confused about port area, tuning, length

So the further into building a box I'm getting the more confused I get. With the specs I have from a manufacturer a 1.95 vented box should have a port area of 25sq in and a length of 17.75 for tuning at 40hz.

I was making some adjustments and thought it maybe easier to use a round port since I have 2 3" port kits. I read somewhere that I would not change the length of the port to keep the same tuning and if I use a larger area port I would need to use a longer length to keep said tuning. Yet every calc I use has the 3" port anywhere from 3.88-3.91 in length wit the same tuning. So what am I missing? There area of a 3" port is 28.27"

Should I just build it to the specs I get from the calcs and forget the 17.75" length?


Thanks
Isk

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99' Dakota ex cab
IN PROGRESS: Premier P980BT, Alpine 1.1000, 4.150, 2.150, Oz Audio 380CS, ME12.4, Infinity 652.7i
220amp alt, big 3, Red Top
AutoPage RS-1000 OLED

area of two 3 inch round ports is a TINY 14.14 sq inches of port area...NOT 28.27 sq inches.

where did you get 28.27 sq inches??

Arghhhh. I used diameter instead of radius. Thanks for pointing that out, it shows me right there in winisd too.

I'm still somewhat confused. The specs I was given were for my truck, I went and dug up more info from their website and it says

Box Volume    1.5
Fb            40
F3            34
Round Port    4 x 10.5
Diameter / Length
Square Port   12.56 x 10.5
Area / Length

Using the 4" port and the specs using winisd are pretty close, Fb is off by .92hz or the port length should be .62" longer.

So I should be okay using a single 3" at 5.71 since this is what I get with winisd? Everything else looks like it would work. This is all with a 1.5cf box.

Isk

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99' Dakota ex cab
IN PROGRESS: Premier P980BT, Alpine 1.1000, 4.150, 2.150, Oz Audio 380CS, ME12.4, Infinity 652.7i
220amp alt, big 3, Red Top
AutoPage RS-1000 OLED

https://www.the12volt.com/installbay/forum_posts.asp~TID~89047~PN~1~FID~2~PR~3

Do yourself a favor:  when you start a new thread for a project you have previous threads open on, reference those previous threads (using links) in the first paragraph of your first post.  Members can get a better idea of what you are doing and you'll get more response.

Like your question preceeding this, for example.  Readers will likely say, "3 inch port...how much power?...what sub?...how is it used?...does he know about port noise?"...and so on...

Use the WinISD tools to see about the port noise issue in this case. 

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Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

Yeah, I even thought maybe I should keep this in the thread I had about the enclosure since this seemed to be a separate topic but it's kinda steering in the same direction...

Oz Audio ME12.4
Alpine PDX 1.1000
SQL, hip hop w/ quality

I've been using winisd but I'm having a major problem with it. No matter what I do this sub will not model out well, it won't even take the specs without an error message. I realized that a 3" port is going to have more port noise but even the specs from OZ are 170ft/sec w/ a 4", keep in mind I can get jack to model out.

Part of this whole problem is I have is conflicting specs from Oz. The manual and the tech dept have different values for the t/s parameters. With the specs out of the manual I have a 7db peak at 56hz.

So really I need some help from somebody who HAS ME12's so I can know what they did for modeling.


Isk

-------------
99' Dakota ex cab
IN PROGRESS: Premier P980BT, Alpine 1.1000, 4.150, 2.150, Oz Audio 380CS, ME12.4, Infinity 652.7i
220amp alt, big 3, Red Top
AutoPage RS-1000 OLED