Re: Time aligning subs to flown mains
I don't think there is a perfect answer to this question because of physics; but would like to ask some advice.
The key question I have is where do you take your measurement from ? There is a triangle between where the line array (A), subs (B) and mic position (C).
Now A & B are not moving, but you can put your mic anywhere; and because the distance between A-C, B-C will change, this means the time and phase alignment will be different. I'm aware of wanting to match the phase traces between the mains and subs in the crossover region.
So you've got to pick a point which I guess is a decent approximation for most of the audience ?
Anyone, anyone.... Bueller ?
Andrew
I think the question is a very good one
i would try to share my understanding (first rather theoretically) of this problem:
First of all, i would divide this problem into two very different parts:
1. The flown system is a fullrange system playing down to ~35...40Hz This of course includes systems, where part of the subs, or one kind of subs are flown with the system.
2. The flown system is HPfiltered at the same freq, where the stacked subs are LPfilterd (xover)
Generally, we need to consider two things when we do want to allign a system:
- relative amplitude
- relative phase
of the two subsystems.
we can say, that we could be statisfied, if we can reach an allignment, where the phase difference of the two subsystems are within +/-90 deg relative to the perfect match at every point of the venue. This - if the amplitude of the two subsystems would be the same - would result in a 0dB summation (and the perfect match was of course +6dB summation).
But of course the amplitude of the two subsystems do change also a lot at each point of the venue. We can in some cases even use this phenomen to reduce the effect of the non perfect allignment...
In case 1. We absolutely need to take care of the best possible allignment, because when we get cancellation, we could get it wideband in the whole subbass range... Despite this fact, many times this setup could give you a better result, because
the flown subs could throw longer without the problem, that the crowd stops the sound as a wall (which could happen with stacked subs)
In case 2. We would "just" get a dip around the Xover frequency, if we are unalligned.
For example in case 1: if we are closer to the stacked subs, we will hear more level from them, than from the flown boxes. (If they are flown high enough) so at the point, where we start having more than 6 dB hotter signal from the stacked subs, than from the flown PA the perfect allignment is not that important anymore, and where this difference is already 10dB or more, it will not even harm, when you are completely out of phase (180deg) Of course even then we still get the dipright around the LPF freqency of the Subwoofer, whis is not good, but not as catastrophic as a wide band cancellation.
we can clearly see, that because of the physics, the lower the Xover frequency (or the start of the overlapping range) is, the
longer the wavelengts are, and so the easier we can reach our best allignment or better formulatad, the wider the area will be, where we can make the allignment good.
Also we can see, that adding a tribune into the equotation makes the allignment even harder, as the variation of the time arrival difference between our two subsystems will be even larger. There could also help to have fullrnage flown system/ or flown subs making a long enough line to get a reasonably focused (vertically) LF energy even down to the lowest octaves.
togethere with the crowd bass shadowing effect (stacked subs) this can result in a scenario, where we get more sub SPL in the farfield, and especially on the balcony from the flown system's bass (relatively to the stacked ones), and at the same time in the nearfield we get less SPL from them (the flown system's bass) relatively to the stacked ones.
so in order to find the best place for doing the measurements for the allignment, we have to consider all of the above circumstances.
that also shows, that there is not such a thing that is a perfect place for doing the allignment for. It changes with the event geomety and the used PA.
anyway, you could play around these data described above in the prediction software of your PA, or if you do not have such a thing, you could even write a fast & dirty Excell sheet for that, using the in erse square law for the stacked subs and the well known equotations for the flown subs taking its total lenght also into account to get a picture about the amplitude differences of
the two subsystems, and generating the following two graphs as the result:
A/ subsystem1 & subsystem2 amplitude. Vs distance (here if you want you can include the non flat tribune as well)
B/ path lenght difference of Subsystem1 and subsystem2 vs distance.
based on these graphs you will clearly and easily find the best position for the alignment.
you might even calulate it in Excell.....
of course in a room or even in open air where we have big reflecting surfaces this will be even more complicated.
that would be so long to describe, that i do not even try it now.
at the end you might find considering all of the abov, that almost everytimes the best position is near the Foh
position.
Sorry for the long post possibly with lots of grammar errors
Tamas
