# Port volume calculations



## Morganpartee (Jul 2, 2009)

Hey there guys. I've had a question on my mind for a while now. When calculating the length/width/height of a port, does one need to take the volume of the port into account? 

Like, would you find the total volume of the box, then subtract the volume the port occupys inside of the box? It seems like that'd be counter productive, but is it neccissary?

Thanks!


----------



## Austin (Mar 12, 2009)

You add it. Say you need 1 cu ft of box for your sub and your port takes up .2 cu ft. You would need a 1.2 cu ft box essentially.


----------



## Morganpartee (Jul 2, 2009)

All of the port size calculators, like the one the12volt.com for example, asks for the box volume, to figure out the port depth. 

Why do you add it?


----------



## Austin (Mar 12, 2009)

It sounds like the way that calculator is doing it is backwards which there isn't anything wrong with that. Just make sure the net volume after the port has taken its space is enough for the sub.

I just look at the calculator and i'm pretty sure you add the port volume to the box volume...


----------



## Morganpartee (Jul 2, 2009)

Thats interesting. What is the formula for port tuning? I haven't been able to find it!


----------



## Austin (Mar 12, 2009)

That formula on the12volts.com works. Just calculate the volume of the round ( PIEr^2 *h) or slot (Length*Width*Height) port and add it to the original volume of the box you typed into the calculator. If you make a slot port make sure you calculate for the thickness of the port walls if you use mdf to make it. Add that volume also.


----------



## Morganpartee (Jul 2, 2009)

Alright man, thanks.


----------



## DaveRulz (Jun 22, 2006)

The calculators work correctly, you just need to understand what you are actually doing. 

When you are designing a box, if you calculate out that it needs 1 cubic foot of air space and a 3 inch by 12 inch round port then you work backwards to put your box together. You take the 1 cubic foot, the .05 cubic feet that the port takes up and then also add the volume displaced by the sub. If you are inverse mounting the subwoofer, or mounting the port externally to the enclosure, both of which are perfectly acceptable, then you adjust accordingly. 

So basically, you cannot calculate port tuning frequency easily(if the port is going to be inside the box) if you have the external dimensions of your box already determined. 

The best way to go about designing a ported enclosure is to determine the correct volume for the subwoofer, use that to calculate your port size, then add the displacement of the port, the displacement of the woofer, and the required enclosure volume to get your total interior volume of the box. Then use that number to get box dimensions to fit the space you have for it.


----------



## Austin (Mar 12, 2009)

^^ what he said


----------

