From what I have read here the port area per square foot rule is just a myth, port area has nothing to do with the size of the box. I tried searching more and all I could find was that port area is determined by the parameters of the sub, tuning, and amount of power. So I am curious what formula is used to actually calculate how much port area you need? 

 

I am currently designing a new enclosure for my 15" Fi SP4 which is run off an AQ3500. My old box was designed to fit in a trunk but now I have a Tahoe so size is not as big of a restriction for the new box. It was 3.83cuFt @32.25Hz (after displacement). I like the low end but actually would like to try tuning lower. I want to move some air too. I can probably safely say that my port area was far too small (only 46sq in). It was hard to get more port area because then the tuning would go up and I didnt have enough space to work with, but now I do. 

 

My new design:

Is this too much port? Also is the box size optimal or too big for being on an AQ3500? The width of 24" works good to fit the box in my third row of seats and I only need to remove one rear seat as opposed to both (just in case). It takes a lot of port length to tune that low with that much port area and I wanted to do sub up port back, but I don't know if the loading of the subwoofer doesn't work because it is too close to the port? (See the below sketch) Also would the rear hatch have the effect of a longer port in this setup? And thus with the hatch open or closed would slightly change the tuning? I tried to leave room between the rear of the box and the hatch so that the port is not restricted. 

 

 

I'm also debating on how to run the speaker wire into the box, last time the port was not long and close enough to the amp where I just ran it through the port so that the box was 100% airtight besides the port. The black Photoshop add in is where I was thinking of possibly having some kind of bolt that wires could attach to and keep the box as airtight as I can while still being able to take the box out of the vehicle if need-be. I am not an expert at box designing, so any input, info, tips, or advice would be appreciated. I would like to at least attempt to design this correctly to achieve decent results. 

dv= 39.37 (fb x Vd) ^1/2

 

dv = minimum diameter of vent in inches

fb = tuning frequency in Hz

Vd = cone displacement volume in cubic meters (sd x xmax in meters)

 

NOTE xmax is not the parameter most manufactures list.  Most manufactures list xmax at 70% BL.  Xmax is defined by Small as coil overhang.  For a 30 mm sub overhang will be in the 18-    22mm overhang range.

Ok thanks! I thought I might have seen that formula before somewhere on these forums but it didn't have all of the units labeled so it was impossible to make sense of. 

 

15" SP4 xmax spec is 33mm so I will use 22mm overhang to estimate

sd is 810cm^2

 

I ended up with 65.7528 sq inch minimum area for the new box

and my old box should have had at least 69.9613 sq in (I could tell that it was too small even without this)

 

That is interesting how the cubic meters gets converted to square inches, 39.37 must be a magic number

 

So this is the minimum that you should use, is there a maximum? I want to move some air, but I suppose that if the port is too large then the port velocity will be too low? Now I probably need a happy medium because 90 sq in of port might be too much (maybe?). Learning new things about car audio! Its interesting to have an understanding of how this stuff works. 

 

I can probably reduce the port area and length a bit now, but I am curious with the way it is in that diagram, would the sub be utilizing all of the airspace in the box? Or would it be unloading right into the port? The placement of the sub is probably another key factor in a good box. In theory should it be as far away from the port as possible? In the center of the open area? Probably not right next to the port I would assume