I got this formula off caraudioforum long time ago
the equation for the tuning frequency is like this
L=(4651704*Pa)/(Fb^2*Vb)-.823*sqrt(Pa)
where
L is the port length in inches
Pa is the port area in square inches
Fb is the tuning frequency of the box
Vb is the internal volume of the box in in^3
if you wanted to make a 1.5cf (2592 in^3) box that had a 30hz tuning frequency and 20in^2 of port area, you would need the port to be
L=(4651704*Pa)/(Fb^2*Vb)-.823*sqrt(Pa)
where
L is the port length in inches
Pa is 20 in^2
Fb is 30hz
Vb is 2592 in^3
so
L=(4651704*20)/(30^2*2592)-.823*sqrt(20) = 36.2 in
so with a 1.5cf box with 20 in^2 of port area, you would need a 36.2 in long port to get a tuning frequency of 30hz