5" long.

Dont forget you need to account for the woofer displacement, any internal bracing displacement, AND the port displacement. (airspace inside the ports does not count toward that 10cuft)

If you did a single 16.5"x6.5" port, it would only need to be 20.3" long. Doesnt sound like much of a difference, but you are making a huge reduction in port surface area (a good thing) and saving a little space by using less material inside the box. (.75" x 16.5" x 24" long board =0.18cuft)

[smokeyand thebandit]

So what your saying is one big port would be better than two small ones. I was going to divide the speakers in the center and have them in Separate chambers. Hmmm Decisions. Thanks for the info

[audiophyle_247]

There are pros & cons to doing it either way, but I think the benefit of a single port will out weight the pros of individual chambers & ports.

That 4" of port length difference is 400+in^3 of extra airspace, on top of the space gained by loosing that center dividing wall.
There is also the issue of air turbulence, which will be lower with the single larger port (less surface area inside).