I've spent the last week having my arse handed to me over on Gearspace
over. I don't have the energy to summarise what's being discussed on my
thread over there but if your interested you can head over and have a look-see.
Something that maybe of interest to the pfm community is the effect of simply placing two 600mm x 1200mm x 170mm broadband bass absorbers on the back wall, centred behind my listening seat. This changes the perceived tonality of my speakers significantly, making them sound both bassier and at the same time brighter. Output from 70Hz-120Hz increases (confirmed by the measurements) and the midrange output is less prominent (not confirmed by the measurements but I'm trusting my ears on this one!).
This necessitated some adjustments to my EQ to restore the speakers' tonality to what I had previously, and I'll have to keep this in mind when planning my room treatments because the more broadband absorption you add, the more you increase the ratio of direct-to-reflected sound and the more you are tuning the response at the LP to the speaker's anechoic response (at MF and HF at least).
The increased bass might be welcome as it may allow me to pull the speakers away from the walls to improve their imaging without losing too much bass reinforcement. I've still to try this.
I'd noted previous in another thread that placing a 600mm x 1200m x 170mm absorber behind my listening seat helped to reduce the deepness of a 75Hz null caused by SBIR. The improvement was small when the absorber was mounted on the wall but improved significantly when I pulled the panel forward by 60cm so that it was halfway between the wall and the back of my chair.
This time I measured the effect with two absorbers instead of one, and raised up off the floor by 40cm so that the centres of the panels were now at ear height. Placing the absorbers against the wall reduced the 75Hz null by 3.5dB, a small but audible improvement. Pulling them 60cm out from the wall shaved an impressive 7.5dB off the null (the change was measured without applying any smoothing to the graphs, that's why it doesn't look as impressive on the graph below with 1/12 oct smoothing applied).
Of course, having two big monoliths standing 60cm off the wall isn't exactly practical, it also cuts out a ton of daylight from my room, but it was an interesting experiment. If I'm serious about going after this SBIR then a more practical choice may be to use the "FlexRange" version of the Monster trap which is tuned to 80Hz and designed to work best against the wall.
Here's the FR before & after adding the two absorbers behind the LP without adding any EQ: