Shape: Port recalculation?

[electroaudio]

How do i convert the length on a square or circular port calculated in winisd or similar program to the length of four triangular ports?

What i want to do is to use B&C's new 21inch sub but recalculate the tuning to 32Hz and recalculate the triangular portsizes to 17cm instead of the suggested 20cm so the boxsize will fit better with D&B Q-sub.

http://www.bcspeakers.com/suggested.php?id=0000000011

 

[AndrewT]

you do not need to do any recalculation.
Make the 4 ports the same area as the single port.
The length remains the same.

However the volume used by four separate ports will probably end up larger than a single port and this will require the box to be slightly larger to compensate.

 

[electroaudio]

you do not need to do any recalculation.
Make the 4 ports the same area as the single port.
The length remains the same.

Are you really sure?
I was under the impression that the area of the portsides where the air moves much slower than it does in the middle, did affect the size of the port?