Theatre Design & Technology - Feb 1972 - 13

brought in for (a) special stereophonic effects, (b)
amplification of massive choruses or on-stage orchestras, and (c) maximum level with minimum distortion for rock and other popular music.
A variation on the broad-front system utilizes
loudspeakers mounted in the orchestra pit railing
for main floor coverage. This procedure is only
successful when the loudspeakers are more than
seventy feet from the first rows of seats, as in a
large outdoor installation.

DISTRIBUTED LOUDSPEAKER SYSTEM
A distributed loudspeaker system is not a random
or arbitrary placement of various loudspeakers
along theatre side walls, without consideration of
the phasing and time-delay problems involved.
Good distributed loudspeaker systems either place
the loudspeakers in the ceiling, directed generally
downward, or in the backs of seats (as in the
Kremlin Hall in Moscow and numerous church systems in North America). A well-designed distributed loudspeaker system, with loudspeakers located to insure 50-degree coverage with some
overlap, can provide high-quality intelligible sound
in a theatre, ceiling-mounted if the ceiling height
is no greater than forty feet. In less critical, nontheatre installations a gO-degree coverage is satisfactory. Higher ceilings rule out the use of ceiling
distributed loudspeakers, because echo effects can
be heard in the front of the theatre, where live
sound arrives at the listener's ears well before the
amplified sound.
A delay system, either pipe, tape-loop, or allelectronic, should always be used with a distributed system ina theatre for loudspeakers other
than those at the front, to insure that amplified
and live sound arrive at the listener's ears at approximately the same time (usually the live sound
a few milliseconds early for directional realism).
Distributed systems are most frequently used in
new theatres as supplements to central systems for
deep underbalcony spaces, and a delay system is
essential for such a distributed system to have a
positive, rather than a negative effect. Use of such
a system can provide complete naturalness, with
intelligibility, for the underbalcony area, while the
remainder of the theatre is covered by a central or
broadfront system.
(Note that delay systems can also improve directional realism for proscenium loudspeakers in
certain cases.)

Choice of Basic Loudspeaker Equipment
Central and broad-front systems in new theatres
In North America use one of three different types
of loudspeaker systems. Two-way horn-type clus-

THEATER DESIGN AND TECHNOLOGY

FEBRUARY, 1972

ters are most frequently used; column or linesource loudspeaker systems are used by certain
engineers; and smaller studio monitor two-way
loudspeaker systems have been used successfully,
particu larly in smaller theatres and aud itoria. Each
of these loudspeaker types has specific advantages.
Finally, nearly all distributed loudspeaker systems
use cone-type direct radiator loudspeakers, usually five-inch or smaller for seat-back and very-low
underbalcony applications, eight-inch for most ceiling applications, and often 1 5-inch for most mainceiling applications.
Horn or line-source loudspeakers are used In
distributed systems when restricted coverage is
desired from each individual loudspeaker.

TWO-WAY HORN SYSTEMS
Two-way horn systems employ a cluster of highfrequency multi-cellular, radial, or sectoral highfrequency horns with short, front-loaded low-frequency horn enclosures containing one, two, or
four low-frequency loudspeakers. Crossover is usually at 500 Hz. The basic advantage of such systems, if correctly designed and installed, is excellently controlled directional characteristics. Such
systems can provide very even coverage, with little
change in frequency response in various seats, especially in the range above crossover frequency,
usually 500 Hz. Another advantage of this type of
system is high efficiency; twenty or thirty watts
being adequate to drive a directional horn system
to 100-dB levels in a typical 12,000 seat theatre.
One disadvantage is the large size of these clusters and the large grill areas they frequently require
when hidden. Also, some engineers profess to be
bothered by a specific "horn quality" of such systems, which makes them reluctant to use them for
music amplification. The answer to such a comment is that nearly any system can profit from intelligent equalization of overall frequency response,
and that properly equalized horn systems can sound
very natural indeed.

COLUMN OR "LINE SOURCE" SYSTEMS
Column systems consist of lines of direct radiator cone-type loudspeakers. They are usually lighter
in weight and easier to install than two-way horn
systems and can require less grill area. They are
less efficient than horn systems, and 200 watts of
power amplification can easily be required for a
typical 1200-seat theatre. In certain cases, where
high levels for modern popular music are required,
line-source systems can prove inadequate and require supplementary horn-type systems. Linesource systems can provide very controlled directional characteristics in the vertical plane, but their
horizontal directional characteristics will vary over

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