Phase and polarity in audio are often misunderstood as both terms are used interchangeably when in fact they are both quite different. This video will explain both in simple terms, so you can understand how to use them to your advantage in every mix.
Learning how to mix is more than learning how to use EQ compressors and effects. At some point, you've got to take a step back and make sure you can trust what you're hearing to turn out better and consistently good mixers, you need to ask better questions of yourself and what you're hearing to make more informed decisions, and you can't do that if what you're hearing is flawed and it's not providing you with a true representation of the instrument recorded. Whilst polarity and phase aren't as sexy as reviewing the latest fancy EQ plugin, it is vitally important to understand what each one is and how they have the potential to ruin and mix if ignored, and especially the ramifications on the low end of your mixers.
Polarity and phase are two different audio and electrical concepts that are often confused and misunderstood. So today I want to explain both because getting them working for you instead of against you is the first step in getting any EQ and compression processing to work to your advantage.
Musical instruments create sound by modulating the air producing sound waves. Sound waves are the pulling and pushing of the air molecules, and these changes in pressure vibrate our ear drums in the same way and are interpreted by our brain into the sounds we hear a microphone capsule, sensors these same pressure changes, converting them into positive and negative electrical impulses, which are fed into our recording equipment. The pushing of the air molecules is called compression, and that produces a positive voltage, and the pulling is called rarefaction and produces a negative voltage in a circuit.
Unfortunately for us, there are a myriad of places that this simple concept can get screwed up. A cross-wired cable on one speaker, for example, means that the speaker cone is pulling when it should be pushing air, and if only one out of your stereo speakers is doing this, then your speakers are said to be out of polarity and the sound is impaired in some way, typically like narrowing the sound stage or reducing the bass.
This isn't just reserved for speaker connections. It can be any connection between any piece of equipment can cause this switch in polarity. So polarity is the positive and negative aspects of an audio signal, and we see this as the plus and minus across a median line here shown using a simple sine wave just to simply illustrate the point. So polarity has nothing to do with time or frequency. Just think of the positive and negative terminals of a speaker or an amplifier and the forwards and backwards motion of the speaker cone, that push and pull of air. Positive values push the speaker cone out and the air towards you and negative values pull the speaker comb back and the air pressure away from you. And Conversely, the capsule of a microphone works in exactly the same way, reacting to the changes in air pressure that a sound source will produce. And when a piece of equipment is out of polarity, it's cross wired, so positive is attached to negative, and negative is attached to positive, and this means that the speaker cone is pulling when it should be pushing, and this can happen anywhere.
Cables inside equipment, even microphone cables can be wired incorrectly, and sometimes they can be a real struggle to try and find.
Fixing this switch in Polarity is relatively straightforward. The button with the diagram of a circle with a line through it on a channel strip or a mixing console, switches the polarity of a signal 180 degrees, flipping it to the mirror image of itself. So if the red line on this diagram was the original signal, the polarity button will flip this inverting it so that the red line then becomes the blue line. If you get two identical signals being played back at the same time and one has the polarity flipped in this way they will cancel each other out.
Remember, these are positive and negative values we're seeing here voltages that sum together to get you right back to zero again and zero is silence.
One area typical of needing this polarity flip is on a snare top and a snare bottom microphone. They are more often than not out of polarity with each other. One is pushing the bottom whilst the other is pulling the top. Think of the stick hitting the top skin of a snare. The skin itself depresses away from the top microphone and is therefore pulling and at the same time pretty much the bottom skin is pushing it's, bowing out with that shift in air pressure inside the body of the snare, and therefore the bottom microphone is picking up that pushing movement, and this results in that thin or hollow sound as frequencies are being cancelled out between the two microphones.
And if the top snare mic is in the overheads, then hitting the polarity button on the channel strip or an EQ plugin, for example, inverts the bottom snare track, and it will magically bring back that lost body fullness of the snare. So how do you identify if something has been recorded with the polarity inverted? And how do you identify if it really masters? When you look at the waveform, you start to see the similarity between that sine wave diagram that I showed you earlier where you can see that the median line runs through the middle of our waveform and everything above it is the positive voltage or signal and everything below it is the negative.
And when you look at the leading edge of the signal, you can see that the polarity of this signal is heading down in the downwards direction.
Therefore, the leading edge is of a negative value. Now, when you see this on a snare drum. Your instinct may be just to decide that this is out of fear or out of polarity, whichever phrase you happen to have habituated into your head. And this is the main clue that should trigger some sort of thought process where you might need to cheque if this is actually causing you a problem with the sound sourcing question. So when it's a signal on its own, it's not a problem, but when it's a signal that's been picked up by multiple mics, it can be a problem here.
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