Recording - January 2017 - 60

Part 2:

Comb Filtering And Polarity
Last month we dove deep into the topic of phase, defining important terms, quantifying how phase is measured, and introducing
comb filters. For this second chapter, we'll outline the three comb filter
rules and investigate the use and semantics of polarity reversal.
Remember, comb filtering occurs when an audio signal is mixed
with a delayed version of itself. This is commonly encountered when
two or more mics are used on a sound source. Since sound takes
time to travel, the closer mic captures the sound before the farther
mic. When the two mics' outputs are mixed together in a console or
other audio device, the time lag between the two mics creates a situation in which some frequencies are "in phase" and increase in level
and other frequencies are "out of phase" and reduce in level. The
amount of level change is unique for each frequency, and changes
with phase difference. Figure 1 demonstrates the comb filter created
when one mic is one foot farther from a source than another mic.
Rule 1: Time/Distance = Frequency
The behavior of comb filters is governed by three simple "rules."
The first rule is that the greater distance/time offset between two signals, the lower in frequency the comb's peaks and valleys will stretch.
This is because lower frequencies have longer wavelengths/periods,
and longer delays are thus required to make significant impact in
terms of phase difference. The one-foot difference between the mics in
Figure 1 creates a comb whose lowest null (lowest frequency of complete cancellation) falls at 555 Hz. Doubling the distance between
mics to two feet would lower the comb by an octave to 277 Hz.
A perfect example of this first rule is flanging, the classic effect
heard on thousands of recordings. A flanger is simply a comb filter
whose offset time is slowly varied. When the delay time is long, the
comb is low in frequency, and its multiple ripples stretch upwards
across the frequency spectrum. When the delay is shortened, the
comb slides upwards, affecting high frequencies only. By slowly varying the comb's delay time, the effect makes a swooshing sound similar to a jet airplane taking off.
Rule 2: Equal Amplitude
In a perfect comb filter, some frequencies outright cancel and others double in level to be 6 dB louder. The second comb filter "rule"
states that in order for complete cancellation and perfect +6 dB summation to occur, both signals must be of equal amplitude. Sure, whenever two signals are offset in distance or time a comb exists. But in
order to form a perfect comb in which some frequencies outright disappear and others rise by 6 dB, both of the signals must be at equal
level.
The classic example of this is the three-to-one rule. Often discussed
in the audio world, the three-to-one-rule states that when miking a
source with two separated microphones, the mics should be separated from each other by at least three times an individual microphone's
source-to-mic distance. This rule ensures one mic is closer to the
source than another. Following the rule guarantees that sound
60

RECORDING JANUARY 2017

By Eric Ferguson
encountering the closer mic will be sufficiently reduced in
level by the time it hits the second mic. This is because
sound pressure level drops over distance. The combing
between mics will still be present, but since the signals are
not equal in level, the effect is reduced in severity.
As a side note, I must mention that the three-to-one rule
does not have to be followed religiously. Feel free to put
your mics wherever you like! If you hear combing, either
move the mics, or simply reduce the level of one of them
via fader or other method to minimize the comb filter. The
three-to-one rule is especially useful, however, when the
two mics must be at equal level, such as when using a
stereo microphone pair to capture a choir.
Rule 3: More Combs = Weaker Combs
The third "rule" of combing states that the more sources
of similar sound present (mics, speakers, or acoustic
reflections), the greater number of comb filters, and the
more minimized the affect of a single comb. This is what
happens in a real acoustic environment, and this is why
perfect combing is rarely heard in nature. A classic example of this occurs when recording drums with both close
and far "room" microphones.
Imagine miking a snare drum tuned with a 200 Hz fundamental. Desiring attack and proximity effect, you place
a close mic near the drumhead. Since the project is rock
and roll, you also place a room mic away from the drum
set to capture the room's natural ambience. Of course, the
distance between the close and room mic creates a phase
issue. The close mic, inches from the drumhead, captures
200 Hz at the beginning of its wave cycle. The room mic,
14 feet away, catches the frequency at the 180˚ point in
its cycle. The close and room mics sound awesome by
themselves, but when you raise the level of the room mic
to near equal level to the close mic, the 200 Hz fundamental thins in level. Figure 2 demonstrates this situation.
Luckily, while the combing is noticeable and possibly
undesirable, it does not cause complete cancellation of
200 Hz. This is because the room mics also capture sound
bouncing off walls, the floor, ceiling, and other surfaces.
With multiple arrivals of the frequency, the comb softens,
as each additional reflection creates an additional comb
filter between itself and the direct sound and other reflections. This lessens the severity of the primary close-mic-toroom-mic comb. Combs soften other combs.
The relationship between close and room mics is only the
beginning of the phase quagmire faced when miking a drum
kit. In today's productions, it's common to use lots of drum
mics-eight to twenty of them. While having the option to
adjust all aspects of a drum sound later when mixing adds

Table of Contents for the Digital Edition of Recording - January 2017