Theatre Design & Technology - Winter 1995 - 27

Also, since locating loudspeakers within the stage picture
may place restricting demands on their size, larger speakers
are sometimes added to provide additional punch to the
underscore. This is usually not necessary for underscore,
however, which is generally played back at a low enough
level to remain "under" the actor's voice. Sometimes you
may also want a near and a far pair to match the distance of
the actors from the audience.

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* LOCATING SPEAKERS I THE AUDIE CE
When loudspeakers are located in the audience, two types of
loudspeaker systems may be required: a system such as
employed in film surround sound, and a system in which the
audience is able to identif}T the auditory sources involved in
the surround sound.

The guidelines for film surround sound systems have undergone extensive testing and development in the last ten years.
The development of the THX criterion for surround sound
systems includes many useful guidelines for live theatre surround sound systems that attempt to emulate the techniques
of film. Tomlinson Holman summarizes these requirements:
Given the need for broad dispersion and the fact that the
speakers are used in multiples so no one speaker need handle
undue power, the systems we have found most appropriate
are modified versions of home bookshelf models.
The number of surround speakers, their spacing along
the side and rear walls of the theater, and their height are calculated using formulas developed to ensure uniformity of
coverage. The final requirement for the surround channel as
a whole is that it have the same power-handling capability as
one screen channel. An additional formula for calculating
the required amplifier power is based on the number of surround speakers, their sensitivity, the average distance from
the speakers to the center of the room, and the maximum
sound pressure level required. To further minimize the need
for post-installation tuning, surround speakers are now
available with high-frequency response tailored to provide
the ISO 2969 curve. 9
Holman also discussed other requirements of film surround sound systems in a paper for the Audio Engineering
Society:

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1. A time delay was arranged such that the "surround"
sound arrived later than the direct sound from the
main-channel loudspeakers by 10-20 ms to utilize
the precedence effect to promote localization to the
front loudspeakers by suppressing localization of
the later arriving signal.
2. The additional loudspeakers were wired our of
phase with respect to one another to promote delo-

T D & T

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WI I N T E R

Table of Contents for the Digital Edition of Theatre Design & Technology - Winter 1995