Whether we're dealing with sound or with audio, we're dealing with waveforms that have peaks, troughs, and polarity. Microphones act as transducers that convert sound into audio and, therefore, also deal with polarity and phase.
What is microphone polarity, and how does it affect mic signals? Microphone polarity is thought of and noted in two ways. The first compares the movement of the diaphragm to the direction (positive or negative) of the outputted audio signal (AC voltage). The other tells us which pin (2 or 3) of the balanced output carries the positive signal versus the negative.
What is microphone phase, and how does it affect mic signals? Phase refers to the amount that a wave has passed through its cycle. Phase does not have a major effect on single mic signals but will affect two or more mic signals when mixed together. Two identical signals in-phase sum together while two identical signals out of phase cancel out.
In this article, we'll discuss the importance of microphone polarity and phase and their relationships to sound, audio, and, of course, microphones.
What Is The Difference Between Microphone Polarity And Phase?
Polarity and phase are often misunderstood or understood to mean the same thing. Let's take a look at how they are different.
Phase refers to the shifting of a waveform in time relative to another waveform or another point in time. We measure this shift along a 360° cycle.
Polarity refers to positioning the waveform “right-side-up” or “upside-down.” We measure polarity as being either positive (right-side-up) or negative (up-side-down).
Let's look at a photo of two sine waves to help illustrate these points:
In terms of polarity, we can see that the blue and pink sine waves are in opposite polarity. They are the flipped image of one another. When one hits its peak, the other is in its trough.
Let's say that the blue sine wave has a positive polarity and the pink sine wave has a negative polarity.
In terms of phase, we can see that these two waveforms are the same but have been shifted by half a full cycle. Using the 360° measurement, we can see that these two waveforms are 180° out-of-phase.
It's critical to note that reverse polarity does not mean 180° out-of-phase, though both yield the same visual and auditory result.
Once again, polarity refers to the flipping of positive and negative amplitudes, while phase refers to the shifting of waveforms along the 360° cycle.
For more information on phase and polarity as they relate to audio, check out my article Audio: What Are The Differences Between Polarity & Phase?
Microphone polarity describes 1 of 2 things:
- The polarity of the output signal when the diaphragm is pushed in versus the polarity when the diaphragm is pulled out.
- The polarity of pin 2 versus pin 3 at the microphone output. This is a typical specification of balanced microphones.
Before we move on, let's quickly describe balanced and unbalanced audio:
What is balanced audio? Balanced audio carries the same audio signal on two pins: one pin carries a positive polarity version while the other carries a negative. A differential amp sums up the differences between the two pins at a balanced input, effectively ridding any common interference noise on both pins.
What is unbalanced audio? Unbalanced audio is carried on one conductive wire with one return/ground/shield wire. This method of carrying audio is highly susceptible to noise and interference and cannot carry audio for long distances without signal degradation.
A microphone accepts sound and turns it into audio.
With unbalanced microphones, a displacement of the diaphragm inward should typically produce a positive amplitude in the microphone signal. This is because the start of a transient should produce sound pressure that moves the diaphragm backward. This is referred to as positive polarity.
Balanced microphones are the same, but this is only seen once the microphone signal passes through the preamp's differential amplifier. At that point, we typically see a positive amplitude when the diaphragm is moved inward, and a negative amplitude when the diaphragm is moves outward. This is also generally referred to as positive polarity.
Conversely, loudspeakers accept audio and produce sound. They should, therefore, be wired to push outward with positive amplitude and pull backward with negative amplitude.
This is how microphones and loudspeakers are typically set up for all equipment to work well together in terms of polarity.
Omnidirectional Vs. Directional Microphone Polarity
We've established that diaphragm displacement causes mic signal amplitude and that positive polarity means that an inward movement means positive amplitude. Let's now look at how this plays out with omni and directional microphones.
Omnidirectional (pressure) microphones only have one side of their diaphragm open to sound pressure and are designed to be equally sensitive to sound from all directions.
A sound wave from any direction will push and pull the front side of the diaphragm only. Therefore sound from all directions is in the same polarity.
Bidirectional (pressure-gradient) microphones have both sides of their diaphragms equally open to sound pressure. Think of ribbon microphones.
These mics are equally sensitive to the front and the back, with a ring of silence around their sides. Equally sensitive to the front and back, but these mics react with opposite polarities to sounds from the front and back.
A sound wave from the front could cause the mic diaphragm to move backward and then frontward. That same exact sound wave would cause the diaphragm to move frontward and then backward if it was coming from behind.
Therefore, bidirectional microphones have positive polarity in the front and negative polarity in the back, even though their diaphragms are equally sensitive to the amplitude of sound waves.
Unidirectional mics may or may not have opposite polarities. Cardioid, for example, has only one polarity for sound waves though it has a null point in the back. The rear lobes of sensitivity in highly directional microphone polar pattern are opposite polarity to their front lobes.
Balanced Microphone Audio Polarity
Polarity is a microphone specification that shows up often enough on manufacturers' datasheets.
With balanced microphones, polarity is generally given as pin 2 of the microphone output having positive polarity and pin 3 having a negative polarity.
From what we've learned above, this means that the microphone diaphragm moved inward causes a positive amplitude on pin 2 and an equally negative amplitude on pin 3. The diaphragm moved outward causes a negative amplitude on pin 2 and an equally positive amplitude on pin 3.
If the opposite is true, it is up to the manufacturer to let us know in the mic's specifications sheet.
A Note On Stereo Microphones
Microphones generally output mono audio signals—even stereo mics output dual-mono signals rather than stereo signals. To achieve the proper stereo image, these signals are then panned in a recorder, mixer, or DAW.
The microphone elements in stereo microphones are properly set up to have the same polarity and be in phase.
In-Phase Vs. Out-Of-Phase
Let's now discuss phase relationships with microphones. Phase is a bit more complicated than polarity.
Phase refers to the amount that a wave has passed through its cycle. Because sound and mic signals are waves, phase is important. Two identical waves/signals in-phase sum together while two identical waves/signals out-of-phase cancel each other out.
Phase generally applies to waveforms of the same frequency. If these waves are in phase, they line up along their entire form. If they are out-of-phase, there will always be a difference between the two cycles at any given point in time.
Audible sound and audio, however, have a frequency range of 20 Hz – 20,000 Hz. To have audio or sound be truly 100% in phase is practically impossible unless we duplicate the audio digitally or use synthesizers.
In nature, sound and acoustics have very complex relationships, and even a pair of mics placed very closely together will not be 100% in phase. That being said, we can get close to in-phase, and that's good enough for us (our ears rarely if ever, hear sound perfectly in-phase.
There are two main reasons why sound at two or more points is practically never in-phase in nature:
- Direct distance from the sound source: The sound wave will be in different phases of its waveform at different distances from the sound source.
- Sound is made of many different frequencies: Even if two microphones are placed equidistant from a sound source, the chance of all frequencies lining up is practically nil. This is particularly true for the higher, more directional frequencies.
There are plenty of other ways in which sound will be different at two different microphone locations. These include reflections/reverb, other sound sources, and microphone direction.
So phase really only matters when there are two or more waveforms. Mic positioning, then, is crucial for phase coherence when using more than one microphone.
Tips To Avoid Phase Issues
Out-of-phase audio signals cause destructive interference and have the potential to really ruin the sound of a mix. In-phase constructive interference, however, can really make a mix shine when mics are placed properly.
For more information on phase issues and how they affect the mix, check out my article Mixing: What Is Phase & Why Is It So Important To Get Right?
Let's look at some tips we can use to minimize phase issues with our microphones:
- Do not point microphones at one another
- Use microphone preamps with phase flip switches
- When stereo miking, place microphones equidistant from low-frequency sound sources
- Use boundary microphones near surfaces
- Use cardioid mics rather than shotgun mics for booming indoors
- The 10 dB rule
- The 3:1 rule
- Try Mid-Side stereo miking over XY
- Use your ears and listen for potential phase issues
Let's discuss each of these briefly.
Do Not Point Microphones At One Another
Pointing two microphones at one another can de detrimental to their phase relationship. Let's look at a stereo example and a common mono example of top-and-bottom-miking a snare drum.
Pointing a stereo pair of microphones toward one another is a recipe for phase issues.
Any sound sources between the mics (unless the source is precisely in the middle) will enter one mic before the other.
If the source does happen to be in the middle, it may send a compression-first wave toward one mic and a rarefaction-first wave toward the other mic. I'll clear this up when we discuss the snare drum example.
Finally, if a sound source is behind one of the mics and the mics are unidirectional, the sound will hit one mic first but be louder in the other mic. This is a recipe for a bad phase relationship and an awkward stereo image.
Let's say we're miking a snare drum with a top mic (pointing downward) and a bottom mic (pointing upward). As the snare is hit, the skin of the top head (and, by design, the bottom head) is pushed down. This causes a “positive polarity” sound to head toward the bottom mic and an opposite “negative polarity sound” to head toward the top mic.
Use Phase Flip Switches
But many engineers mic snare drums with top and bottom mics. They avoid phase issues by flipping the phase of the bottom mic.
Many preamplifiers allow you to flip the phase of an audio channel. Alternatively, most mixers and digital audio workstations allow this to be done in the mix.
If your microphones are set up and sound great individually, but there are still phase issues, it can be effective to flip the phase of one or more mics to hear if the phase coherence improves or not.
This strategy obviously works best when two microphones are nearly 180° out-of-phase.
Place Microphones Equidistant From Low-Frequency Sound Sources
Low frequencies have long wavelengths. This is both a blessing and a curse.
It means we can't rely on their phases to sometimes match up when microphones are positioned poorly. This is a benefit of high frequencies and their very short wavelengths.
However, if we place our microphones equidistant from a low-frequency source (like a kick drum), we can bet that the bass energy will be in-phase, nice and full. This is partially because of the long wavelengths of low frequencies and also due to their omnidirectional nature. Low frequency sounds project outward uniformly and works well with evenly spaced mics.
Use Boundary Microphones Near Surfaces
Opt for boundary microphones when you need to mic close to a surface to avoid the comb filtering effect of reflections. These mics are designed to pick up the sound from the source without picking up any reflections from the near-surface.
Use Cardioid Mics When Booming Indoors
This is, again, to avoid the phase issues that come from rear lobes of sensitivity and the reflections of the close surfaces.
Shotgun mics positioned close to surfaces pick up these reflections, and the opposite phase of the rear lobe really messes with the mic signal. Be aware of this when positioning your boom microphone(s).
Follow The 10 dB Rule
The 10 dB rule is a general rule of thumb that states the following: when multiple microphones in close proximity capture the same sound source, it’s best to have at least a 10 dB signal difference between the dedicated mic and the auxiliary mics in order to minimize phase issues.
Follow The 3:1 Rule
The 3:1 rule of thumb states when two mics are capturing a source (or multiple sources in one space), the mics should be separated by a distance 3 times that of the shortest mic-to-source distance.
Another 3:1 rule says a distant-mic should be at least 3 times further from a source than a close-mic.
Try Mid-Side Over XY
XY is a coincident pair and rarely has phase issues. However, there are some instances when collapsing the XY stereo image to mono causes some issues.
In these rare cases, or if you know for a fact that you're going to be tasked with collapsing a stereo mix into mono, try the coincident mid-side stereo technique.
Mid-side has two microphones:
- Mid microphone: this is a cardioid mic that points toward the centre of the stereo image you plan to record.
- Side microphone: this is a bidirectional (figure-8) microphone that points to the left and right (90° and 270°) from the Mid mic.
In the mix, pan the mid mic centre. Take the side mic and duplicate the signal. Pan one copy to the left and pan the other to the right. Flip the phase of the right copy.
Now when you collapse the stereo image to mono, the side (left and right) information will cancel out completely (180° out-of-phase), and the mid (centre) will be strong.
Use Your Ears
My final piece of advice is to use your ears.
If your microphone setup sounds out-of-phase, troubleshoot a solution.
By the same token, if the microphone setup is not perfect, but the sound is excellent and in phase, then go with it!
What is a microphone polar pattern? A microphone polar pattern is a way of representing the directionality of the mic. In other words, polar patterns describe the directions in which a mic is most sensitive, least sensitive and all the points in between. Polar patterns can be qualitative (certain titles) or quantitative (with polar response graphs).
For an in-depth look into microphone polar patterns, check out my article The Complete Guide To Microphone Polar Patterns.
What happens when a speaker is wired in reverse polarity? Wiring a loudspeaker in reverse polarity will not damage the speaker but will likely have negative effects on playback. Transient info may be lost as the speaker moves inward rather than outward. The big issues happen with phase cancellation when two or more speakers are wired in reverse polarity.
Choosing the right microphone(s) for your applications and budget can be a challenging task. For this reason, I've created My New Microphone's Comprehensive Microphone Buyer's Guide. Check it out for help in determining your next microphone purchase.