Slow down, sparky.
"The measurement you posted had the first mode centered at 45 hz (20' wall), and is wide enough to stretch 40-50 hz (maybe a tad more), which is a Q (bandwidth) of 11. "
Can you explain this a bit more?
That is an example of how I learned to look at resonances. Peaks and dips are all resonances, when they can be attributed to room dimensions. The predicted room mode for a 20' wall is 56 hz. (1131/20= 56.55) Doors and windows allow flex in boundaries which can lower the actual resonant frequency (mode) from predicted. Your measurements indicate that resonance is actually most active at 45 hz. That is the center of the peak, which would be the frequency used in creating a DSP Filter.
Next, you have to measure the bandwidth, Q. That is the width of a resonance, measured in Hz. That resonance appears to be 11 hz wide, so Q= 11
Amplitude is how many dB above or below the relative (average dB across operating range) sound the offending frequency is playing. You would apply however many dB is necessary to bring that resonant (mode) frequency back to the relative level.
Post #41 and #99 show steady improvement, but without knowing what inputs you've made, any other suggestions would be guesses.
The other part I mentioned is I do not know what resolution you are measuring in.
For bass, it really has to be 1/24 octave with phase, with and without room correction. to ensure you capture the exact amplitude and Q of said resonance. Lower res can mask offending frequencies. So that 45 hz resonance, could be higher in amplitude, or have a different bandwidth than is indicated in that measurement. Being sure of that, saves time on the guessing of amplitude and Q.
Yet another part with multiple subs is ensuring they are all playing at the exact same time. I've only setup a few multi sub rooms, but I've never NOT needed to adjust the delays, even with identical subs with identical cable lengths. And this is where I am in my learning: I do not know how to confirm subs that are not playing together in measurements.
For that, I listen. I use music with strong punchy drum playing (Rush) and really strap in for critical listening. Turning my head side to side (intramural cross correlation - your ears are not symmetrical, which means your own physiology can interfere with how and when you hear certain frequencies) and moving between seats I listen for subs that can be easily located (bass is omni directional, you should not be able to locate the source of the bass, if you can, it's too loud) and try to achieve the best balance at all seats. The delay of each sub should be set so that when a drum should sound snappy, it does. It's immediate, and does not rumble or resonate.
*Disclaimer: in my experience, the Q being measured in single Hz only seems to apply to frequencies within the subs operating range. Minidsp can be used between preamp and separate amp for passive speakers as well. Subwoofers have circuits to protect them from being over driven, where passive speakers do not. Anyone applying my advice about treating subwoofer frequencies to higher frequencies should be aware that they need to be certain of what they are doing as their passive speakers will not be protected from being over driven if the amplitude of any frequency above the subwoofer, is increased.
