Phase is an important concept and tool to be used by the modern producer. The relationships between difference perspectives of a source can make or break a recordings clarity. The theory behind phase relationships isn’t complicated, but its easy to forget how important phase is as a coherency of a recording.
There is more to phase than just polarity switching.
How Sound Gets Around
Acoustic Audio moves as propagations and rarefactions of air pressure. A sound will move throughout the air from its source and will be periodically compressed in a positive pressure and spread apart by a negative pressure.
Microphone diaphragms mimic our eardrums; moving one way for a positive pressure and the opposite way for a negative pressure. Where a microphone is positioned relative to this acoustic cycle will affect the resulting phase of the digital audio.
Phase can be defined as how far the signal is in its cycle, measured in degrees.
0° is neutral pressure
90° is positive
180 is neutral
270° is negative
360° is a complete cycle and once again neutral
Phase isn’t a concern until 2 or more signals are added together.
Adding it Together – Mo Mics, Mo Problems
Phase becomes a concern when more than one microphone is used to record a single acoustic source.
When two signals are IN phase, the two signals add together and become a louder, and bigger overall image.
When two signals are OUT of phase, the two signals will sound thin, wimpy, and disjoint.
This is because whenever the first signal is sending a positive pressure, the second signal is canceling it out with a negative pressure.
Frequencies present between the two mics are canceled as a result of:
Comb filtering
Comb filtering is caused by adding a delayed signal to the regular signal, causing destructive interference. The frequency response of a comb filter consists of regularly occurring notches in the signal’s spectrum, which looks like a comb. This destructive phase canceling usually removes a lot of low end information from the sound.
Phase has to do with time of arrival.
Flipping polarity will create a mirror image of the audio, making what is negative, positive and vice versa. (flipping the audio 180 degrees) but it is possible to be more precise with our phase relationship.
2 identical waveforms can be nudged out of phase to create a unique sound with more character. Destructive interference will drastically alter the frequency content of a recorded sound. This can be used as a way of altering the resonance or formant of a sound.
A formant is a vowel like resonance that exists in all oscillations. Whichever formant frequency is loudest will determine the vowel sound of the resulting audio.
This is great for creating Time of Arrival differences between left and right guitar tracks. Phase cancellation can be used aesthetically and isn’t always considered a bad thing. Use your ears and see what new sounds you can create with phase adjusting.
Well, What If I Want the Bass?
While blurry, hazy tones might sound great on the electric guitar, we want to be a bit more conservative with sources of bass frequencies.
- Bass guitar
- Toms
- Kick Drum
These sources will sound best if we preserve the phase relationship between any mics we use. Adding more microphones will increase the potential for phase problems. #MoMicsMoProblems
When using 2 microphones on a single source (ie. Top and bottom of snare drum or tom tom.) It is assumed that the 2 microphones will not be an equal distance away from the source. It is a common practice to flip to phase of the bottom microphone to correct this issue.
Listen to the fullness of the low end of the signal; this will disappear when the signals are out of phase. Experiment with mic placement and polarity until you get the sound you’re looking for,
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