Stupid question about tuning

[mr_f4ke]

Hi guys, i have two Cvr 12" and i will build my own box. I want more spl than sq. So when somebody tell me that i have to build a 2.5cuft ported box tuned to 38hz i know that 38hz means the size of the port, but how i will know the size of the port? there is a formula for that? plz help me guys, thx.

[Moocow16]

you can download WinISD Pro and input all the info and it will tell you the dimensions of the port. well that's a pretty good guess, it's not perfect, but it's good enough.