Is this port length correct?

I'm getting ready to build my sub boxes and was figuring out the port length for a slot ported box for a 15" Soundsplinter rl-p.  I was going with 3 ft^3 tuned @28 hz.  It was suggested (by the manufacturer) to go with 35 in^2 of port area so i decided to use a 2.5" x 14" port opening.  Now I used the formula pi*(d^2)/4=35 and solved that to get a round port diameter of 6.676 and I entered that and the other specs. into the port length calculator on this site which gave me a port length of about 35".  That seems like a long *** port to me but I think it's correct.  I double checked with the port calculator on carstereo.com or something like that which gave me about 34".  I just need a little reassurance...

Long port issues is a topic I touched on when I wrote the WinISD guide.  I would look at your actual power applied to this sub and determine if 35 sq in of opening is necessary to stay below 10% Mach.  A reduction in opening will shrink that length, and you know that the total port structure has to be considered as a separate volume from the 3 cu ft you need for the driver.  Do yourself a favor and run this through the WinISD program.

Whenever I see a port length that long it clues me that the driver might be better off in a sealed box, or the box volume may not be right, or the port opening area could shrink smaller without excessive port noise.  35" of 2.5 X 14 plus the thickness of an additional port divider baffle adds up to a lot of displacement that has to be added to the total box volume.

Also, when looking at this in WinISD you'll note that with extra long port lengths, the difference in tuning freq with the port at an inch or two shorter or longer in length is barely negligible.  It is with the short length ports that an inch of difference makes an appreciable difference in Fb.