when two microphones, at different distances are used on a single source...the sound waves of that source arrive at different times to each microphone. Phase denotes the current state at which the wave form is in.
Let's take an example...One microphone is 2 feet away from the source and the other microphone is 5 feet away. Because of the difference in distances, the source's wave form will reach the first microphone before it reaches the second one. Sound is not instant...it take a little bit for sound to travel through the air, so it arrives late in time to the second microphone. When these are recorded you can view them graphically in a DAW and you will see one wave form begin and then a little while later the second one begins. These two signals are considered out of phase with one another.
Sometimes this can be pleasing to the ear and may not even be noticeable. However the further away you move the mic, the worse it may start to sound. We know sound waves have a positive and negative state to it's wave length (also called the waves polarity). It is possible to capture the postive motion of a wave with one microphone and capture the negative state at the same time with the second microphone. When you add two positive numbers together you get a bigger signal...or more amplitude away from our zero crossing. When you add a positive and negative number together, the number gets smaller...the closer our amplitude gets to our zero crossing.
When a signal is said to be 180 degrees out of phase, this is when the positive and negative sides of the wave form line up in time exactly and have the exact same amplitude. Create a sine wave and make a copy of that sine wave. When you play back the two together (started at the exact same time) the sound gets louder. Now turn (invert/polarity reverse) one of the two 180 degree...so now it's the opposite of the original. What happens? The sound disappears!! This happens all around you. Have you ever been in your living room and walked around listening to TV to notice that the sound seems quieter in one part of the room than it does in the other? Sound is canceling right there....frequencies are clashing with one another.
On mixers there is a polarity reverse/invert button that can help with this phenomenon when you are recording. It's not always needed, only when you hear it causing a problem. What this button does is flip the polarity of our electrical signal and makes the positive negative and the negative postive. Our wave form is now 180 degrees the other way to help correct the cancellation of sound we are hearing when combining channels together. In other words, since the wave form is coming in out of phase already (can be up to 180 degrees off), we just flip it back around another 180 degrees to equal 360 degrees.....which gets us closer to looking like our other signal.
{the above I stole from a previous post of mine}
Like I mentioned, this usually only has to do with mics at different distances. With drum overheads, if you use an XY pattern, it will be avoided because the mic capsules will pick up the sound at the same time.
An example of another out of phase issues would be when micing a snare. If you use a top and bottom mic, the sound of the snare would be PUSHING one mic while PULLING the other mic's capsule. So you would need to use the polarity button on the mixer to compensate for this.
I have some visual samples
HERE for this. Sorry, the audio samples on that link have expired...and I doubt I have them anymore.
