View Full Version : Port Length Forumla Help

12-07-2006, 10:16 AM
Can someone post an example of how to figure out port length? Please show how to figure out the equation in depth solving the equation.

Say a enclosure that is 3 Ft^3 tuned to 33 Hz with 39 In^2 port area.

Below is the formula I am using but ended up with answers that I know are incorrect. I thought I had it but then I read that the vb in the formula is for the boxes net volume in cubic inches not cubic feet and now I am getting odd answers. It has been a number of years since I have even figured out an algebra equation, I am getting old.

Lv = Av*1.84*10^8/[vb*1728*(Fb/0.159)^2] - 0.823*sqrt(Av)

12-07-2006, 10:19 AM
i need the width and height of port.

12-07-2006, 10:24 AM
i need the width and height of port.

3 x 13 slot

12-07-2006, 10:25 AM
give me a minute.

12-07-2006, 10:29 AM

click square port and slot port, enter cu.ft., desired tuning, width and height of port. click calculate and it gives you the length.

by the way what are the dimensions of the box?

12-07-2006, 10:31 AM
i got 25.52 inches for a port that is 3 inches wide by 13 inches high. if it is too long for the box try making it 2 inches wide instead

12-07-2006, 10:42 AM
Thanks for taking some time to help! It is going to be an l-port box. I read up on how to measure port length and I am okay with that. I just cant seem to figure out the port length formula correctly and I need to see it solved in each step of the equation (long hand is what we use to call it back when I had math classes) and I know about that online calculator, that is what I am putting the formula up against and on how I know I am messing up.

w x h x d
30 x 14.5 x 19.5

12-07-2006, 11:14 AM
ok so the enclosure is 3 cu ft. and you want it tuned to 33 hx. correct? the port will have to be 13x2x17.24 to be tuned to 33 hz

12-07-2006, 11:24 AM

step 1: click on this link http://www.carstereo.com/help/Articles.cfm?id=31

step 2: scroll down to where it says "calculate vent length"

step 3: where it says "select your port type" click square

step 4: where it asks " is this a slot port" click yes

step 5: where it says qty of ports click how many ports there will be, in your case 1

step 6: next to where it says "enter enclosure volume" type in 3

step 7: next to where it says "enter desired tuning frequency" type in 33

step 8: where it says square vent enter the height and width of the port (note that the port width will have to be 2 inches in order for the box to be tuned to 33 hz, if you type in the 3 inches which is what you planned it will give you a length measurement that is too long for your box.

step 9: results: you should have gotten 17.24 inches

hope that helped

12-07-2006, 12:40 PM
Not really, but thanks for taking the time to help out. This is something I already know how to do, I need someone to solve for the port length using the actual formula and show it in a step by step process on how they derived the answer.

I donít get why you say that a 3 inch wide port wonít fitÖthe l-port will fit as it is only 25.52 when I use that calculator from carstereo.com

Measuring the port in the middle it will go up 17.25 inches and then over 8.25 inches and will be 25.5 total length.


12-07-2006, 01:00 PM
If you want to do it free hand you have to remember to calculate the Radius of the port and add that to the equation you gave.. here you go:

Lv= (1.463x10^7xR^2) / (tuning frequency)^2 x (target box volume)) - 1.463 x R

For R, you will need to know you r max height of the box and subtract the wood thickness time 2.
So for example:
Max Height = 14”
Wood thickness = .75
Slot height = 14 – (.75 * 2) = 12.5”

So now calculate R for the Slott port:
R (slot) = SQRT( (slot width x slot height) / PI )

Feed R, Fs (tuning freq) and Vb (box volume) into the equation and you will get the same output as the calculator mentioned above.

12-07-2006, 02:08 PM
Ok, so what you’re looking for is the actual length in the box, not the calculated.

You will need the following:
Lv = Port Length Calculated
Wt = Wood Thickness
Sw = Slot Width
Bd = Box Debt

Then calculate:

Sl1 = Slot length 1 = BD - ((Wt * 2) + Sw)
Actual = Wt + Sl1 + (Sw / 2)

Sl1d = Sot length 1 d = Wt + Sw

Sl2 = Slot length 2 = Lv - (Sl1 + Sl1d) + (Sw / 2)

Now this will depend on the design, lv, Bd, etc.. but you should be albe to get wha tyou want from this.

12-07-2006, 03:17 PM
You do not need "R" in this formula, and it change Vb from ft^3 to in^3 hence the "Vb*1728"
Lv = Av*1.84*10^8/[vb*1728*(Fb/0.159)^2] - 0.823*sqrt(Av)

Lv = 39*1.84*10^8/[3*1728*(33/0.159)^2]-0.823*sqrt(39)

Lv= (7176000000/223305090.778)-5.14

Lv = 26.995

I get ~1.5" more but it is due to the rounding I did. From what I understand it is better to be long than short. In your drawing the 4.5" part should be 6" to give you 8.25" to center of the 3" slot. Hope this helps

12-07-2006, 06:47 PM
Thanks guys!

That is very close to what I got the first time until I read about Vb being in cubic inches not cubic feet, here is the post where I read that at.


and it change Vb from ft^3 to in^3 hence the "Vb*1728"
Thanks, missed that lol, I see it now.

Now here is what I got the first time.

Lv = Av*1.84*10^8/[vb*1728*(Fb/0.159)^2] - 0.823*sqrt(Av)

Lv = 39*1.84*10^8/[3*1728*(33/0.159)^2]-0.823*sqrt(39)

Lv = 7176000000/[5184*43075.83]-0.823*6.25

Lv = 7176000000/223305102.72-5.14

Lv = 32.14-5.14

Lv = 27

Now I rounded everything to two past the decimal point as I remember that was the way to do it… correct (because of how ours differ)? Would someone mind going over my math and making sure it is correct as I haven’t done this in a good number of years (yes I am old). Also this differs from what I get on the calculator over at carstereo.com, so what do most people go by… the formula or that calculator? Personally I would rather trust a formula, but which one is more accepted and to be true?

Moving on to measuring the length why do you say it needs to be longer can you explain that, I believe I am measuring correctly.

15 (vent side) + .75 (baffle thickness) + 1.5(1/2 of port width) = 17.25

4.5 (vent side) + .75 (vent side1s thickness) + 1.5(1/2 of port width to connect to the middle going over) + 1.5 (1/2 of port width to connect to the middle going up) = 8.25

giving me 25.5 total for port length

12-07-2006, 07:03 PM
This was suppose to be an edit to the above post. Is there a way to delete a post?

12-07-2006, 07:09 PM
I dont understand why you add 1.5 (1/2 of port width to connect to the middle going up). This is how I got it:

Your External Dimentions 30 x 14.5 x 19.5

19.5-.75 (board thickness)-1.5(1/2 port width)=17.25


8.25-.75(slot 1 bord thickness)-1.5(1/2 port width)=6

I dont think the 1.5 (1/2 of port width to connect to the middle going up) is right.

I could be wrong.

12-07-2006, 07:14 PM
Okay what you are showing me makes sense, maybe somebody else can pitch in and confirm this as I am now questioning if I measuring the port length incorrectly. But now at least, thanks to you, I know that I was doing the formula correctly.

12-07-2006, 08:49 PM
Okay so what you said got me thinking and I grabbed one of Moes Lesters designs to retest myself on how to measure port length and this is what I came up with.


18.75 - .75 (board thickness) - 1.75 (1/2 port width) = 16.25
34.75 - 16.25 = 18.5
18.5 - 1.75 (1/2 port width) = 16.75

But on that design it says that piece should be 15 inches long.
So where did the 1.75 go?

Now if you do it like how I do it, it works out.

18.75 - .75 (board thickness) - 1.75 (1/2 port width) = 16.25
34.75 - 16.25 = 18.5
18.5 - 1.75 (1/2 of port width to connect to the middle going over) - 1.75 (1/2 of port width to connect to the middle going up) = 15

or (if you already know your cut pieces)

13 + .75(baffle) + .75 (port piece 2s thickness) + 1.75 (1/2 of port width) = 16.25
15 + 1.75 (1/2 of port width to connect to the middle going over) + 1.75 (1/2 of port width to connect to the middle going up) = 18.5

And that is what I did before on my example, just added it up going from the beginning of the port. Because I already figured it out by subtraction to get my length for the port pieces.

Does this make sense, can someone clear this up?

12-07-2006, 11:23 PM
Now you got me thinking. So I went to the JL website and found this


If you do it the way I do you get the 8.875" that they do if you do it your way you get 7.875".

12-16-2006, 12:56 AM
Can someone clear this up for me are we both right?