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There are two meanings of 'phase' in common use...
One refers to cyclic phenomena.
Suppose Mr. A works during the day on a 12-hour shift. So does Mrs. A.
So they are in phase and can enjoy their evenings and nights together.
But the Mrs. A's boss puts her on the night shift. Now they are out of phase. More precisely we could say that they are 180 degrees out of phase, 180 degrees being half of a full circle.
And then that nasty boss changes Mrs. A's shift again so that it starts 6 hours after her husband's. 6 hours is one quarter of a full cycle of 24 hours, so we say they are 90 degrees out of phase. We could go further to say that Mrs. A lags her husband by 90 degrees. Mr. A therefore leads Mrs. A by 90 degrees.
You can apply the above logic to any repetitive phenomenon. Nocturnal animals are 180 degrees out of phase with diurnal animals for example.
If you take a single frequency of sound, which would be a sine wave, then clearly two sine waves of the same frequency can be in phase, out of phase, shake it all about of phase if you please.
Phase only has practical meaning to us when there are two things that are repeating, and they are repeating at the same frequency. We use phase to compare how much one leads or lags the other.
Now take two loudspeakers...
Wire them up to your power amplifier, but take care to connect one of them red-to-red, black-to-black, and the other one red-to-black, black-to-red.
Many people would say that you had wired them out of phase.
Play some music through them. It will sound odd.
But is the music in some sense out of phase?
Well it isn't really the same thing. It would be more proper to say that the sound coming out of one speaker is inverted in comparison to the other. There is no lead or lag involved. But so often you will hear the term out of phase that it's probably best just to get used to it and infer the meaning from context.
There is a close link between '180 degrees out of phase' and 'inverted'. When two signals of the same frequency are 180 degrees out of phase, one indeed does look to be inverted in comparison with the other.
If we wanted, we could say that the signals are in opposite phase, or even in anti-phase.
So why is anti-phase bad, so bad that you want to get rid of it?
Well here are two examples...
One is the example of the loudspeakers. When one pushes the air, the other pulls. This causes one of your ear drums to be pushed in while the other is pulled out. There is nothing in nature that can cause this effect so the brain has no way of interpreting it. It sounds bad and no-one has yet come up with any kind of artistic creation that uses it in a positive way.
The other is where a microphone picks up a signal direct from the sound source, plus a reflection from a nearby hard, flat surface.
Because the reflection has longer to travel, it will be delayed. At a certain frequency it will be delayed sufficiently that the reflection lags the direct sound by half a wavelength. At that frequency, the direct sound and the reflection are 180 degrees out of phase. When they mix in the microphone they will partially cancel. This is sometimes called phase cancellation and it is bad because it affects the frequency response of the direct signal with a phenomenon known as comb filtering.
In music and audio, both examples show up anti-phase as a bad thing to be gotten rid of.
But there are situations where anti-phase can be put to good use...
For example if you have a situation where there is a noise source that is repetitive, like an aircraft with a propeller engine. Then if you capture that sound and beam it in anti-phase through loudspeakers in the cabin, the noise can be reduced to a worthwhile extent.
Anti-phase can be used in music and audio in noise-canceling microphones, noise-canceling headphones and of course the phaser pedal effect.
So how to get rid of anti-phase?
Back to the question, how to get rid of anti-phase.
Usually it is enough to be aware of situations where phase might be a problem. Then when something doesn't sound right you can almost always do something about it.