So i notice some guys here need help designing box. well may i suggest you try and give it a shot yourself first? ill help ya
Step 1: Measure your trunk too see what your max dimensions are. Then subtract twice the thickness of the mdf your gonna use (usually 3/4" so subtract 1.5"). This gives you the internal dimensions you need for the upcoming equations. example, your max dimensions are 10" high and 20" wide so so your internal dimensions will be 10-1.5=8.5" and 20-1.5=18.5".
Step 2: Now remind your self of what internal box volume you need, pick a tuning frequency, and pick a port area.
Step 3: Figure out port length using this formula
Lv = Av*1.84*10^8/[Vb*1728*(Fb/0.159)^2] - 0.823*sqrt(Av)
Lv is port length (what your solving for)
Av is your port area (height x width of the port face)
Vb is the internal volume of your box
Fb is the tuning frequency you want
Step 4: subtract half of your port width from your port length(Lv) to get physical Lv
Step 5: now you need to figure out how long your second wall is gonna be, so pick ANY number shorter than pLv. you can do this because your displacements always gonna be the same no matter how deep your box is.
Step 6: now take your physical Lv and subtract the external depth ( the number you decided on in the previous step +1.5" for the mdf. ex: if you picked 12 then your external depth is 13.5) from it. now this will give you the length of the 2nd wall.
Step 7: take this length (step 6) and multiply it by your port width+.75 and then multiply it by your internal height and finally divide by 1728. this gives you the displacement.
example, your 2nd wall length is 20", port width is 4", and height is 10"
so you get this 20*4.75 (since you need to add the thickness of the mdf to the port width)*10 for height/1728= .5497ft^3
Step 8: Now find the first wall length which is internal depth-port width. like in step 6 lets use 12. but this time since its internal depth we do not add the 1.5" for the mdf. example: 12-4=8. 8 is the length of your first wall. now do step 7 with these numbers to find the displacement. example: 8*4.75*10/1728=.2199ft^3
step 9: now find out how much your sub displaces by lookin in your owners manual or the company website. when you find out how much is displaces, well use .21 (15" xxx). so now you add all the displacements.
example: .5497+.2199+.21+(the internal box volume you chose earlier, well use 4 cubes) so the gross volume is 4.9796ft^3. this means your TOTAL box will be 4.9 cubes including port, this way your sub gets its reccomeneded 4 cubes (15" xxx in this example)
Step 10: Now we need to find out how deep we need to make the box in order for the sub to get its 4 cubes. well i know what your thinking...
"we used 12 inches for internal depth earlier in the example, how can it change now?" well like i said, displacement always stays the same so we just chose a number inorder to find the displacement, now we need to figure out how deep we need to make it to actaully fit within the 4 cubic foot reccomeneded volume.
so to do this we need to do the gross volume (step 9)*1728/(HxW).
example: 4.9*1728/(10x20) and we get 42.336. so the internal depth would be 42 inches for a box that is 10 inches high and 20 inches wide, in order for it to be 4.9 cubes total(gross volume).
Step 11: find the real port wall lengths.
for port wall one, its just internal depth-port width. example: 42-4=38" is your first port wall length.
for port wall two its your phyisical port length - (internal depth + 1.5")
example: lets say your pLv is 20 and your internal depth is 10 so you get 20-11.5=8.5". thats the length for your second wall.
and your DONE . now of course all the numbers in this post are screwed up cause i was just makin them up as i was going along and not actually tryin to make a box, so the numbers wont match. either way try it out. good luck!