What a great reply. I agree with all these points. Maybe not so much the measuring part, I'll admit. But for sure very good point about the decoupled system absorbing low frequencies.
I'm a little surprised about the measuring part. If you're talking about measuring the ambient noise level, I don't see the point. If we are simply talking about sound proofing, then all it will do is show what you have achieved. I'd say that's worthwhile, mainly for the sake of curiosity. Especially when one wants to see what happens at 50 Hz! Probably a bit disturbing ...
The most benefit in measuring will come when the room is actually done, and it's time to work on the bass.
This is a simulation of a room that I used in the past:
The dotted line shows worst case scenario if it were built like a bomb shelter/garage. The magenta line shows a more typical room. This would be easy to eq mostly, except two deep nulls. The red line shows what we want to see - a room where the modes are sufficiently damped that we have a good chance of getting it right with eq.
Of course, you can't rely on simulations, but they are illustrative still.
These settings were used to show the eq required to get the bass flat:
Note the actual room response is the inverse. As you can see, it's pretty tame. This is a lossy room with timber floor and light timber framed drywall as well as double glass doors and reasonably large windows. No bricks or concrete.
The two charts above are for the same room. Keep in mind the eq/measured chart is more complex also as it has mains and subs overlapping, the mains able to get down to 23 Hz.
So if you get the room right first, this should be the first thing for the bass. Then I'd experiment with eq and placement. There's a good chance this will get a good result. Ideally if funds permit you'd do this in conjunction with a multi sub arrangement. Here is a measured example:
Geddes suggests 3 subs are ideal. You can start with one sub which mostly meets your performance demands on its own. It's placement is less critical since it will most likely operate below problemmatic zones. The other subs will give some boost, but their main function is to improve the room response by "spatial averaging." They can in fact be compact, and this is a good place to diy since you can cleverly hide them if desired, and integrate them into the room creatively. You could have a stair riser sub, coffee table sub etc.
If after doing all this you are still having a case of audiophilia perfectionitis, then it's time to think about adding bass traps.
Do you like to use theater risers as bass traps? I'm not an in-room acoustics person but this topic comes up frequently.
The idea makes sense, but I haven't done it due to practical restraints. I also don't actually need add on bass traps. Chances are I will experiment with them to see if I can get an improvement. In that case I'd certainly measure before and after.