altie diyAudio Member

Join Date: May 2010 Location: Portland, OR

OK, I found some options that work - measured. Those results to come soon. The text version:

- there is quite a bit of distortion coming from the ports of these speakers at the rated lowest frequency of 45hz and nearby in the next few 10s of Hertz. At a volume I estimate to be a bit below xmech just based on my ears, the port and whatever is seeping into a close directional mic from the woofer cone is around 15% THD with strong 3rd-8th order components. It's quite audible.

- straws actually harm the output with these speakers, increasing THD from 45-55Hz (possibly elsewhere, but this was the band I was the most interested in on the assumption that the port contributions would be the greatest here). The failure of the straws to improve the situation may be due more to difficulties getting them to sit squarely in the port and cut cleanly at the ends than to any deficiency in the principle of a flow straightener in the port.

- I say that last bit because a folded up, pressure-fit piece of cardboard with the corrugations running front to back along the port allowing airflow actually worked quite well. This resulted in a couple of percent reduction in distortion at 45Hz to around 13%, and cut it nearly in half at 55Hz to around 7%.

- By far the most effective approach I tested was the press fit cardboard combined with a 1/2" thick open cell foam ring placed around the port exit. This reduced distortion to 2% at 45Hz and 3% at 55Hz.

- Nearly as effective as the foam gasket was a set of what I call "eyelashes" mimicking the Boeing 787 engine exhaust chevrons, made out of painter's tape. This plus the cardboard measured 3% at 45Hz, 4% at 55Hz.

- Cardboard + foam + eyelashes: 1.9% at 45Hz, 2.6% at 55Hz.

- The simplest/easiest/best bang for buck approach turned out to be a set of vortex generators made out of painter's tape placed near the port exit. With no other treatments added, just the vortex generators by themselves: 7% @ 45Hz, 5% @ 55Hz.



In principle all 4 of the approaches outlined here could be applied: cardboard stuffed deep in the port, followed by vortex generators closer to the mouth, with eyelashes at the mouth (ick, bad choice of mixed metaphors), followed by a foam gasket. It seems possible that the vortex generators and eyelashes would not work well together, but who knows. I've actually been surprised at how well all of these approaches are working so far.