Sound System Design 131934.6.1.5 Electronics
Electronics for portable systems are generally the same
as for installed systems. Choose the electronics for rug-
gedness, however, in addition to performance. The large
power transformer on a power amplifier, for example,
must be securely fastened to its chassis to avoid physi-
cal destruction of the power amplifier during the jolts of
traveling over rough roads.
Most portable equipment will be rack mounted.
Thus, the equipment must be designed to survive the
jolts of travel when mounted in a rack. Experienced tour
companies often support the rear of large rack-mounted
components to help prevent damage. The racks them-
selves, of course, must be rugged and travel well, and
small racks can mean heat buildup so that extra cooling
fans should be considered.
Some manufacturers offer electronic packages
specifically designed for traveling. The powered mixer,
designed for smaller portable systems, is a good
example. Some powered mixers include a full-function
mixing console, internal effects, one or more graphic
equalizers, compressor/limiters, and one to four power
amplifiers. For a small- to medium-sized portable
system, these powered mixers are often the only elec-
tronics needed.
Mixers and other nonrack-mounted equipment must
be carried in padded road cases. Similar cases can be
used for microphones, cabling, and system accessories.
34.6.1.6 Multichannel Portable Systems
The primary problem with a multichannel portable
sound system is that, ideally, each member of the audi-
ence should be able to hear all of the individual chan-
nels. That means, ideally, each loudspeaker channel
must cover the entire audience. The large system
required to make this happen prevents true stereo sound
in most portable systems. Stereo-type effects, however,
can be achieved with traditional left and right loud-
speaker systems, and side and rear loudspeaker systems
can be successfully used for fill and special effects.
34.6.1.7 Electrical and SPL Safety in Portable Systems
Electrical safety in portable systems is complicated by
the uncertain condition of the ac power system of each
building. One excellent way to bypass this problem sim-
ply is to carry a portable ac power distribution system,
which should be designed and constructed by a quali-
fied, licensed electrician. Have a local licensed electri-
cian connect this portable system to the house ac power.
Often, the portable ac system can be connected directly
to the building ac service entrance (a local qualified
licensed electrician must perform this connection). This
not only bypasses any potential safety problems in the
house ac system but also provides a (relatively) clean ac
power system for the noise-sensitive sound system elec-
tronics.
High LP (high sound-pressure-level) hazards are
often overlooked but are, nonetheless, dangerous. It is a
well-established fact that high LP , over an extended
period of time, can cause permanent hearing damage.
Hearing protection is a must, especially when
performing high LP equalization or other testing or
when checking out individual loudspeakers before or
during a performance. It is possible to wear concealed
ear plugs (the expanding foam type are comfortable and
very effective) during a concert performance, even if
you are mixing the performance. The human brain
adjusts the hearing mechanism to the point where things
begin to sound right again after a short period of
wearing hearing protection. The situation is similar to
wearing sunglasses. After a period, colors begin to look
right again. This type of hearing protection is important
especially for the extremely high LP encountered during
stage monitor mixing. When in doubt about whether or
not you need hearing protection, listen to the ringing in
your ears after a performance. This ringing, known as
tinnitus, is the human body’s way of telling us that the
sound level is too high. Prolonged exposure to these
levels, of course, will almost certainly lead to perma-
nent hearing loss.34.6.1.8 Performance CriteriaAny system for entertainment must be designed to
accept and reinforce an LP of 100–120 dB. These levels
represent amplifier power output and loudspeaker
power handling in the neighborhood of 1000 times that
of a speech-only system. Microphone input levels are
such that electrical output from a high-sensitivity micro-
phone may be as high as 0 dBu on peaks. Thus, mixers
must have high-input capabilities and lots of head room.
Ideally, system head room should be as high as
20 dB, although, economically, this may be unobtain-
able. Extensive use of compressor/limiters can make a
10 dB head room system sound almost as good as one
with 20 dB of head room (and the loudspeaker and
power amplifier costs are lower).
The frequency response of a system designed for
popular music must extend down to at least 40 Hz (or
lower) and at least as high as 12–16 kHz. One way to