Does anyone know of a good exercise to learn to identify what phase cancellation sounds like in the field? Is there a spectrum to this problem, or is it kind of binary? I don't think I've ever really run into phasing issues, but I'd like to learn what to listen for when I'm using a lot of mics. Thanks!
The first thing is a loss of bass, this is the easiest to hear. If you are summing lots of mics then the easiest thing to do is invert the phase on each mic as needed, if the bass disappears then switch it back, if the bass suddenly appears then leave the phase switched. Always leave the most important mic in phase and then work outwards from there.
As already mentioned above, when summing a number of signals (from stereo to mono) phase cancellations can be made more apparent and evident through a change in the quality of the sound.
Phasing is basically the result of comb-filtering - effectively and addition of a delay copy of the sound to itself (except for polarity reversal where the results can be more dramatic). An excellent 'naturally' occurring example of which is when a broadband noise source arrives at the receiver (mic or ears) both directly and while simultaneously reflected off a flat surface. Walking towards or away from an air-conditioning unit for example it is pretty obvious. You can even stand facing a hard surface (like a wall) make a constant sshh sound and move gradually toward and a way from the wall (1 foot to a few inches). After a while you should clearly hear the characteristic sweeping effect which is created by a change in frequency characteristics of the sound by the addition/subtraction of a delayed 'copy' (a function of the distance and the speed of sound).
Once you get a good sense of the effect, it becomes a little easier to pick up even without it needing sweep up and down the spectrum. And then it will start to stick out and sometimes bug you like a speaker moving closer or further from a desk or table while being miked/recorded ; ) I realize this is not the precise scenario you are talking about, but learning to recognize the characteristic qualities of comb-filtering may help to pick it up more readily in other situations.
I think there are two types of phasing errors in aesthetic sound (i.e. this is not a definition in sound physics).
1) You sum one signal to another and hear that it affects something in the original signal negatively (e.g. some bass or other frequencies cancel out). When it happens to bassy sounds, it can sound like the sound "loses its power" or "punch". This can be only identified by comparison (i.e. soloing the original source and then listening to how summing another one to it changes the whole).
2) You sum multiple similar signals (e.g. mics that record the same source, but from different positions or two synthesized signals of which another one is delayed) and you hear a kind of smearing or sweeping EQ/filtering, which may be periodic. The audio effect "Phaser" takes this to the extreme and I believe this type of phasing always sounds kind of unsettling/unnatural, especially in stereo (because you get these Haas-type effects in the stereo field), unless it's been specifically added as an effect.
It's important to note that a "phasing error" means a negative change in sound (something is lost, the sound sounds worse) that results from summing/combining multiple signals. Phasing itself happens in everything where multiple signals and channels are summed.