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April 7, 2022

Vance Powell checking polarity on the mics




When Vance Powell says he’s going to “Click out the mics,” in the excerpt from “Start To Finish: Vance Powell Episode 10: Guitar Overdub Setup,” he’s referring to checking for polarity issues using a two-piece hardware device called the Galaxy Audio Cricket. Vance uses the Cricket to test a pair of mics on each of two amps getting mic’ed for the guitar overdub session for the song “Live and Learn” by The Weird Sisters.

Hearing Crickets

The Cricket system features two pieces: a Send unit and a Receive unit. It allows you to test polarity both wirelessly and with cables plugged into it. It also functions as a cable tester.

The excerpt begins with Vance in the control room with the Receive unit, with his assistant Mike in the live room with the Send unit. They’re setting up two different amps for the overdubs, a Marshall head with a 1x12 cabinet in the live room and a Fender Princeton in an iso room. Vance uses the same miking configuration for both: A Neumann U67 tube mic and Shure SM57.

On each amp, the two mics are aimed at the sweet spot between the center of the speaker cone and the edge. The capsules of the two mics are about as close together as they can be without touching and as close to the grille cloth of the amp as the U67’s shock mount will allow. Keeping the distance as short as possible between the mic capsules close helps assure that there won’t be phase issues between the two mic signals. However, a miswired cable somewhere or a component with a phase-reverse button left in reverse mode could cause one of the mic tracks to be out of polarity, which is why they’re clicking them out.

Vance’s mic setup on the Fender Princeton.

Only a Phase

The word “phase” is often used interchangeably with “polarity,” but they’re not completely the same. Polarity deals with voltage; it can only be negative or positive. The audio signal from sound waves converted to electricity by a transducer such as a microphone will have positive and negative peaks. If the polarity is reversed, the positive peaks become negative, and the negative become positive.

Phase, however, is about time. That is, how sound waves are aligned in time, relative to each other. Unlike polarity, two signals or soundwaves don’t have to be 180-degrees reversed to be out of phase. They could be as little as one or two degrees out of phase. That said, the larger the phase difference, the more likely you are to hear comb filtering.

Even if you don’t have any reversed polarities in a multi-mic setup, you could have phase issues based on mic placement. If the mics are a different distance from the source, they’ll reach the microphone at slightly different times, also degrading the sound. Drum mic setups are the most tricky because they feature multiple mics in different positions covering the same source. That’s why engineers experiment with the position of phase switches on various channels when setting up for drum recordings.

However, in this guitar amp mic’ing situation, Vance’s setup puts the mics so close that significant phase shifts are unlikely. However, it’s possible they’ll encounter cables or outboard gear with their polarities reversed.

We Really Clicked

Mike goes into the live room and places the Send unit in front of the capsule of the U67 on the Marshall. He turns on the unit, which starts clicking. The clicks are picked up by the mic and reproduced on the studio monitors in the control room where Vance holds the Receive unit up to the speakers. If the Receive unit’s indicator light turns green, there’s no inversion of the signal’s polarity. If it’s red, there is. In this case, Vance gets the green light, which he subsequently gets for the 57.

The front and back panels of the Cricket Send and Receive units as pictured on the Galaxy Audio website.

Next, they check the mics on the Princeton, which are going through outboard Neve 1073 preamps. The Cricket indicates a polarity reversal so Vance goes over to the 1073 and presses the phase-reverse button, which had been left in the reversed position, to change the polarity back. “That’s why we check these,” he says.

After that, Vance mutes the first guitar bus, channels 17-18 and checks the polarity of the signal coming through the second guitar bus, channels 19-20, which is a stereo channel in Pro Tools. Afterward, he takes the clicker unit, plugs it into his console, and sends it to the Marshall’s input to make sure the polarity is correct in the signal path going to that amp —the guitarist will plug in his pedalboard in the control room. The signal will go through the input path that Vance just checked to get to the amp.

What’s the Trouble?

In a multi-mic recording situation like the guitar overdubs Vance was setting up for in the excerpt, polarity and phase are particularly important if the tracks are summed to mono. The signal degradation resulting from an out-of-polarity component is less noticeable in stereo, but the stereo image can shift unevenly.

In the following example, you’ll hear a four-measure rhythm guitar part recorded with two mics in a similar configuration to what Vance used and summed to mono. When it repeats, one of the tracks is switched out of polarity.

Notice the way the level drops off from the comb filtering.

Next, here’s an example of the stereo shift that can happen with tracks panned left and right. In this one, recorded with the same mic configuration as the previous example, the track polarities match the first two measures. In measure three, one of the tracks has its polarity flipped, and in measure five, the polarity is again flipped so that they match again.

The image shifts pretty significantly. You might be interested to know that image processors often use controlled phase shifts to create their effects.

No Cricket, No Cry

You can use plug-ins to check and adjust polarity and phase. For the former, most channel strip plugins offer a phase reverse button (which is really a polarity reverse button). If you’re working with a source recorded on multiple mics, listen to the sound of the mics summed to mono and try flipping the phase button on one of the channels. If it sounds a lot better when you do, it’s possible the polarity was reversed somewhere in the recording or playback signal chain or the two tracks were out of phase.

The UAD Little Labs IBP

If you want to make phase adjustments smaller than 180-degrees, you’ll need a plugin like the UAD Little Labs IPB, which lets you reverse polarity and adjust phase with continuous knobs. That makes it possible to experiment with minor phase changes to see if you can dial in a better-sounding result. It also has a phase invert switch that you can use.

Another option is Waves’ InPhase. In addition to phase reverse, it gives you a graphic display of the waveforms from each track and lets you line them up visually if you want. Both plug-ins are quite powerful, and you can find other plug-ins available that allow for phase adjustments.

Waves InPhase.