| Port area required is related to the cone area and excursion of the driver; this is what excites the air mass in the port. In an ideal situation, the maximum port area can be calculated using the enclosure size (Vb), the port's resonant frequency (Fb), the number of ports (Np), cone area (Sd), and Xmax (maximum linear excursion). However, Xmax is not always an appropriate limit when considering excursion capabilities. Here is a general way of figuring it out with a round port (which I prefer, although harder to utilize for a low frequency Fb in a small enclosure).
Firstly, we want to calculate the maximum linear displacement of a volume of air by the driver we are utilizing (Vd). This is determined by Xmax and Sd, both of which should be published by the manufacturer.
Where:
Xmax = maximum linear excursion (cm; note that this is often supplied by the manufacturer as a measurement in mm, so divide by 10 to convert to cm)
Sd = effective radiating surface of the driver (cm^2)
Vd = maximum linear displacement of a volume of air (litres)
Vd = Sd*Xmax
That's quite easily calculated. Our next step is to calculate the mimimum diameter of the port. This uses our calculation of Vd and other variables that you will have decided. Let's get going there.
Where:
Vd = maximum linear displacement of a volume of air (litres)
Np = number of ports (unitless)
Vb = volume of the enclosure (litres)
Fb = port's resonant frequency (Hz)
Dmin = minimum diameter of the port (cm)
Dmin=(20.3*(Vd^2/Fb)^0.25)/Np^.5
It seems complicated, but is really quite easy if you've passed grade 10 math. If you intend to work with slot ports, I can review that as well, or I can cover flared ports, though I will warn you in advance that calculation of a flared port proves very challenging, although the roughly 60% decrease in velocity might well be worth it.
Hope that helps get you started. I'm putting together a php calculator on another audio forum I'm on, so if you can wait a month or so, that might make the process easier. You can also use very general rules if you would like, such as those listed above. | 
Quote:
thats very general... may work out well. 
Linear Mode
