for one 15 what would be better one single 4 inch port or 2 3 inch?

 

IMO I think you'll need more port area...a slot type port would be easier to fit a bigger port.

But to be sure, there are a couple of factors that determine the necessary port area. i.e. What sub? Size of enclosure? Tuning? Power?

 

I dunno a lot about boxes, but from what I do know, you're gonna want two 6in. ports or four 4in. ports.

The big SPL boys are running that kind of setup...

And I just built a new SQ box, with dual four's. They're so long, they're half an inch from the sub basket.

She's tuned to 31hz.

 

I vote (2) 4" diameter vents or more.

 

pretty hard to say without knowing what speaker it is.

you can also use the more traditional way to determine the minimum port diameter.

Sd = pi*(D/100)^2/4
Vd = Sd*Xmax/1000
MinimumDiameter= 100*(20.3*(Vd^2/Fb)^0.25)/Np^.5

this is with xmax in mm, diameter in cm, Fb=resonant (tuning) frequency of port, and Np=number of ports you will use.