Phase is the time relationship between two or more sound waves. Phase is measured in degrees and starts at zero and ends at three hundred and sixty degrees. This also completes one cycle. So one cycle starts at zero degrees and ends at three hundred and sixty degrees. A quarter cycle is known as ninety degrees and half a cycle is noted as one hundred and eighty degrees. It is best to use a sine wave to visually understand what is happening. If two waves are out of phase from each other then the peaks of the waves are not aligned. This creates cancelation in the sound. When the peaks are inline they reinforce each other and create a better sound.
Waveforms are very complex and are not alike especially with different instruments. If they are identical waveforms they are a duplicate. Knowing this comes in handy when using two different microphones on the same instrument. Phase – coherent is when two wave forms start at same time. Out of phase or phase shifted is when the two waves start at different time. To correct this you must nudge the waveform until they have the same start time. Phase inverted wave is when two waves have the same start time but there amplitude is exactly opposite or inverted. To correct this you must invert one of the wave forms one hundred and eighty degrees. If two exact waves were one hundred and eighty degrees from each other you would hear complete silence. These minor timing differences in waves can make or break a great performance or a mix. It’s always important to check phase as not all out of phase material is audit able.
Izhaki, Roey; Mixing Audio; Concepts, Practices and Tools; Second Edition