1288 Chapter 34
34.4.4.5 High-Pass and Low-Pass Filters
High-pass filters, also called low-cut filters, pass high
frequencies and attenuate low frequencies. Low-pass
filters, also called high-cut filters, pass low frequencies
and attenuate high frequencies. A high-pass filter and
low-pass filter with the same 3 dB frequency make a
simple crossover network, Fig. 34-45.
Most sound reinforcement systems should include a
40 Hz, 18 dB/octave high-pass filter to reduce subsonic
frequencies that might otherwise damage loudspeakers.
For systems that include subwoofers, use a 20 Hz,
18 dB/octave (or steeper) high-pass filter in the
subwoofer circuit and a 40 Hz high-pass filter (or
higher) in the main loudspeaker circuit. Many graphic
equalizers, and virtually all mult-function DSP devices,
include a variable high-pass filter for this purpose.
34.4.4.6 Delay
Delay units are also called signal delay or digital delay.
The term time delay is inappropriate since the signal is
being delayed, not the time. Delay is useful in many
sound reinforcement applications as detailed in Section
34.5.2.1. The most common example is a combination
cluster and under-balcony loudspeaker system. The sig-
nal to the under-balcony loudspeakers is delayed to
allow the sound from the cluster to catch up and avoid
an artificial echo. Delay can also be used to line up the
wavefronts from the high- and low-frequency compo-
nents of a packaged loudspeaker system or to line up the
wavefronts of the multiple loudspeakers in a cluster.
For sound reinforcement, choose a high-quality
delay unit with dynamic range of 96 dB or greater and
adjustment increments of 20μs or shorter. For delaying
the components of a packaged loudspeaker or a cluster,
choose a delay with 10μs or shorter increments.
Delay is a normal module of a multifunction DSP
device. Most have varying increments of delay and a
total delay limited only by system memory.
34.4.4.7 Electronic CrossoversA crossover network routes high frequencies to the
high-frequency loudspeakers (tweeters) and low fre-
quencies to the low-frequency loudspeakers (woofers).
The use of crossover networks is discussed in Section
34.3.4. Electronic crossovers should conform to the
general specifications presented in Section 34.4.1 and
are often included in multifunction DSP devices.34.4.5 Digital Signal ProcessingMost audio signal processing now takes place in the
digital domain. Common DSP devices can be pro-
grammed to emulate a group of their analog counter-
parts arranged in a configuration chosen by the sound
system designer. Some include mixing and output signal
routing. There are two general types of digital signal-
processing devices now in use in professional and com-
mercial audio systems. (Also see Chapter 31.)34.4.5.1 Multifunction DSPThe multi-function digital signal-processing system,
typified by Peavey’s Media Matrix, Biamp’s Audia, or
the BSS Soundweb, can become an entire sound system
up to the power amplifiers and loudspeakers. This type
of DSP device includes mixing and automatic mixing
capabilities, output signal routing, and all types of sig-
nal processing (compression, limiting, equalization,
delay, crossover), Fig. 34-46.34.4.5.2 Power Amplifier DSPPower amplifier DSP performs most of the same func-
tions as all-in-one DSP. However the DSP devices are
attached to the individual channels of a power amplifier.
Because of this location, amplifier DSP is not suitable
for mixing or output signal routing. However, it can per-
form all signal-processing functions and can also super-
vise and control the amplifier channel. Some power
amplifier DSP devices are optional. Others are included
with the amplifier and located inside the amplifier chas-
sis, Figs. 34-47 and 34-48.34.4.5.3 Loudspeaker Processing DSPMost DSP functions needed by a loudspeaker can be
performed by a multifunction or amplifier DSP. How-
ever, some manufacturers offer special-purpose DSP
devices designed to perform loudspeaker optimization
functions not available in these other devices. ForFigure 34-45. High-pass and low-pass filter curves.