So since I am in the process of building a HW
and am scaling it for 18" drivers to be used for PA work I have been thinking about the ports. The bottom is easy to understand. They are situated as to use the floor to extend frequency. But then we have the top port. First it is centered as all ports should be. But then it is out of phase with the bottom ports.
I wish I had some software to model
this and measure the differences between the direct radiated centered versus say centered frontal exit versus  in phase. Etc.

I do have the idea that the top port is
tuned higher to give a tighter sound but that is about it in my head.

For an install. The top port could be positioned
on a particular side in order to use a bounding wall just like the bottom 2 do the floor.


So it would be nice if the plans shared scaling along with
different port locations and their effects on the output for a given speaker.

Anyone want to share some input on the subject?

Well high vs low frequency will depend on how a person loads
the cabinet. Which ever port is the frontal port will be the most efficient low freq port do to the direct radiation.  That with the actual tunning of each port.

I just suspect the reasoning behind the top firing
of the upper port is phase alignment. But sense I do not have any software to model this
and do not know an equation to figure it out. It is just theory.

the top port isn't "out of phase"

its just firing off axis