Sebastian wrote
Now forgive my ignorance with the seemingly obvious point raised numerous times on the phasing issue, but isn't that solved by as simply as inverting the phase of one of the mics if such an event were to occur?
No, it's not that simple.
Firstly, I don't want to be a stickler but the correct term is "inverting, or flipping, the polarity". This is an amplitude operation that is used to shift the summed response of two similar but slightly delayed waveforms into something potentially more usable and more "in phase". It is not always a remedy. I'm going out on a limb here but it is most effective when the difference in phase of two similar waveforms is 90/270 degrees or less/more, respectively.
Now, phase is related to distance which in turn is related to time because sound travels at a [relatively] constant rate. The rule of thumb is that sound is delayed 1ms per foot, which is helpful to remember when creating slap back delay/ambience manually with microphones. However, we're often dealing with values smaller than that in double close miking a guitar amp, maybe a few ten thousands of a second, and this is when things become tricky. This is because frequencies will start to cancel each other out in regular intervals (as described above by ryanguit) depending on the discrepancy in distance between the two mics and this is how a comb filter is introduced. This can thin out a guitar tone very quickly. It can also be a very helpful tone shaping tool, depending on what you're doing and whether or not you KNOW what you're doing. Then there's the issue of peripheral/room sound that both the mics captured that will also be inherently delayed because of the nature of reflected sound. Those sounds too will interact with each other, which can bring another dimension of difficulty to this already complicated can of worms.
In the case of a guitar amp recording, if you ensure both mics are the same distance from the cone/s you should not have a problem with phase issues. Once we get down to differences of millimeters the delay between the waveforms approach a few hundredths of a millisecond, which is almost negligible.
Like I said before, it is way more preferable to me to use two mics with complimenting responses at equal distances from the cones than two of the same mic put in some "clever" configuration for the sake of it. I also vastly prefer one mic setups. Not to say that I don't experiment - I do - but for me, the simplest solution is often the best one.
Cheers ?
