Calculating Port Size, Confused!!!

[wof131s 10]

Lets say you have a 6.0 cu.ft. box and you have woofer displacement of .38 cu.ft. Now you have a net volume of 5.62 cu.ft. before port displacement. How do you know how to calculate the port length if you dont know the exact box size since port displacement have to be considered. I know how to calculate port displacement but i am not sure about calulating port length with a changing box volume as the port size vary. Can someone help me understand how this is done please. I know to some of you this is a dumb question but am just looking for some help and wanting to understand how its done and not mess up any more designs. Looked through the forum but couldnt find the info i needed. Thanks in advance for any help.

[Kanaworm$ 292]

Take a look at this ... http://www.carstereo.com/help/Articles.cfm?id=31

[wof131s 10]

Take a look at this ... http://www.carstereo.com/help/Articles.cfm?id=31

So where it says enter enclosure volume (cu.ft), is this the total volume minus the subwoofer displacement only (net volume)? If so then the calculator take into account the port displacement and gives you the total volume of the enclosure after all displacement?

[///M5 2,833]

Do it the other way around

1) Figure out the NET space you are targeting.

2) Figure out the GROSS space your port will take (including the wood)

3) Figure out the space your bracing will take

4) Look up the volume that your sub will displace

5) Add all four together and now you know what your box needs to be internally.

[wof131s 10]

Can you give me an example?

[KU40 173]

If you want 4 cubes after all displacements, plug in 4 cubes for the enclosure volume. Then plug in your port info and calculate port displacement with the equated length. Add that to your net volume. Adjust either port size or net volume in the equation until the work out.

You want 4 cubes. Plug that into the equation.

You want a 40 sq. inch port and a tuning of 32 hz. Plug those in. It gives you port length of 21".

Calculate port displacement. With a 10" x 4" slot port, you'd take 10" x 4.75 (adding in port wall) x 21" for a port displacement of 0.57 cubes.

4 + 0.57 + 0.38 (sub displacement) is only 4.95 cubes. So increase the net volume in the equation to, say 4.5 cubes. Recalculate.

4.5 cube box w. 40 sq. inches tuned to 32 hz gives length of 18". Recalculate displacement.

10 x 4.75 x 18 = 0.49 cubes.

4.5 + 0.49 + 0.38 = 5.37 cubes total. Getting closer.

Rinse and repeat until you get the correct volume.

[antonmiller 8]

i did this right up for a guy on youtube one time. by no means is it perfect. but this is how i design an enclosure for some one. i should also add that this was a sub up port back enclosure.

"if your doing a box that is 5 cubes you take 16 x 5 which equals your port area. then you figure out how tall you want it to be. then you figure out the port height. i would say your going to want the box to be 17" tall then you minus the double baffle and the bottom plate. so 17-2.25 which equals 14.75 then you divide that by your port area. so (16 x 5)/14.75= 5.423728 so lets say your port will be 5.5 by 14.75. (this is for a sub up port back driver side box) then you type that into this calculator

http://www.carstereo.com/help/Articles.cfm?id=31

(bottom half)

so you type in 5 cubes square port, what you want your tuning to be say like 35 hz, make sure to check the slot port yes thing. then it gives you the number.

so your port will be 14.75 tall by 5.5" wide by 25.5 long. ok you have your tuning done.

now you just add your total port displacment with your sub displacment and then add the interior volume you want.

so... 5+.15(sub displacement)+ [(14.75x6.25x25.5)/1728)]=6.50 cubes total interior. (rounded)

you do the 6.25 wide because you need to incorperate the piece of wood that runs along side of it.

so you then take your 6.5 x 1728 (1ft^3=1728^3in) and you get 11232

then divide by 14.75 because thats how tall your box will be on the inside,

so you get 761.5 ish then you just decide how wide you want it or how deep you want it. dont forget this is internal volume. you need to accommodat the .75 wall thickness on each side of the box to fit where you want it to fit. so if you want the box to be 42 total outside then you minus 1.5 from that and then divide by that number.

so say thats what you want the box to be width wise, divide 761.49/40.5=18.8

so your total interior dims are 14.75 tall by 40.5 wide by 18.8 deep.

check all your math and times them numbers together and see how you came out. so 14.75x40.5x18.8=11230.65 then divide by 1728 and you get 6.4992187

very close to what you wanted! this is a good thing!

now if you think you need bracing just go and get some 1"dia round wooden dowels and put one or two in there. also put corner pieces in the corners. dont need it but it helps. yes these do remove some volume. but not enough to change the tuning too much.

now you build and enjoy the sweet sweet sound and poundage!"

also i posted it here first.

[wof131s 10]

Ok I think i got it now. Thank you guys for all your help and input. Hopefully i would messup any more of my designs. Thanks again.

[wof131s 10]

Ive got my dimension sorted out, wasnt as hard as I was making it to be Thanks guys. Another question, I was going to uses 3/4" MDF plywood but it just so heavy with the size enclosure and woofers that I have. Does anyone know of another option of a lighter plywood that is as effective as the MDF. I was wanting to get the materials so that i can get this done this weekend. Thanks in advance.

[///M5 2,833]

Baltic Birch VOID free or Trupan Light....but your check book won't like you.

[wof131s 10]

Where can i find either of these? LOWE'S maybe?

[wof131s 10]

Found a store that sells it for $23.00. Thanks

[wof131s 10]

Here is a link that shows a sketch of the enclosure (without the brace), followed by the enclosure information. Just needing some feedback. Yes? No? Modifications? These are the max dimensions that i can fit. Thanks in advance for the response.

http://s883.photobuc...re.jpg#comments

Wedge Enclosure Dimensions (Exterior)

Height: 26”

Width: 43”

Top Depth: 8”

Bottom Depth: 20”

Total Volume: 7.576 cb.ft. (No Displacement)

Port Dimension (32 Hz)

Internal: 24.5”H X 3.25”W by 21.0”L

External: 24.5”H X 4.0”W by 21.0”L

Port Displacement = 1.191 cu.ft.

Sub Woofer Displacement = 0.38 cu.ft. (2 15” Fi Qs)

A 6 cu.ft. Enclosure is what’s desired after all displacement, so we have 6 + 0.38 + 1.191 = 7.571 cb.ft.

I plan on using one strip of plywood as if making chambers for each woofer with large 6” holes to maintain strength but decrease bracing displacement. Am using the Baltic Birch and its pretty stiff. What do you think about the design? Will it be sufficient?

Edited November 25, 2009 by wof131s

[wof131s 10]

Any input?

[wof131s 10]

Had some miss calculations, will post recalculation later.