Port calculations

[Inductor]

As you see it's about 3m/second with one vent and about half of that with two. So if we go to Flare-it and say that "Required" is 5m/s@40Hz it says Ok you are cleared because it´s way from the 15/13m/s that are problematic, and that you don't need to flare anything. Green light on that, here:

 

[Inductor]

So now, I just went back to WinISD and asked with no flare correction on the ends and it gives some very long 2xports of Vent diameter=10,20cm and Vent lenght=17,39cm; End correction~two free ends (no flare).

If you use the first approach with 2 vents of 10,20cm diam. and one flared end (End correction) on each it's only about 16,19cm of vent lenght. If you think the vent is very long, then or you flare the two sides of the tube (lenght=15cm) or you use only one, or you go to a smaller size let's say 2x8,20cm=diameter, and 10,26cm=lenght and two free ends (no flare).
I guess, if you work with the cross area instead, a slot with be better than very long tubes in a big number, let's say 2, 3 or more. If you need help just ask.
As you see, no formulas (I'm sorry).

 

[Brett]

and the air velocity:

That's at what signal level? 1W?

 

[Inductor]

That's at what signal level? 1W?

Yes, well said Brett.
Now, for 300W (max. output at 70Hz for this alignment), we can see that minimum would be 3->4 vents. With 3 we are still in the noisy area. That's a plus maybe for a slot or 4 vents. Also start looking at flaring... (and required air velocity).
Notice that for 4 vents (or a slot) flared on one side, your venth lenght should be ~40cm. Use Flare-it for flare size.