1286 Chapter 34
34.4.4.1 Compressors and Limiters
Compressors and limiters are devices that control a sys-
tem’s dynamic range (dynamic range is the difference in
dB, between the highest and lowest LP levels in any
audio program). A limiter reduces the signal level when
the level rises above a preset threshold. In this manner, a
limiter helps minimize system damage from dropped
microphones or other transients, Fig. 34-42.
A true compressor reduces too-high signal levels but
it also increases very low signal levels to keep them
above the ambient noise. Compression ratio is the ratio
of output level change to input level change in dB.
Although true compressors are used in broadcast and
recording applications, sound reinforcement systems
seldom use true compressors. Perhaps for this reson the
terms compressor and limiter are often used inter-
changeably in sound reinforcement literature.
34.4.4.1.1 Sound System Applications for a
Compressor/Limiter
In a paging system, a true compressor can keep the
average level of the voices of different announcers more
constant so that paging can reach noisy areas of a fac-
tory or airport more consistently. In addition, because of
reduced dynamic range, peaks are lowered, reducing the
chance of clipping distortion.
In a large sound reinforcement system, such as a
concert tour sound system, a limiter can reduce the
chance of peak clipping and can thus help avoid ampli-
fier or speaker damage from large turnon/turnoff tran-
sients or from sudden, loud feedback.
34.4.4.1.2 Problems with Compressor/Limiters
While useful, compressor/limiters are not cure-all
devices. The compressor makes its decision to begin
compressing by continuously monitoring the program
level. Unfortunately, the highest levels are usually low
bass notes. Thus, the compressor/limiter may compress
the high frequencies needlessly when it detects a bass
note that is too loud. One solution to this problem is to
use a compressor on each output of an electronic cross-
over on a biamplified or triamplified system so that the
compressor acts only on the frequencies in each band.
Another solution is to use a separate compressor on each
mixer input that may receive excessive program levels.
Perhaps the best solution, for quality-conscious sys-
tems, is to use the limiter just to limit peaks. Set up the
limiter with a high compression ratio and a high thresh-
old so that it begins limiting only on potentially danger-
ous peaks and then limits them hard. With this setup, the
limiter should be inaudible at normal program levels.34.4.4.2 EqualizersAn equalizer is a device that controls the frequency
response of a system or an individual source. An equal-
izer could be considered to be a large number of tone
controls, operating at different frequencies, all in one
device. Chapter 23 provides details of equalizers and
their design.
There are two types of equalizer commonly used in
sound reinforcement, the graphic equalizer and the para-
metric equalizer, Figs. 34-43 and 34-44.34.4.4.2.1 Graphic EqualizersGraphic equalizers usually have a series of slider-type
controls that boost or cut each frequency. When the con-
trols are adjusted up or down they resemble a graph of
the unit’s frequency response, hence the name graphic
equalizer. Graphic equalizers are commonly available in
1 octave types or in -octave types. An octave-band
equalizer has controls that are spaced 1 octave apart. A
-octave-band equalizer has controls that are spaced
of an octave apart. Some manufacturers offer
-octave spacing for part of the frequency range.
Octave-band equalizers are useful for adjusting the
frequency response of an individual source. For
example, to mellow the sound of a nasal-voice singer,
use an octave-band equalizer connected to the insert
points on the mixing console’s input channel.(^1) » 3
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Figure 34-42. Compressor/limiter with threshold and compression ratio adjustments. Courtesy dbx Professional.