Phase is the term used to describe what stage a wave is in, within its its period cycle. As sound waves are emitted from the sound source they interact with other reflected waves, or waves from other sound sources. When they interact, issues arise dependent on the waves signature, as I stated before issues with frequency and amplitude occur.
A key problem when negative phase issues occur, is with mic placement. As the mics picks up sound from varying distances from the same sound source, the harmonics of the sound interact, can cause problems called comb filter, which generally not a desired effect.
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