Handbook for Sound Engineers

1262 Chapter 34

Left-center-right clusters are commonly used for
dramatic performances where actors’ voices are panned
from one side of the stage to the other. In this type of
system it is imperative that every listener be able to hear
all three clusters.

34.3.2.6 Exploded Clusters

Technically, the term exploded cluster refers to a con-
ventional central cluster where individual packaged
loudspeakers have been moved outwards (exploded)
along radii from the original position. The term is often
used, however, to describe any system where several,
smaller clusters are used in place of a single, central
cluster. In this sense, a left-center-right cluster system
could be considered an exploded cluster.

Exploded clusters are often used in auditoriums with
very wide stages and relatively shallow audience areas,
Fig. 34-19. Many so-called mega churches in the United
States are of this design. A central cluster cannot cover
the sides of the room toward the stage effectively. A
split cluster (including line arrays) cannot cover the
center of the room toward the stage effectively. An
exploded cluster is often a good compromise for this
type of facility. Typically, left, center, and right clusters
will be supplemented by an additional cluster between
the left and center clusters and another between the right
and center clusters.

To minimize comb filtering, try to design each
cluster to cover a specific audience area and minimize
overlap. For example, if there are four audience areas,
separated by aisles, try to design the system with four
clusters. If the room is fairly deep, consider a second
ring of delayed clusters, located outward on imaginary
radii from the near-stage clusters. An exploded cluster
system can also be combined with delayed
under-balcony speakers.

34.3.2.7 Rear and Surround Clusters

Rear and surround clusters are primarily used for spe-

cial effects in live dramatic presentations. Design these

clusters for the facility and presentation requirements.

For example, if a rear cluster is to reinforce the spoken

voice of an off-stage actor, it must cover the entire audi-

ence clearly. However, left and right clusters used for

ambience effects may not need to cover the entire audi-

ence evenly.

34.3.2.8 Designing a Central Cluster in a Simple

Rectangular Room

Most sound reinforcement system designers now use

EASE or some other loudspeaker system design soft-

ware to help them design their systems. The following

method, used before this type of software was available,

is presented for educational purposes and for historical

completeness. In addition, some of the concepts pre-

sented here, such as methods of choosing loudspeakers,

will be of value to a designer using EASE.

34.3.2.8.1 Evaluate the Room

If the room exists, measure its reverberation time and

physical dimensions (drawings will help in physical

measurements). Calculate the room volume and total

surface area. Using the Sabine reverberation time equa-

tion, derive the average absorption coefficient, , as

follows:

(34-27)

where,

V is the room volume,

S is the room surface,

is the average absorption coefficient,

For metric distances, replace the constant 0.049 with the

constant 0.161.

From this equation, the average absorption coeffi-

cient, , can be found with

(34-28)

For metric distances, replace the constant 0.049 with the

constant 0.161.

If the room is in the planning stages only, estimate its

average absorption coefficient, total surface area, and

volume from the architectural data.

Figure 34-19. An exploded cluster loudspeaker system.
Courtesy Community Professional.

a

RT 60 0.049V

Sa

=-----------------

a

a

a 0.049V

SRT (^60)
--------------------=