calculating vent size

I am currently going to be building a sub box for a single Kicker L7 12" sub. The demensions are 20"x15"x18" which has 3.125 cubic feet. I want to know how to calculate, build the vent for this enclousre becuase i made another box for this sub which the vent is all screwed up and the air flow out the vent makes a muffled sound as it comes out, i think it is becuase the vent is too small but i am not sure.. So i will be starting over from scratch but actually trying to calculate the information of the specs of the box so i can achieve the best sq i can get from the sub... THx for your time!

I'd suggest atleast two 4 inch round ports. Here is a simple calculator for you....mind you that you have to subract the vent displacement from the gross internal volume port tuning calculator

kmgraphix wrote: I am currently going to be building a sub box for a single Kicker L7 12" sub. The demensions are 20"x15"x18" which has 3.125 cubic feet. THx for your time!

I don't wish to be the bearer of bad news but I bet you didn't take the volume of materials used into account when calculating volume. You'd be surprised at how much space wood takes up. If you used 3/4 inch MDF, you need to shave off an inch and a half off the width, height, and length.