Can someone explain how to find the square inch per cubic feet when designing port?

[osiris10012]

I have been seeing a lot of suggestions for ports based around 15-16 square inch's per cubic foot when designing a port. I am confused on this subject. I have been looking but just can't figure it out. I am designing a box that has a gross volume of 1.24 cu. ft. The port I am thinking about using is an L design and measures 2"Wx5.25"Hx17"L which should give me the 35 Hz I am trying to tune it to according to the calculator I used. Now when I do the math for how many sq. in. it consists I get 178.5 sq in. Am I doing something wrong? Sorry just confused and trying to make sure I am designing this box correctly. Also it is for a sundown audio E10V2 and its displacement according to sundown's site is .1 displacement. Also if it would help I can attach a picture of the enclosure I designed.

[Souldrop]

You're calculating the volume of the port. You need the face area of the port which in this case is 2" x 5.25" = 10.5" sq.in of port.

Now you need to subtract the volume of the port and displacement of the sub from the volume of the box. so 1.24 cu. ft - ~0.10 cu ft - 0.1 cu ft= net box volume ~1.04 cu ft.

Then Port area/Net Box Volume = sq in port per cu ft.

[Joe X]

Yes you multiplied the port area by the length of the port, the result is no longer an area but a volume. 178.5 cubic inches is the airspace inside the port.

Also consider no less than 16 square inches of port area (width x height) for that sub.

BTW sundown now offers ready made cutsheets for their subs in their website.

[osiris10012]

awesome info. I appreciate it. I saw the build sheets on sundowns site but I am trying to make a box that will fit under the rear seat of my truck. So are you suggesting I make the port even larger? The reason I decided on the 2" wide is because any larger is going to drop the net volume below 1 cu. Ft. And I figured that would be to small for the sub and I can't make the box any larger. What would you guys suggest doing? The overall dimensions I am working with are 6.75Hx53Wx12D. Then I removed a 19Wx3H section out of the middle for the transmission tunnel.

[Souldrop]

If you just make it 1" wider you will only lose ~90 cu in. of box volume which isn't all that much. You will still be fairly close to a cubic foot after displacement.

Added: You will only be losing ~.052 cu ft.

[Junior Russell]

here dude i found this a little bit ago when i was looking for how to calculate weird boxes

http://www.the12volt.com/caraudio/boxcalcs.asp#por

[osiris10012]

If you just make it 1" wider you will only lose ~90 cu in. of box volume which isn't all that much. You will still be fairly close to a cubic foot after displacement.

Added: You will only be losing ~.052 cu ft.

After looking at my available dimensions I realized I can actually go 2" wider on each side of the box if I do a forward firing port. I then got to looking at the sundown designed box for the e10 and saw the port area is 18.75". If I go with a 3.5"x 5.25" port it will put me at 18.38". So with the gained volume from the added width I should be at right around gross total of 1.38 cu. ft. If I do the 3.5" port in an L port tuned to 35 Hz (3.5"x5.25x25.75" after added length for bend, 24" before the added length) and I did my calculations correctly the displacement would be .27 cu. ft. So that with the sub displacement would put me at a net volume of 1.01 cu ft. Do you all think it would be worth squeezing that extra couple inches and run the 3.5"x5.25" port or stick with my previous 1.24 cu ft design and go with the 3"x5.25" port.

here dude i found this a little bit ago when i was looking for how to calculate weird boxes

http://www.the12volt.com/caraudio/boxcalcs.asp#por

thanks thats actually one of the calculators I have been using. very helpful tool

[Souldrop]

I really can't weigh in. I've never personally dealt with the drivers enough to tell you to go one way or another. Maybe someone with more experience can chime in.

[osiris10012]

Its all good I am just trying to figure out if it would be beneficial to up the port area to 18.38" with a 3.5" port in the larger box compared to the 15.75" with a 3" port in the smaller box. I am very new to porting. I just keep seeing the 15-16" of port area everywhere. I really appreciate your help it has got me this much further. I originally didn't even know how important port area is.

[Souldrop]

I personally would go with more port area. If you're really unhappy it's a whole lot easier to experiment by covering a hole already there and filling up box space with blocks as opposed to making a box bigger