More complex phase issuesYou can get phasing issues which are more difficult to get rid of due to reflections in your recording room.
When you sing into a mic, the majority of the sound goes directly into the mic. Some of the sound will go indirectly into the mic, first bouncing off a wall/window, floor or ceiling. This sound will arrive delayed and therefore you can get cancellation. It is a problem in relatively small rooms with no treatment and is usually referred to as 'comb filtering', since complete phase cancellation occurs at various frequencies across the spectrum - if you open up a frequency analyser on your track, it looks like a comb
. Big troughs at regular intervals between the peaks.
Obviously, close-miking will mitigate some of these effects since the direct signal will be much louder than the indirect one. However, you don't need to get too far away before it becomes a problem.
I found an old sound-on-sound article referring to guitar cabinets which said:
"
If the distance from the cabinet's speaker cone is only six inches, and the floor is a foot below the mic, the direct and reflected sounds of the cone will meet at the mic capsule with around 1.5ms delay between them. In theory this will give a comb-filtering effect with total phase cancellation at around 300Hz, 900Hz, 1.5kHz, 2.1kHz, and so on.
"
This is assuming that the reflected sound is more or less identical to the direct sound, which of course isn't the case in the real world, so you never see total cancellation, which is why the effect can be difficult to spot.
But, you can certainly here the difference once its been pointed out and corrected!
You can't really treat this kind of phase problem in the mix - the only reliable remedy is acoustic treatment. A blanket behind the singer can work wonders.