Jeff,
I have heard SB1000 port chuffing, but I don’t know if they had the additional round ports.
There are several interesting things going on in the SB1000 that make for more variables than a simple model.
The rectangular duct is coupled to an expanding opening, so it’s tuning is not the same as it would be on a flat surface.
The round ducts are roughly 18” in front of the rectangular duct, if they do “choke” at high power they would also change the phase a bit too, since they would lead the other port by a bit.
Your previous statement from Sat, 17 October 2009 08:40 “Actually, the enclosure tuning shifts up in frequency (when the small round port 'chokes' at high power levels)”
Is incorrect. More port area makes for a higher FB (tuning).
Drew Gandy was also mistaken
“wondering if a downward movement of the tuning is actually what you want when the vc heats up and/or the woofer is really moving”.
Electro Voice called covering one of the ports for a lower tuning the “step down” mode for the TL cabinets. Of course, the smaller port area then made for more “chuffing”, but that is a different can of worms.
My guess on the whole mess is the large “venturi port” tuned a bit lower than EAW wanted, and an easier fix than re-tooling or re-building existing cabinet shells was to simply add a couple of ducts to raise the tuning, giving more of the 60-80 crap everyone seems to want.
And if you wanted to make the cabinet the same as the others, you could simply cover the round ducts.
Everybody’s happy.
And if Phil’s theory is correct,
“At high output levels the resistive component of the port impedance rockets up due to turbulent flow conditions (which you can estimate from reynold's number calculations). This effectively shunts these small round ports out of system, leaving the tuning set by the large slot port....a variable box tuning that dynamically shifts depending on the velocity of air in the smaller ports.”
It would do no harm in the process, as the “chuffing” would not be noticed with all the raging HF that would generally accompany the sub being pushed to the point where the air is blown out of the smaller ports.
Art Welter