Originally Posted by rock_bottom
Not a dig at all. I'd love to have a DEQX, but their previous lowest-cost device seems to have been discontinued and replaced with one that's out of my price range.
I am not saying exactly that. But the relationship is more straightforward for linear-phase FIR filters than for IIR, so let's talk about them first. For such filters, the group delay, which is proportional to the negative of the slope of the phase vs. frequency, is constant for all frequencies (because phase shift vs. frequency is a straight line, so its slope is constant for all frequencies). If you were to look at the impulse response of a linear-phase FIR low-pass filter having a delay of, say, 1 msec, its peak will occur 1 msec after the impulse is applied.
Now consider an IIR or analog low-pass filter. Its phase vs. frequency is not linear, so its slope of phase vs. frequency is not constant with frequency. Therefore its group delay is not constant with frequency either. The group delay is just a mathematical construct though, and it doesn't always match up with intuition. Sometimes it has a clear interpretation in terms of the impulse response (such as the linear-phase FIR low-pass filter example of the previous paragraph). Sometimes that interpretation in terms of the impulse response is not so straightforward. If the delay depends on frequency, where is the peak of the impulse response? Which delay should be used? The one at very low frequencies, very high frequencies, mid frequencies, or some other one? They're all different, so which is the right one? What is the time domain interpretation, in terms of the impulse response, of a group delay that varies, sometimes considerably, with frequency? There is no straightforward answer to that. The upshot is that dealing with IIR and analog filters in the time domain in the context of crossovers may not always be productive.
I am not saying that either.
As mentioned before, there are at least two entirely different contexts to be considered. Consider two low-pass filtered subwoofers only, no mains, with one sub delayed with respect to the other. Apply some kind of transient signal to the subs and vary the delay of one relative to the other. At which delay value do the sounds appear distinct between the two? I don't know, but maybe Earl's 30 msec figure is right.
The other context is integration of mains and subs, which is an entirely different problem. A statement like "delay is inaudible" has no meaning without context.
I am not saying that "delay is inaudible" in either context BTW. What I am
saying is that the most productive use of relative delay of mains and subs, and/or multiple subs with respect to one another, is to optimize frequency response flatness.
thanks indeed for the time you took! (ha, maybe I only view how much effort people put into their posts by my own...I have to correct so many damned typos it takes me forever! You OTOH maybe could type three hundred words/minute without error haha)
I have to admit (which could be frustrating to the more knowledgeable here) that in certain cases I just hunker back down to 'forget the theory, what actual
steps do I take?' To be fair the correct counter to that might be 'well, if you understood the theory that will guide you'.
Anyway, I think from some of the responses here that at least some fall into my camp on that. I guess there are just some areas that I am not that willing to take the time to study to come to a 'full intellectual understanding'.
I mean it is all well and good to say 'earle has some proprietary and secret way to overcome all this'. We're (us dummies) are not really any better off. And a further subset of those dummies (me as but one) are either not able or unwilling to delve into what can be viewed as difficult intellectual territory.
Even on a philosophical level I can't quite grasp the 'it is inaudible in sub territory' when it clearly is, if not in terms of 'oh I can hear that one or both of those subs are leading or lagging each other or the mains' but certainly in terms of 'oh, I can hear the non flat response that has resulted'. It IS a simple experiment to (ha, well only if you can of course) do a sweep, then delay/advance-even by a small amount- a sub, or both differently, and do another sweep and see the often widely varying FR that results.
I did manage to remember what photo hosting service I used, but even then my appalling lack of documentation means I have to guess (a few years later) what graph is what!
I know I did the above experiment but don't recall exactly the set up back then, so take these as illustrative only, just to show the point.
(oh, even tho I am quoting you RB, this is just general talk)
Now to remember how to post graphs! wish me luck
These are kinda in a sequence so it is reasonable that they are together to show the point I was making
Firstly, here is the FR of two sweeps, note the differences
Ok, that worked. The crossover point looks about right, so am confident I have got the right pics. If not, just view them as demonstrating the point.
We could see the 'poor' response, decide that we do not need to bother with delays, which DOES seem to be the general thrust, and if the system is capable enough apply eq and correct the response. Of course in doing that if we find it is a room null then we don't boost.
Here are the two measurements of the impulse arrival times that precede that graph, the arrival times 'as is' and then adjusted. They correspond to the two different sweeps just shown
(ah, when I inserted one of them it did have the title 'subs aligned', so am even more confident that these are the correct graphs) I had already aligned the subs with each other, then as a group aligned them to the mains. It would have been even more stark had the two subs not been aligned.
As I said, if you can do it it is a very easy experiment to simply change the delay on a sub and compare the resulting FR.
So even if the ONLY bottom line is to fix the FR, it makes sense to align the bass sources prior to eq.
It actually drives me insane in the bass if things are not aligned. I am aware that I seem to be running counter to the prevailing view that 'different arrival times 'down there' are unimportant' (oh yeah? tell that to your wife haha), yet I am also aware that we can fool ourselves. It is hard to imagine it is all expectation bias or whatever, yet it is equally hard to ignore my reactions to non aligned bass.
Oh, this being a home theatre forum perhaps I'd better also say this is all about 2 ch? I don't have a HT set up.