Which way to flip it? That's the question at hand in this excerpt from the Puremix video "Jacquire King Mixing James Bay, Part 1." The "it" in this case is the polarity of the bass on the song, “Let it Go.” We pick up the action with the mix already underway and King working on the bass sound. It was recorded to both a DI and miked-amp track, and both are routed through a single bass bus.
POLAR OPPOSITES
When King is talking about polarity here, he's not referring to phase differences between the DI and the mic tracks, but rather the polarity of their combined signal that's coming out the bass bus.
He says that accidental polarity reversals happen a lot with electric guitars and basses because sometimes when you put other pedals in line, one might be wired backward and that causes the polarity to flip. Reversed polarity means that the electrical signal representing the sound is negative instead of positive and the waveform is inverted.
Before dealing with the polarity, King checks that the individual DI and mic tracks are not significantly out of phase with each other ("out of phase" and "out of polarity" have different meanings, which we'll get into a little later in this article). He tests this by flipping the polarity button on a Massenburg DesignWorks MDW EQ5 plug-in that's inserted on the bass mic track while listening to both tracks. The result is a thinner sound, so he concludes it was better before he changed it, and he flips it back. Although they're not perfectly in phase, he says, they're close enough. He uses the phrase, "in agreement."
Polarity-reverse buttons are often found on channel strips and EQs, such as the Massenburg DesignWorks MDW EQ5 plug-in, which King uses on the bass tracks.
He points out that while the phase relationship between elements is essential on a multi-miked source, the polarity of their combined sound should be positive, so that the first transient pushes the speaker out, rather than pulling it back. If the polarity gets accidentally reversed, it will start by pulling instead of pushing the speaker.
If the bass and kick are hitting together and one is pushing, and the other is pulling, it will have a "nullifying effect," according to King. In a case like that, he says, you'll end up "chasing your tail" trying to figure out how to make things sound like they're sitting correctly together, and you'll have difficulty getting the low end to glue together. It's an issue that he pays a lot of attention to.
Finally, he plays a section of bass again and compares the sound with the polarity flipped and unchanged. He observes that the flipped position sounds deeper and comes forward more and works better overall, so the bass was out of polarity before he flipped it.
PHASE VS. POLARITY
You've probably heard the term "flip the phase" or "phase reverse" or "phase invert" a lot. Although they use the word “phase,” those terms actually refer to polarity. Polarity refers to the positive or negative voltage of the signal. Reversing the polarity results in the inversion of the waveform, so positive is negative and vice versa.
The polarity of these two waveforms is the same.
The bottom waveform is out of polarity.
Phase, on the other hand, involves time. Let's say you use two mics to capture a snare drum. The waves will arrive at slightly different times to their respective microphones, so they'll be out of phase with each other, because their starting points are somewhat different, and when they play back together, an effect called "comb filtering" will cause the signal to cancel at certain frequencies and be reinforced at others.
These waveforms are in phase with each other. Their peaks and troughs are aligned.
The bottom waveform starts later than the top, so they're out of phase.
FILTERING DOWN
So what does comb filtering sound like? Not too pleasant, that's for sure. Let's take a listen.
Example 1: Here's a snare drum, copied to two tracks. For the first four beats, the tracks are entirely in phase. For the next four, one of the snares was shifted forward by 100 samples. For the last four beats, the same track was shifted forward by another 100 samples.
In this screenshot of the Example 1 audio, notice that the second and third groups of snare hits drop in amplitude as they get more out of phase.
What would happen if you took the two identical tracks and reversed the polarity on one of them? You'd end up with nothing because they'd cancel each other out. We'd include an example, but all you'd hear is silence.
Now let's check out an example of the issue King was most concerned with, a situation where a polarity mismatch causes the kick to push the speaker while the bass is pulling, or vice versa. As he said, it's not an absolute rule that they must have matching polarities in order to sound good, you might like the out-of-polarity sound in some situations. But it is something to check for if you're having trouble getting the bass and kick to sit well together.
Example 2: This features bass guitar and a kick drum. For the first four measures, their polarities are matched. After a slight pause, they repeat, this time with the polarity reversed on the bass. In this case, they sound a little different, but it’s hard to tell which one is better.
