Over the years, certain manufacturers have started to use the terms polarity and phase interchangeably. Although there are some similarities, there is a difference between them, and understanding that difference can help you make the most of any session, whether in a live sound situation, or in the studio recording, mixing, or mastering.
Polarity
Any electrical signal has polarity, and it is a reference to the signal’s position or voltage above or below the median line. A device that inverts the polarity of a signal will simply swap positive voltage to negative voltage and vice versa. Electrically, it is as simple as reversing the positive and negative terminals. Electrically summing a signal with an inverted-polarity time-synchronous copy of itself results in complete cancellation. On a mixing console, this is equivalent to swapping the “hot” pin (pin+2) with the “cold” pin (pin-3), which reverses the polarity. This is useful if you have two microphones on the same source but facing each other. For instance, you use two microphones to capture the sound of a snare drum. With the top microphone catching the snap and rim of the drum, while the bottom microphone captures the snare rattles. If we leave the polarity the same for both microphones, the results may be weak. If we reverse the polarity on the bottom microphone, then the waveforms will reinforce each other, allowing for a full and accurate sound recording.
Digging a little deeper, if we could see what’s happening when sound is generated, we’d find air molecules being disturbed and vibrating. Reacting in a cyclical fashion, the air molecules are pushed then pulled, then pushed, then pulled and so on. Those air vibrations are the force that move a microphone’s diaphragm or element. When the air pushes against the diaphragm, there is a rise above zero or “median line” in the waveform. When air pulls the element, there is a drop below zero. This pushing or pulling of air molecules is polarity. If you reverse the polarity, a push against the mic element will cause a drop below zero instead of a rise above. It will also cause a rise above zero for a pull of the mic element instead of a drop below. The point is, nothing moved earlier or later in time. We simply inverted the push/pull relationship between air pressure and its electrical/sampled representation.
Phase
Most mid-level to high-end mixing consoles will offer a polarity button, which is often labeled incorrectly with a ⌀ symbol or 180. Phase is delay and results in the offsetting of a signal in time. This can be the product of acoustical, electrical, or mechanical operations. It is a relative concept and can be visualized as circular. Comprehensive mathematics tells us that a circle has 360° of rotation before returning to the origin. The phase of a signal, relative to the starting point, is expressed in degrees. A 90° phase shift, as in a circle, is a quarter-rotation or a quarter of a wavelength. It follows that a 180° phase shift is a half-rotation or half-wavelength and a 360° phase shift is a whole rotation or full wavelength. This increase will continue infinitely; a 720° phase shift is two full wavelengths (or two rotations) and a 1080° phase shift is three wavelengths (three rotations) and so on. If you had two sine waves at the same frequency and you shift one of the signal’s phase by 180°, then the signal would cancel the original. Flipping the polarity would correct it. However, it is unlikely that we would be aligning two sine waves. We would be working with complex frequencies with lots of harmonics, which result in different phase timings depending on the frequencies being generated. If you were to use two mics to record a single sound source, with one mic at the source and the other mic some distance away, the sound would reach the farther microphone a moment later in time. This phase difference will generate some anomalies as the close mic’s signal and delayed “room” mic is arriving at two different times. Switching the polarity might help, but a dedicated phase tool, which allows you to adjust the phase variably between 0° and 180°, might be your ticket to getting the signals aligned and alleviating any sonic weirdness.
Tips and Tricks
Mid-Side recording uses polarity and allows you to record two microphone signals and then process them to create a super-wide soundscape that folds down to mono for broadcast applications. The “Mid” microphone is set up facing the center of the sound source. Typically, this mic would be a cardioid or hypercardioid pattern. The “Side” mic must be a figure-8 pattern. This mic is aimed 90 degrees off-axis from the sound source. Both mic capsules should be placed as closely as possible, typically one above the other.
Once recorded, you will need to decode the signal, which involves splitting the side signal into two separate channels. This can be done either in your DAW software or hardware mixer by bringing the Side signal up on two channels and reversing the polarity of one of them. Pan one side hard left, the other hard right. The resulting two channels represent exactly what both sides of your figure-8 “Side” mic were hearing. With three channels of recorded audio (Mid center channel and two Side channels), the signals must be balanced to recreate a stereo image. MS decoding works by what's called a “sum and difference matrix,” adding one of the Side signals, the plus (+) side, to the Mid signal for the sum, and then subtracting the other Side signal—the minus (-) side—from the Mid signal for the difference.
The Haas trick is a psychoacoustic effect based on how humans localize sound, and utilizes a simple delay to introduce a phase shift. You will need two copies of a mono track, one panned to the hard right, another hard left and simply delay one of the tracks by 1-35 ms (any longer will begin to create an outright delay). The source will appear to be coming from the non-delayed side, while producing a thick and textured sound.
That about wraps up this article on demystifying the difference between phase and polarity. Hopefully, it helps you with your production work and ensures you are using the correct nomenclature. For more info, tips, and tricks, check out some other articles in Explora or stop by the B&H SuperStore.
