I’ll be honest, I’ve been putting off writing this one. Mainly because this could go down the rabbit hole very quickly and I don’t want to do that. So I’ll say at the start that I’m going to keep this fairly simple and not delve into a deep treatise on phase. I would suggest you listen to our podcast with Bob Heil if you want to learn more about phase. He’s pretty smart in that area. With that out of the way, what’s the difference?
Phase has a time component, Polarity does not.
That’s about as simple an explanation as I can give. Polarity is the reversal the positive and negative terminals in a balanced circuit. As you may recall from our previous discussions of balanced and unbalanced circuits, you’ll recall that a balanced circuit has a positive pole, a negative pole and a ground. It is said to be balanced because the voltage that exists on the positive side is the same as the negative side; it’s just that one is + voltage, the other is - voltage.
When you press the polarity button—which is often labeled with a ⍬ or ⌀ symbol—what you are doing electrically is swapping the positive and negative poles of the input. That has the effect of flipping the phase 180° relative to itself. If you were to have two mic’s right next to each other pointed at the same source, flipping the polarity on one would cause a near total cancellation if you brought both channels up.
Down the Rabbit Hole
Here is a pretty vast simplification of what we’re talking about. Thanks to the desmos graphing calculator for the visuals. Below are two signals that are out of polarity with each other; that is, we’ve swapped the + and - making them 180° out of phase. For every + voltage on one signal, there is a corresponding - voltage, which would cause total cancelation of the signal.
Below is the same thing, only the polarity of both signals is the same. It’s hard to see but both signals are overlaid on top of each other. In this case, the resulting signal would be twice as loud as one of them alone because they would add.
To further our discussion, below is one with one of the signals shifted over in time. This causes a phase shift.
Phase is Time
When I took the Rational Acoustics Smaart course last January, Jamie Anderson said, “Phase is the most demonized and BS term in the industry today.” He also said that, “Filters don’t have phase shift; filters are phase shift.” Phase has a time component, whereas polarity does not. I said that again in case you missed it the first time.
So don’t say, “Flip that channel out of phase, would you?” Instead the correct phrase is, “Flip that channel out of polarity, please.”
Why Change Polarity?
You may want to change polarity for a few reasons. When I mic up a Leslise 122 rotary speaker cabinet that is being driven by a Hammond B3 (we’re getting really technically correct here at CTA today…), I like to put two mic’s on the top horn. I place them 90* to each other and flip one out of polarity with the others. That results in a really wide stereo image of the top horn.
I also find when I have an interview situation on stage polarity reversal can come in handy. Let’s say you have a pastor on a headset mic interviewing someone with a handheld mic. The headset mic may also pick up the other person, but because they’re a foot or two away, it will be out of phase with the handheld mic (phase is time, remember?) Sometimes flipping one of them out of polarity will minimize the phase interaction. Basically, that shifts the phase offset by 180°, which may make it less destructive.
When mic’ing a snare drum on the top and bottom, you’d want to flip the polarity of the bottom mic. The wave front coming from the bottom of the snare is 180° away from the top of the snare, and you need to flip polarity so you don’t get cancellation. That’s a bit of a simplification, but try it sometime. The reason you don’t get complete cancellation is because is because rarely are the mic’s pointed straight up and straight down at the snare heads, so you’re not technically 180° out on either of them. But if you don’t flip the polarity on the bottom mic, there will be some cancellation, which will make the resultant sound very thin.
Well, we ended up down the rabbit hole after all. I made a video a while back that demonstrates some phase shift concepts that may help further explain the concept. Check it out if you want to learn more.