Recording - December 2016 - 51
Comb filtering
Phase difference and associated level
change is often pictured on a wheel.
Figure 8, courtesy of Mauricio Ramírez
and Meyer Sound, demonstrates the
cyclical nature of phase interaction and
its associated amplitude change. Using
the wheel you can easily calculate the
per-frequency level change when offset
sources are mixed. For example, frequencies close to 90˚ offset see approximately a +3 dB level change, while frequencies shifted by about 150˚ see
around a -6 dB change. Frequencies
experiencing close to 180˚ of phase difference almost completely cancel, but as
shift increases further, the wheel moves
from cancellation back to summation.
Frequencies with close to 210˚ of shift
see only -6 dB of reduction, and those
270˚ off see +3 dB.
Although its
ramifications are
everywhere in
audio, phase is a
complex and often
misunderstood
topic.
This circle illuminates the cyclical
nature of phase. In Figure 5, we saw
that phase difference steadily increases
as frequency rises. When converted into
a 0˚ to 360˚ scale, phase difference
(and associated level change when
mixed) continually circles the phase
wheel as frequency rises. This means
that when we mix our two mics together,
their combined amplitude response
slides up and down across the frequency
spectrum, reflecting the change from
complete cancellation to perfect summation. The effect is commonly called a
comb filter, since its amplitude response
looks like the teeth of a comb when
graphed linearly across frequency.
Figure 9 illustrates the comb filtering present between our spaced piano mics.
Next time, we will continue this discussion of phase, focusing on comb filtering
and common situations in which the
effect can be sonically undesirable. See
you then!
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 DECEMBER 2016
51
http://www.kiqproductions.com
Table of Contents for the Digital Edition of Recording - December 2016
Contents
Recording - December 2016 - Intro
Recording - December 2016 - Cover1
Recording - December 2016 - Cover2
Recording - December 2016 - 1
Recording - December 2016 - 2
Recording - December 2016 - 3
Recording - December 2016 - 4
Recording - December 2016 - 5
Recording - December 2016 - Contents
Recording - December 2016 - 7
Recording - December 2016 - 8
Recording - December 2016 - 9
Recording - December 2016 - 10
Recording - December 2016 - 11
Recording - December 2016 - 12
Recording - December 2016 - 13
Recording - December 2016 - 14
Recording - December 2016 - 15
Recording - December 2016 - 16
Recording - December 2016 - 17
Recording - December 2016 - 18
Recording - December 2016 - 19
Recording - December 2016 - 20
Recording - December 2016 - 21
Recording - December 2016 - 22
Recording - December 2016 - 23
Recording - December 2016 - 24
Recording - December 2016 - 25
Recording - December 2016 - 26
Recording - December 2016 - 27
Recording - December 2016 - 28
Recording - December 2016 - 29
Recording - December 2016 - 30
Recording - December 2016 - 31
Recording - December 2016 - 32
Recording - December 2016 - 33
Recording - December 2016 - 34
Recording - December 2016 - 35
Recording - December 2016 - 36
Recording - December 2016 - 37
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Recording - December 2016 - 40
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Recording - December 2016 - 46
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Recording - December 2016 - 70
Recording - December 2016 - 71
Recording - December 2016 - 72
Recording - December 2016 - Cover3
Recording - December 2016 - Cover4
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