Originally Posted by Ricci
I had thought that 6" to either side wall and 8.25" to the back wall would be enough to avoid a big shift. I was wrong. Although undesirable in this case... It seems to be a very effective technique if you are really cramped for space and desire a low tune.
you have modified my original design. your design increased the width of the flange at the internal end of the port. your design essentially terminates the round port not into an airspace but into a network of rather narrow rectangular ports formed by the bracing.
if you stuck closer to my sketch you would still get significant loading at the port end and thus significant reduction of tuning frequency but it would at the same time be smaller. i sort of assumed that you wanted that kind of loading given that you were considering the use of that polk style port that you used on the XXX sub.
your analysis in the above post however is slightly wrong. it is not the proximity to any one boundary that has the major effect here, but rather the fact that you have several boundaries enclosing the port exit on on basically all sides. my sketch had that too - but to a lesser extent. the point is however that if you were to take out all of the bracing ( especially the flange ) the tuning would increase with the same port length in the same box.
as far as effectiveness of this technique i would say that using a large flange should be an excellent way to increase port performance because it lowers tuning ( very slightly ) without obstructing the straight-line airflow path and in fact REDUCING turbulence by switching a 180 degree edge for a 90 degree ( so less pronounced ) edge.
placing the port exit up agains a wall is in itself probably a BAD idea because it will result in more turbulence than tuning frequency reduction.
however the real tuning reduction is to be had from using both simultaneously - that is sandwiching the port exit between the back wall and an enormous flange effectively loading the round port into another slot port. which incidentally is what happened with your subwoofer.
the mistake here was in not properly calculating the magnitude of this effect. not that it would have been easy, but we should have put more effort into it. guess one of the reasons we didn't do it is because we couldn't agree on the optimum tuning frequency to begin with.