# Recording - January 2017 - 37

```Note that there's less than a 6 dB
drop in the first octave below 1 kHz.
The slope doesn't reach the theoretical
6 dB/octave until below about 100 Hz.
Also, the amplitude doesn't reach 0 dB
(flat) until about 10 kHz. This isn't a very
sharp filter, but it's perfectly suited for
some applications. The lesson here is
that a lowcut filter on your mixer that's
specified as "80 Hz, 6 dB/octave" isn't
going to stop much kick drum bleed.
You'll probably need an 18 dB/octave
or steeper filter to help there.
With a shelf or shelving filter (Figure
4), the amplitude drops with frequency
when moving away from cutoff, but levels off at some point. The result is that all
frequencies in the stopband are attenuated by the same amount. A shelf can be
either low- or highpass.
Q, which represents the passband
width, is an important part of a bandpass
filter's specification. It's a calculated value
equal to the center frequency of the passband divided by bandwidth, which for a
bandpass filter is the difference between
the frequencies above and below the
peak at their -3 dB points. There are no
units for Q; it's just a number.

for the purpose of a specification, Q is
always measured at the filter's maximum
boost or cut which, for this equalizer, is
15 dB. Figure 6, showing a 3 dB boost
at 3 kHz with maximum and minimum
Q, illustrates why.
Maximum Q for this case calculates out
to 0.9, but at minimum Q, without extending the measuring beyond 20 kHz, we
can't tell where the curve intersects the
-3 dB line. Guessing that it would be
around 25 kHz, that gives us a Q of
about 0.1. Don't panic! EQ works fine
with small boosts or cuts, it's just that without knowing the -3 dB points, neither
bandwidth nor slope can be defined.
Equalizers with Q of less than 1 are
often described as "musical." An equalizer with Q in the 1-2 range is often
termed "British," and Q of 5 or greater
is often described as "surgical EQ."
Understand that these are marketing
terms, not technical definitions.
A "Constant Q" equalizer is one in
which the bandwidth changes with the
gain so the curve doesn't flatten out for
small boosts or cuts. It can be handy for
live work, but once applied, constant Q
equalization can't really be undone.
3

A parametric equalizer is one that
allows Q to be adjusted along with the
gain and center frequency. Figure 5 illustrates a 3 kHz boost at maximum (red)
and minimum (green) Q settings for a
commercial parametric equalizer. The
frequencies that define the bandwidth
are shown, where the frequency
response curve crosses the -3 dB line.
For maximum Q (red):
Bandwidth = [3.2 kHz - 2.5 kHz] =
0.7 kHz, and Q = [3 kHz / 0.7 kHz] = 4.3
For minimum Q (green):
Bandwidth = [7.5 kHz - 1.2 kHz] =
6.3 kHz, and Q = [3 kHz / 6.3 kHz] = 0.5
This equalizer specifies that Q is
adjustable from 0.5 to 5, which is close
enough to my measurements to believe.
An important thing about Q is that,
while you can calculate it for any case,

Lastly, bandwidth is sometimes
expressed in octaves or fractions of an
octave. The one-third octave graphic
equalizer is the most common example,
but a few parametric equalizer plug-ins
have picked up on this labeling. I can't
think of any reason why, other than to
be different.
And then there's phase shift
Though you'll probably never see a number for phase shift on a filter spec sheet, all
filters introduce a phase shift between input
and output. This is a result of the characteristic time lag between the current and voltage when an alternating current is applied
to a capacitor or inductor.
The most common filters are characterized as minimum phase. As the name
RECORDING JANUARY 2017

37

```
http://mojaveaudio.com

Contents
Recording - January 2017 - Intro
Recording - January 2017 - Cover1
Recording - January 2017 - Cover2
Recording - January 2017 - 1
Recording - January 2017 - 2
Recording - January 2017 - 3
Recording - January 2017 - 4
Recording - January 2017 - 5
Recording - January 2017 - Contents
Recording - January 2017 - 7
Recording - January 2017 - 8
Recording - January 2017 - 9
Recording - January 2017 - 10
Recording - January 2017 - 11
Recording - January 2017 - 12
Recording - January 2017 - 13
Recording - January 2017 - 14
Recording - January 2017 - 15
Recording - January 2017 - 16
Recording - January 2017 - 17
Recording - January 2017 - 18
Recording - January 2017 - 19
Recording - January 2017 - 20
Recording - January 2017 - 21
Recording - January 2017 - 22
Recording - January 2017 - 23
Recording - January 2017 - 24
Recording - January 2017 - 25
Recording - January 2017 - 26
Recording - January 2017 - 27
Recording - January 2017 - 28
Recording - January 2017 - 29
Recording - January 2017 - 30
Recording - January 2017 - 31
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Recording - January 2017 - 72
Recording - January 2017 - Cover3
Recording - January 2017 - Cover4
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