Originally Posted by primetimeguy
This thread is not intended for the Pros/Cons of bass management. I understand those pretty well. What I'd like to do in this thread is better understand the "inner workings" of bass management. There are many people on this forum smarter than me that probably have the answers to my questions. So here goes....
I've always sort of felt a bit of "bass buildup" or the soundstage moved forward to the front of the room (where my two subs are) when running my system with all speakers small and an 80hz crossover. Setting the crossovers lower and/or running some (fronts) full range removes this apparent "bass buildup" and the soundstage appears more enveloping if you will. Now maybe that is because I'm starting to lose some bass since my speakers can't produce it and that just sounds better to me. But to date my measurements don't really tell me that and so let's assume that is not the case for now.
I've been doing some reading about differences in an acoustical sum (think full range speakers) vs electrical sum (think bass management). And along with that the term coherent bass comes up, at least I think that is the correct term but others can help me here. So coherent bass would be the same bass signal coming from say the front left, front right and center speaker all at the same time, right? So I'm wondering how bass management deals with this summation. And how big of a difference could this "perfect" electrical sum be than the acoustical sum in the room? Is it enough to make a difference, say 1-2db? And could that be what I am hearing (or imagining)?
So along those lines I took some measurements using Omnimic, and then I imported them into REW since there are more tools to use there. The legend for both charts is:
Purple = Left speaker
Red = right speaker
Green = both left and right speaker
Magenta (or light purple) = REQ sum (A+B) of Purple and Red
charts are 1/6 octave smoothing
My goal is this would compare the acoustical sum (measured green trace) with the electrical sum (magenta trace). I was hoping, at least from an answering my question standpoint, I'd see a 3db difference in bass between the two. Well, I didn't. Charts are below.
This is with fronts set to full range
This is will fronts 80hz crossover
Acoustical vs Electrical sum appear pretty similar in the bass range but not at high freq? Is this just because of all the comb filtering going on up there?
What I see is is a peak at 80 Hz with your fonts set to full range.
What I don't see is your opinion of the comparison between the two.
I don't see any details about how you set up the system before you took these measurements.
So, whether you like it or not, intended it or not, I see a trick question. I decline to answer.
So a lot of questions buried in there and hoping people can shed some light. My suggestion being that bass management induces some error in the bass level because acoustical sum vs electrical sum is not the same.
Expecting an electrical sum and an acoustical sum to be the same is a faulty expectation. They almost always aren't. If you do everything right, maybe they will be similar enough. Room acoustics and loudspeaker transmission errors preclude such a thing unless they are carefully accounted for and managed near-perfectly.
How does bass management deal with a signal being sent to left, right and center channel together?
Bass management done right looks at the few limited basic things that you tell it about the speakers and takes an educated but somewhat simplistic shot at doing the right thing. The best thing about bass management is that it is usually (but not always) better than no management or stupid management.
If is sums them to a speaker in one location wouldn't that yield a level increase of 6db since you are co-locating 3 signals? How does it compensate or even know it needs to?
No way does every bass management system do the same thing. So, this is an area where only demagogues and fools make very broad statements. ;-)
For example, if your front speakers are set to large, and there is no subwoofer, it would be stupid for the bass from all channels to be summed into one common signal and then split up. OTOH, it would be wise for the LFE channel to be spit up among the fronts.
If there is a subwoofer, and the fronts are set to large, then things get a little more complex. I would still expect that the bass from the front channels is kept separate and routed to the front. I would expect the LFE to be sent to the bass, and it gets to be a judgement call whether we want some of the LFE to be trickled to the fronts. Some AVRs let you do this, but they provide a control so you can keep it from happening.
And so on. There are a large number of reasonable permutations for the speaker configurations, and the bass maangement system should make good but usually different choices for each of them. The system optimization system tries to smooth out the larger of the rough edges that are left over by the bass management system.
In modern times AVR bass management and system optimization systems like YPAO, MCACC, and Audyssey work together to further enhance the fact-gathering behind what they finally implement.
The reference cited in post 2 looks overly-simplistic to me.