Otto asked me a very good question via PM, and I thought it was worth answering in some detail here. He asked why I included only two port plugs when there are three ports, given that I've included as many plugs as ports on my previous Thunderchild models.
Well, there's actually more to it than me just growing cheap in my old age!
Briefly, one port open (the center port preferrably) very closely approximates a sealed box, and in fact is arguably a bit better than a sealed box.
Okay, let me put my geek hat on:
With all three ports open the tuning frequency is 46 Hz, the -3 dB point is 59 Hz, and -10 dB is 39 Hz.
With two ports open (center port plugged), the tuning frequency drops to 37Hz, the -3dB point rises to 65 Hz, and -10 dB drops to 37 Hz.
With one port open (top and bottom ports plugged), the tuning frequency drops to 27 Hz, the -3 dB point rises to 73 Hz, and -10 dB bumps up to 38 Hz.
So as we lower the tuning frequency, the -3 dB point moves up, and the slope of response curve below the -3 dB point becomes more and more gentle.
Now here's the interesting part about one port open: From 100 Hz down to 25 Hz, the curve looks exactly like a sealed box with a Qtc of .57 (which is an unusually low Qtc). The -3 dB point is a bit lower with the single port than it would have been if all ports were sealed, which shifts the curve downward a little bit relative to a true sealed box, but the shape of the curve itself from 100 Hz to 25 Hz is otherwise identical. In addition, the excursion-limited power handling is better than if the box were sealed.
One final thing we want to take into account is port airspeed. If we were expecting to put 900 watts into the fundamental, then our port area would be too small once we start plugging ports. But two things work in our favor: First, the fundamental will be seeing far less than full power; and second, as we shift the tuning frequency lower, we are shifting the frequency of maximum airflow lower, down below where the cab is going to be seeing very high power anyway. Also, if any of you decide to model the airspeed, remember that my 3" diameter ports are flared inside and out, which delays the onset of chuffing by about 2 dB. In other words, my 3" diameter flared ports theoretically start to chuff about where a 3.5" straight-sided port would.
Anyway, that's my story for why I'm only including two port plugs with the TC212 cabs.
