Originally Posted by Alan P
See, now...this is exactly what I was thinking!
Just so I'm clear, Bosso - you advise against gain matching and
against multiple subs being split between far and near field? I ask because there is no way I can fit all 4 subs in the front of my room....at least one will simply have
to go in the back. I'm guessing if I have the ability to set phase on all of them (but not separate distances) I'll still be OK?
I advise against gain matching when the distance disparity is too great, as in one sub at your side and the other sub at 4M. and yes, I advise against such a placement configuration because neither a phase control nor a distance control is likely to be able to reconcile the arrival time disparity.
Remember, a phase control is a simple all-pass filter. The filter must have a center frequency, so it's typically close to the most-used crossover point of 80 Hz and is meant to reconcile the sub with the mains, not separately placed subs. As someone already mentioned, that will help at 80 Hz but hurt at other frequencies.
The best scenario is to begin placement with all sub equidistant to the LP. As you deviate from there, you have to deal with the consequences of a) cancellation/reinforcement from same waves arriving at different times and b) level matching to avoid localization, more so as distance disparity increases.
Originally Posted by sputter1
If i'm following you Bosso, with a 2 sub setup you would suggest one near/one far placement level matched at the main seat?. In my particular room I know that setup gave me the flattest response 13hz-80hz sans eq. I've since co-located my subs for the greater output on a trial basis.
Always level match the subs from the LP. It's just common sense.
Originally Posted by zheka
It seems to me that example Bosso used is exactly the type that gain matching is meant to address. To get these two subs level-matched would require one of them to run 12db hotter than the other. I suspect this much headroom is not an option with most commercial subwoofers.
You have it exactly backwards
The 2 subs in my measurement are perfectly gain matched. To level match them would require the closer sub to be reduced in level, not the other way around.
It also needs to be mentioned that the near field sub will always exhibit a different FR than the properly placed sub. The native response of a sub (should be) tailored to work with room gain to result in a flat response at the LP in a given room. The closer you get the "mic" (your ears) to the sub, the more its response reverts back to a native response. This is clearly evident in the 2 traces on my graph.
Originally Posted by Tack
And that's basically my point. We call certain frequencies non locate-able. Why would they ever become locate-able for reasons of relative volume between subs and mains,
or volume between subs? It should be simply. not. locate-able. Alan is saying he hears where is sub is, not that he hears it first.
I don't know the answer, It just seems important if we want to diag the symptom.
We're not talking about locating the sub vs the mains, which is why I said earlier that locatable frequencies are irrelevant to this discussion. We're talking instead about the same frequencies from 2 different sources, so relative volume is the whole bowl of wax here. Taking equal bandwidth from 2 sources can be applied to the mains as an example. What happens (psycho-acoustically) when the L/R mains are fed a mono signal (as all subs are) if the right main is outputting at +12dB higher a level than the left main?
You could argue what frequencies are more or less locatable, but that's irrelevant. The mains are assumed to be exactly equidistant from the LP. Even in that perfectly equidistant placement scenario, your head would lean to the louder speaker. Move 1 of them right next to you and the other at 4M away and it becomes clear (to me) the futility of gain matching. The balance control of stereo days was to level match the 2 speakers without needing 2 volume controls and is still used in car audio because you sit off center from the 2 sources.
I also wanted to talk a bit about distance delay settings vs phase control settings. Years back, Seaton helped me wrap my head around the similarity, but I had the problem seeing it because people were saying "the distance setting is the same as a phase control". No, it's not. A phase control, as I mentioned earlier, is an all-pass filter. That filter affects a wide range of frequencies, but more so at its center frequency and less so out from that point. An all-pass filter must have a center frequency, which value is never known on the phase control on a sub plate amp, but is usually generally aimed at 80 Hz.
Rod Elliot has an excellent article on building a subwoofer phase control: http://sound.westhost.com/project103.htm
The digital delay distance setting, OTOH, affects the entire signal (all frequencies in the signals BW) equally. The only way you could claim that the 2 methods are the same is in a single scenario; if you apply the same phase adjustment to 2 sources, there would in effect only be a delay and otherwise there would be zero change. Likewise, if you apply the same distance delay to 2 sources there will only be a delay and otherwise there is no change. Once you apply the effect of either method to only one of 2 sources, you open Pandora's box.
Because a multi-subwoofer systems response at the seats is primarily the result of the phenomenon of reflected wave interference, using a phase or distance delay control to attempt to alter that result in a constructive way is like playing pin the tail on the donkey. Placement is the primary tool. Everyone would prefer the press of a button to placement experimentation, but you'll always end up with a compromise in the end if you skip the placement step.
Finally, one of the reasons I prefer the up/down-firing sub configuration vs front firing is that it presents only reflected pressure waves. The front firing sub will always present the direct-radiated wave first and all reflected waves 2nd. When, for example, you apply a PEQ cut to tame a peak, you are dialing a dip in the signal that's sent to the sub. The front firing sub then presents that distorted direct-radiated sound first, followed by the "corrected" reflected sound.
It has become obvious to me over the past 15 years of infinite placement and tweaks variations in the same room that using the configuration and placement to achieve the flattest result is the far superior method to ignoring those factors and "running 'X' auto-EQ" or ham-fistedly distorting the input signal with a dozen PEQ filters. Of course, YMMV, etc.