Welcome to the group.
Have a look in this thread:
Bob Golds entered a very nice link explaining those things.
http://web.archive.org/web/200108030528 ... mTubes.asp
Only take into account that the way they (Bob's link) define the lenght of the tubes for the tangential modes is complete nonsense.
I wrote those guys after Bob started this thread to tell them gently, but as so often they never replied.
On the net it seems often more important to show nice and impressive than accurate things. Sense of responsibility versus site visitors is completely subordinate.
By definition such a tube works as a 1/4 wave silencer.
The way they define the first order tangential (and higher) by relating the 1/4 wavelength with 50% of the diagonal of the room is wrong.
Since they didn't reply when I informed them, I don't take those guys serious anymore, no matter how nice those animations look.
Without absorption inside those tubes, this are very narrowband devices.
Adding wool in it will broaden the bandwidth but will diminish the efficiency of the absorber significantly (depends of amount of absorption). Compare it with putting absorption in a windwood instrument, you block the resonance from building up.
One could make them also with both sides open, then one could make them shorter since it become 1/2 wave silencers.
There is only one annoying problem: both sides must be excited in inverse phase, that should even increase efficiency.
If both sides are excited in phase, the thing won't do a thing.
So that should become a matter of playing with very exact positioning in the modal pattern in a room.
This is more theory than practical, but should be fun to experiment with.
The idea: http://www.physicsclassroom.com/Class/sound/u11l5c.html
Use them without wool but only to solve ennoying remaining problems.
Those things are very easy to tune.
Martin Walker from SOS (technical editor, reviewer) told he used such things without wool, but found them having low rendement. It were those he tested:
I think a better approach is the corner absorption to start with as found in the FAQ.
Use those 1/4 wave silencers if needed to solve specific problems.
But that's my personal vision, you decide.