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Old 27th September 2016
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Hi Jules
I'll answer all your points one at a time
Time Aligned to what?

Interesting question. It depends where you measure from and how the PA is flown or stacked. I should say that I have a system tech who flies the PA and makes all the measurements using measurement algorithms. I'll come in when its as ready as he can get it. I'll then play a track from last nights show and wander around the venue upstairs out wide, all the way to the back and get a sense of how well covered the space is.
Often you will start by time aligning to where the FOH engineer stands to mix. Whichever part of the PA gets there first is delayed to the other(s). As different frequencies have different wave lengths it is never 100%. You often end up with compromises. Each position throughout the venue has a different set of delay time as speakers will reach each position at slightly different times. So its not purely science but also creative. I'll start by listening to the main system and the sub. Most important is aligning the sub with the flown PA. usually my tech will have time aligned a frequency that is in the flown PA as well as in the sub.Say 60 or 80 or 100Hz. Sub has a long wave and is hardest to get an even spread. There are so many ways to align sub. An array with several stacks in the middle of the stage with differing times in each stack pushing the sub for ward in a semi circle. Car dioid with some of the box pointing back at the stage but phase reversed to control the amount of sub going backwards onto the stage. Two big stacks L and R for a big slab up the middle. You may have a strong bottom end coupling up the middle of the space which pleases the engineer but will make for a disappointing experience for people stood outside the "power alley."If possible you want people to all have a relatively equal experience wherever they are listening from.
You often have infills to align to the flown PA, flown sub, ground stacked sub, out fills, delays. I'm honestly not an expert but the one thing thats most important I think is to trust your ears more than any computer readout.

What sort of delay times are used?

we usually work in milliseconds

If everything is time aligned, are the main speakers either side of the stage delayed? or are they the "master"?

see above

If the mixing position is halfway down the field, is there a lag in live mixing fader movements / actions?

Sure sound travels slowly. More or less 3ms a metre. Or a ms a foot for the pre metric folks. If sound travelled at the speed of light our job would be so much easier. So 30 metres down a field I'm hearing the drums 90 ms or so after each strike.I don't consciously make an allowance for this but if I'm spinning in a delay on a word or dub reverb ion a snare I think, with repeated experience, my body just kind of subconsciously allows for the delay. I'll start twisting the aux send knob before the word and bring it down before it ends. It happens so naturally that now the next time I'm doing it I'll screw it up because thinking about this question will make me self conscious:-). Its like driving you don't consciously think time to push down the clutch.

Does out of sync clapping along "do your head in"?
Ha ha not really. I'll sometimes push the snare and hat to try and get the more rhythmically challenged audiences back in time.
Out of tune guitars or singing do my head in more.
Now you've got me started:
-people talking loudly at the FOH whilst I'm working. I'm really territorial during the show and will throw badly behaved liggers, cameramen, family members etc. out of my world.
-those nodding bucket lights that always seem to end up pointing in my eyes.
-strobes whilst I'm placing the mids on stage
-TV crews that arrive during the intro music and hand you a minijack and ask for an audio feed.

I could go on all day being a grumpy old roadie.... I'm joking, I'm actually genuinely happy doing what I do and as my friend Hunter who is Massives LD always says "hashtag first world problems"

Is the audio in your headphones "advanced" compared to the audio around you at the mix position?

No I put a delay on the monitor bus that feeds my cans.

How do you work with 360 degree stages, where you have this big Bowl with the stage in the middle of the hall and your mixing position is fixed to one side?

Never done that but watched my mate Dave Bracey do it brilliantly well with Adelle recently.
I once did a show in Wembley stadium when the Gangnam style guy performed on the roof of the FOH. that was interesting. I also had an artist (who shall remain nameless) who complained about the weird delay we were putting on her voice as she walked out onto a thrust. Quick physics lesson about how slow sound is etc. see above, sorted the problem. We weren't putting a weird Elvis slapback on her voice to mess with her, honest.