port calculation for 2 or more subs

Is there a specific formula for finding the minimum port diameter?

Yes.  This from the Subwoofer DIY Page: 

Minimum Port Diameter

To calculate the minimum diameter of the port required to prevent port noises, you will also need to know the following:

    Xmax = maximum linear displacement (mm)

    Dia = Effective diameter of driver (cm)

    Np = number of ports

Calculate the minimum port diameter from the following equations:

    Sd = pi*(Dia/100)^2/4

    Vd = Sd*Xmax/1000

    Dmin = 100*(20.3*(Vd^2/Fb)^0.25)/Np^.5

where,

   Dmin = minimum port diameter (cm)

Note: You CAN use ports that have a smaller diameter than that given by the equation above, especially if the ports are flared at both ends. However, at higher volumes, you may notice some port noise caused by the air rushing through the ports.

 

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

Me and Jethro Clampett would have to stick our brains together and cipher for pert' near a coon's age to come up with an answer using those formulas.

If that math hurts your head like it does mine, here are easier ways to find the answer.

1.  The manufacturers of some reputable subs will give you that information in the manual in the section that describes recommended enclosures.

2.  There are online calculators that have formulae plugged in that will do the math for you.

http://www.carstereo.com/help2/Articles.cfm?id=31

http://www.mhsoft.nl/spk_calc.asp

3.  There are box-building software programs that will enable you to see a graphic representation of port noise with a given opening size and power applied.  See the featured sticky subject on WinISD above for info on two such programs that are free to use.

Build the box so that it performs well in the worst case scenario and, in return, it will reward you at all times.

The second link does it work for me ( http://www.mhsoft.nl.spk_calc.asp), is it working for you guys?, or is it just me?