For my NSB array I created;

* 4 chambers per tower
* 4 NSB's per chamber
* 1 6" diameter port, 1 3/8" length per chamber in the rear.

I chose the 6" diameter for a few reasons. First it gave me a port
length that is essentially a hole in the rear wall as I have 3/4" plywood + another 5/8" MDF dampening sheet for 1 3/8" wall thickness.

Second, I could only afford ~ 1.5 cu. ft. air space per chamber
and I achieved the desired tuning I wanted, ~ 95-100hz with a nice
+6dB boost centered around 106 hz. The plan was to make a port door
on each port so when you close the door it's now a sealed box and you
attenuate the bass or you can open the door slightly to tune lower but you don't really get much output from tuning lower as NSB's don't
really move alot of air. The 100hz peak is nice as it gives you natural boom without over driving the NSB's and you could mate this to
a subwoofer. I have one of my array's in the corner, rotated and I
get alittle bit more bass due to corner loading. People that audition
the system are amazed at how much bass I get from the array, they assume I have some woofers 'somewhere' but I don't. The last reason
for making a big 6" diameter port is... With all the sound conditioning and dampending I installed, the port is in the 'line of fire' of those NSB's so the rear wave can exit the chamber so reduce
reflections off the back wall that may add coloration. If you pay attention to detail you can make an amazing NSB system, pretty neat for 49 cent speakers.