The absorber was made from two Eltax Atomic 8.2 whose plate amplifiers failed and I mounted the boxes back-to-back, creating a common double volume with two 8inch drivers. I closed the reflex ports with socks (but I'm wondering if efficiency improves if just let them open, have to check that also).
Measurements done by Holm Impulse, indeed, using good USB-mic (dr. Jordan-Design), I attached the exports for you to check (conditions identical). I am also very impressed, I repeat that the improvement is easily to hear, I expected even much more graphic difference before doing measurements, just because of such a subiective improvement. I would quantify this like having at least one more Hofa bass trap in the room (have already two installed).
Hofa basstraps are each in the front corners, behind the main loudspeakers. The electroacoustic absorber was put over a Billy bookshelf to be also in the front, left up-corner. I have two subwoofers, one in the middle in front and the other behind the couch near the middle of the right wall. Mic position was on the listening position on the coach.
Measurements done by Holm Impulse, indeed, using good USB-mic (dr. Jordan-Design), I attached the exports for you to check (conditions identical). I am also very impressed, I repeat that the improvement is easily to hear, I expected even much more graphic difference before doing measurements, just because of such a subiective improvement. I would quantify this like having at least one more Hofa bass trap in the room (have already two installed).
Hofa basstraps are each in the front corners, behind the main loudspeakers. The electroacoustic absorber was put over a Billy bookshelf to be also in the front, left up-corner. I have two subwoofers, one in the middle in front and the other behind the couch near the middle of the right wall. Mic position was on the listening position on the coach.
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Well that's not physically possible, because the sound cannot vary spatialy at any less than the lamba / 4 of it wavelength, and that's several feet or more at LFs.
And the sound variations in frequency can not vary faster than the frequency spacing of the modes, which gets pretty wide at LFs, sometimes a full octave. Of course as the modal density goes up this all changes, but you said "low frequencies", so I am assuming < 70-80 Hz.
Yes, I see, my terminology is not accurate, apologies. What I didn't understand in DorinD's initial post of the FR was the apparent smoothing over the very small intervals because I think the absorber would only work over a narrow bandwidth according to its resonance. I wonder if something else is happening, I don't know what, but it's almost as if less comb filtering is occuring 
Sorry guys, previously I made exports under each graph I wanted to send, now I make it from File>Save_measurements , hope it exports only what is on the screen, not all 25 I have it now. If it exports all the stuff, then please consider only "Unnamed-24" and "Unnamed-25"
The conditions were the same as I know (each of those 3 curves has "raw" in Options) and they are pretty consistent because gave me same results each time even if I switch between those 3 shown graphs.
....Your file of 7.02 MB bytes exceeds the forum's limit of 976.6 KB for this filetype...Let me clean it a bit...no way, it clears everything.
Hope you can download it here: Dropbox - with&without_elac_abs.zip
The conditions were the same as I know (each of those 3 curves has "raw" in Options) and they are pretty consistent because gave me same results each time even if I switch between those 3 shown graphs.
....Your file of 7.02 MB bytes exceeds the forum's limit of 976.6 KB for this filetype...Let me clean it a bit...no way, it clears everything.
Hope you can download it here: Dropbox - with&without_elac_abs.zip
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Let me explain a little bit, the sub-bass (under 80Hz) is mono-ed (summed and 1/2) and both 24&25 measurements are of left channel. The difference is visible on graphic under 200Hz, and to be honest I have expected to see even a bigger one considering how night and day is what I am hearing on each case, with and without absorber. So, there is something here and now I am much more motivated to continue in this direction (like exploring R/L/C shunt). I wasn't expecting such a broadband behaviour with simply a short-circuit and nothing exotic.
I find the small interval smoothing below approximately 55Hz very interesting, I wonder if this is around the resonant frequency of the subs because from my understanding it can only have an effect below resonance, although I presume this could be raised by using a smaller enclosure, maybe you could seal the port a bit better and see if this has an effect. Are you planning to try parallel resistor and capacitor?
Thank you Flaesh for the conversion to REW. As I remember, I did measurements from 20Hz up, no idea why there is something below 20Hz?!
@Scott: Basstraps in front corners, behind main speakers, is a recommended setup by most manufacturers, seems it works fine. My room is pretty long and listening position is still under half of the room lenght. I choose putting most of bass absorbers in the "active end" (sorry I use such naming, I have no recording/mixing facility, just a dinning room) but also thinking of putting something in "dead end" after further investment in acoustic treatment.
I mentioned before but not keeping you update: I have also a pair of EA Evo1 bookshelf speakers (6.5inch woofer) to play with as electroacoustic absorbers but I gave up with them after rediscovering these Eltax decomissioned subwoofers. I'm not saying that Evo's don't work, but Eltax --with their 8inch woofers-- are ways better.
@Scott: Basstraps in front corners, behind main speakers, is a recommended setup by most manufacturers, seems it works fine. My room is pretty long and listening position is still under half of the room lenght. I choose putting most of bass absorbers in the "active end" (sorry I use such naming, I have no recording/mixing facility, just a dinning room) but also thinking of putting something in "dead end" after further investment in acoustic treatment.
I mentioned before but not keeping you update: I have also a pair of EA Evo1 bookshelf speakers (6.5inch woofer) to play with as electroacoustic absorbers but I gave up with them after rediscovering these Eltax decomissioned subwoofers. I'm not saying that Evo's don't work, but Eltax --with their 8inch woofers-- are ways better.
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