Handbook for Sound Engineers

Sound System Design 1287

For overall sound system equalization, as described
in Section 34.5.2.2, an octave-band equalizer may be
acceptable for simple systems in rooms with
well-behaved acoustics. Most sound systems, however,
need the greater precision of a -octave band equalizer
or parametric equalizer as described below.

34.4.4.2.2 Boost-and-Cut Equalizers Versus Cut-Only
Equalizers

Early equalizers were made from passive components
and, thus, could not amplify a signal. These were
cut-only equalizers. When active models were devel-
oped, however, they included electronic amplifiers for
the purpose of buffering the impedance of the filters
and, in some cases, of allowing the frequency response
to be boosted as well as cut at any given frequency.
Either type of equalizer is suitable for sound rein-
forcement system equalization. However, choose a
high-quality equalizer and when equalizing a sound
system avoid boosting any frequency more than about
+3 dB to maintain good system head room.

34.4.4.2.3 Constant Q versus Variable Q Equalizers

Q for a filter is the ratio between the filter’s center fre-
quency and its bandwidth. Early passive equalizers used
variable Q filters. The Q of these filters was low at low
insertion (small fader movement) and increased at high
insertion. Some active equalizers have constant Q filters
whose Q does not vary with insertion. Both types of fil-
ters can be combining filters. A good quality equalizer
of either type is suitable for system equalization.

34.4.4.3 Parametric Equalizers

Parametric equalizers have fewer filters than graphic

equalizers but the parameters of each filter are highly

variable, hence the name, parametric equalizer. Typi-

cally, each filter of a parametric equalizer has variable

insertion, variable Q and variable center frequency.

Some mixing consoles include parametric equalization

on each input. Others include quasi-parametric equal-

ization where the insertion and center frequency, but not

the Q, are variable.

Because of their flexibility, parametric equalizers

with only three or four filter sections can approximate

almost any curve needed for sound reinforcement equal-

ization. For this reason, some designers favor them over

graphic equalizers for sound system equalization.

34.4.4.4 Digital and DSP Equalizers

Most modern systems will have their equalization func-

tions performed by some kind of multifunction DSP

device where the equalization is simply a software mod-

ule. Commonly, these DSP devices are controlled by a

computer and the settings of an equalizer module are

accessible via a user interface or GUI that resembles an

analog equalizer of the same type. It may be possible to

change many of the settings so the system designer can

choose a graphic or parametric equalizer and control Q,

center frequency, insertion depth and even filter design

type. Usually the default settings are acceptable but it’s

a good idea to review all of these settings before using

the device.

Figure 34-43. A two-channel analog graphic equalizer. Courtesy Rane.

Figure 34-44. A two-channel analog parametric equalizer. Courtesy Rane.

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