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Yes, that is exactly the same result I obtained - low to moderate effectiveness of a single HR box for LF absorption. For good results, it require big volume(s) devoted to HR box(es) - very limiting in a typical living room.
It's important to remember that those plots I posted do not show the impact of the resonator on the room acoustics as measured at the main listening position. Rather, they show the sound levels measured with a mic placed right in front of the port, with the port positioned fairly close to the speaker. This rather artificial configuration is intended to allow qualitative comparison of the responses seen with different versions of resonator.
This gives a very detailed depiction of the effect, but it also greatly exaggerates the effect compared with what would be seen at the listening position with the resonator installed as room treatment.
There may be pitfalls in measuring the way I have done, and I'm actually a bit surprised that nobody has come out to criticize this approach or flag some limitations (although I am trying to be clear that these don't provide a quantitative measure of absorption). If there's a way to do it better, or limitations to be aware of, I'd be happy to hear.
From the measurements I've taken, I don't think one can really say if my "best" resonator design (the flared pipe) would be effective or not as room treatment. If I arrayed five hundred of them over my back wall, they might well be effective (keeping in mind that this might not be the most economical or effective approach). It's very clear, however, that a single instance of the "flared pipe" design will have no significant impact at all on room acoustics.
One other point, related to my crude tests where I dropped cotton balls into the tube, is that the placement of an absorber could be important and simply letting them pile up at the bottom of the cavity may not be optimal. It may well be better to affix any absorber close to the resonator neck, where airflow velocities should be higher (as sarumbear has described).
Thanks Vladimir for keeping up the discussion, I just want to make sure it's clear I'm not measuring an actual room treatment.
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