Recording - January 2017 - 61

flexibility, each additional mic doubles the number of intermic phase relationships. Even with phase-fighting solutions
such as polarity reversal, directionality, EQ, level changes,
and mic repositioning, it quickly becomes impossible to
overcome all of the inter-mic combs. For this reason, a simple drum miking setup, with one to four mics, often sounds
better than larger configurations.
Polarity reverse
Because phase differences and associated comb filtering are always present when using multiple microphones, most experienced engineers experiment with
polarity when multimiking to ensure frequency response
is as desirable as possible.
Commonly found on mic preamps, consoles, and
processors, a polarity switch offers a useful solution
when combatting phase issues. In analog equipment,
reversing polarity inverts signal voltage from positive to
negative, which in turn reverses current flow. In digital
systems, reversing polarity overturns the waveform so
positive values become negative and negative become
positive. Acoustically, the effect flip-flops the pressure
wave so peaks become troughs, etc.
Polarity reverse is most easily visualized through a loudspeaker driver. Feed a kick drum through the speaker,
and you will see the sound first move the driver in one
direction. Next, flip the polarity by reversing the positive
and negative wires attached to the speaker. The kick will
now push the driver in the reverse direction. While you'd
think this might create a major change in sound quality,
it's surprisingly inaudible. Our ears and brains are simply
not fast enough to process the difference. Polarity reversal
is nearly impossible to notice under normal circumstances.
However, polarity reversal can cause major sonic
change in multimiking scenarios. The classic example
of this is when miking a snare drum from both the top
and bottom. The top mic excels at capturing the drum's
fundamental and the bottom mic more easily catches
the sizzle. The challenge is that when hit, the drumhead
flexes downward, away from the top mic. Oppositely,
the head moves toward the bottom mic when the drum
is played. Thus the two mics capture opposite pressure
waves and the resultant electrical signal is nearly 180˚
out-of-phase at the fundamental. When mixed together
at near equal level, the tone of the drum usually thins
considerably. Therefore polarity reverse is typically
required on one of the two mics to ensure the fundamental does not phase-cancel. The mics are still slightly
offset in distance and therefore a comb filter will still
exist, but the sonic impact is far less dramatic than
what occurs before polarity reversal.
Reversing polarity changes the phase of all frequencies by
180˚. This can be very beneficial when two microphones on
the same source are offset in time or distance and the perfrequency phase difference creates a comb filter. Inverting
the polarity of one mic changes the phase difference of all
frequencies by 180˚. While this does not remove the comb,
it changes the filter's summation and cancellation frequencies. Depending on the frequency content of the source and
the amount of time offset, the change might improve subjective sound quality. Therefore most pros experiment with
polarity whenever they use more than one mic on a source.
Are you double-miking a piano? Try listening in
mono and flipping the polarity of one mic. Are you
deploying multiple mics on a drum kit? Experiment with
a variety of polarity reverse combinations until the various drums are all as punchy as possible. Two mics on
an acoustic guitar or electric guitar amp? Definitely
play with the polarity switch.

Polarity vs. Phase
On many devices, polarity reverse controls are labeled "phase."
This has led to the common audio engineering expression "flip the
phase." Some in the audio industry argue that this is not a correct
use of the word "phase," as changes in phase other than 180˚
require a change in time. While polarity reverse changes the phase
of all frequencies by 180˚, changing the time offset between two
mics changes the phase of all frequencies by a different amount.
Phase shift and polarity reverse mean very different things, and
Figure 3 demonstrates the difference nicely.
As an audio educator, I strongly protested calling a polarity reverse
a "phase switch", but have softened my stance in recent years.
Reversing polarity does indeed change phase, albeit by a fixed
amount (180˚) for all frequencies... and besides, if Rupert Neve
labeled the polarity switches on his classic preamps "PHASE", who
am I to question that? That said, I do ask that my students call it
"polarity reverse," not "phase reverse."
Semantics aside, polarity reversal is a fantastic tool for solving
phase-related issues. It's not the perfect solution for all phase problems, but it is certainly one all engineers should experiment with
whenever using multiple microphones. This is especially important
when miking drums, as multiple microphones are typically in close
proximity and create numerous inter-mic comb filters. Whenever multimiking a kit, listen with all mics on and experiment with all possible
combinations of polarity reversal. You may find that reversing the
polarity of one mic improves the tonality of another drum.
Next time, we will continue to explore phase and its importance in
the recording studio. Stay tuned!
Eric Ferguson (ferguson@recordingmag.com) spent a dozen years
as a freelance audio engineer in Los Angeles, and is now on the faculty of the New England School of Communications (NESCOM) in
Bangor, ME.
RECORDING JANUARY 2017

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