I have 2 9515 DD's and I am building a new box. I need 75 square inches of port area. The square port I was going to use is 5 x 15 and is suppose to be 13.75 long. Do I measure the port from the front of the wood or the back? Also how far should my port be from the back of the box? I was thinking I might have to turn the port to get the length I need. The box inside measurement is 16" so the only would give me 2.25 inches from the back of the wood and 3.75 from the front.

I have 2 9515 DD's and I am building a new box. I need 75 square inches of port area. The square port I was going to use is 5 x 15 and is suppose to be 13.75 long. Do I measure the port from the front of the wood or the back? Also how far should my port be from the back of the box? I was thinking I might have to turn the port to get the length I need. The box inside measurement is 16" so the only would give me 2.25 inches from the back of the wood and 3.75 from the front.

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Just 75 sq inches for 2 DD 9515s? I would do much more than that if you could. Try 200

But yea, You measure the total length of the port, including the front face. You will probably need to turn the port, especially if you add all that port area.. Maintain the overall width throughout the whole port.

What will this be in? If you need to, make a 200 sq inch port for competing and make a plug that brings it down to 125 or so and a lower freq for daily.

The 75 sq in is per sub and thats what DD says on the website. So your saying 100 sq in port per sub and make a plug for each port about 37.5? I also thought about going with 3 round 6" or 8 " ports and plug one for daily.

Yes, 100 per sub. It really isn't necessary to do 2 different ports, unless length is a problem. It is much easier to just do one really big port that you can stick your head in

If you do the 6" ports, you will need 8 of them to attain 200 sq inches of port. You could set them up to plug 4 for daily and run with 100 sq inches daily at a lower freq, and unplug them for competing and raise the freq to 50ish. That's what my box will have for my 15. Just 4, not 8 though.

Just remember you will need more room for round ports than you will for a slot port. Also, you'll want to keep the end of the port as far away from the back of the box as the port is wide or is in diameter.