(WARNING: Major sound geek talk here. Proceed at your own risk!!!!)
Robert Scovill spent much more time in the master class at Willow this year discussing his concept of coherence then he was able to in the Thriving class last year and it really resonated with me. This one seems to get progressively more difficult to wrap your brain around the more you think about it and try to implement into your room.
The core question is can two like signals be in polarity yet out of phase? We’ll stop here for a second so you can re-read that question a few times.
The answer is obviously yes! The answer to this question is found in signal to noise ratio. I’ve always thought of this in terms of evaluating gear – low noise floor, quiet, etc. But in the world of coherence, signal represents the sound you want to hear, noise is the sound you don’t. Two signals in polarity that are also in phase will be arriving at the listener’s ear at the same time, although they come from two different sources. What is this called? Time alignment.
This concept effects everything we do as engineers and paying attention to it can add a whole new dimension to your mix. Scovi suggests there are three levels of coherence and success means experiencing the perfect storm of these:
1. Sound system. This one is largely scientific – minimal destructive interaction between PA boxes for any one listener, a high signal to noise ratio in the interaction of the designed array with the room, and high signal to noise ratio in the interaction of the PA and the stage. “All instruments are exiting the sound system at the same time.”
2. Performance. Scovi suggests this one is a combination of artistic discipline and scientific challenges/conclusions. This one is more band related – everyone playing in time, good inputs and execution.
3. Mix. Coherence in this area requires attention to detail – mic placement, inputs in polarity and phase with each other, and achieving the highest signal to noise ratio possible in the mix through successful use of gating, compression, and fader moves.
He gave an interesting example of the importance of phase and time alignment/interaction that I’ll never forget. He was mixing Tom Petty or Pink Floyd (I don’t remember which) in Europe as part of a large multi-day festival. The PA sounded awful throughout the whole event and he was dreading having to step up and deal with it in making his band the best they could be. The PA was configured with two rows, 6 or 8 speakers each stacked on top of each other per side, lined up side by side with no splay at all. Because of this, he made the assumption that the primary reason for sucky-ness was phase cancellation and comb filtering.
Just before the band hit the stage, he had the guitar techs go into secret ninja mode and unplug every other cabinet in the PA, alternating top to bottom as well, along with him bypassing the house EQs. The result was magnificent – easily the best mix of the festival. The PA crew was dumbfounded at the difference and in awe of Scovi’s skills when in reality the only thing he did different from all the other engineers was value the concept of coherence. (For the record, the techs plugged all of the PA cabinets back in when his set was done!)