phase is a component of time. It's a total natural occurence in sound waves. However, it can be harmful at times.
Phase denotes the current state at which the wave form is in. It can be efined as: the time position (or angle in the complex plane) within a cycle of a periodic waveform.
The example you are familiar with is when two wave forms are clashing with one another. We say these two wave forms are out of phase. Take for example two microphones set at different distances from their source. 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. Then of course there are reflection around the room that can ALSO arrive later to each microphone. When these waves are recorded you can view them graphically in a DAW and you will see one wave form begin and a 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. And once you get it far enough away it becomes an audible delay instead, and you don't really notice phasing issues.
We know sound waves have a positive and negative state to it's wave length (which is 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. Some software programs call this "invert"
Now, when you do that, 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 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 physically 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.
I personally prefer to physically move the waveform on that track in my recording software BACK in the timeline so that the beginning of the first sound lines up with the beginning of the same sound in the other microphone. That's just me though. Others like the slight delay sometimes and think it gives color to the signal.
