Theatre Design & Technology - Oct 1973 - 12

SPEECH ACOUSTICS FOR THE THEATRE
DAVID l. KLEPPER

KMK ASSOCIATES

An important characteristic of the Greek and Roman
theatres that favored their acoustics was a steep seating
slope. This both gave listeners additional energy reflected
off the stage floor (in addition to sound reflected off the
proskene, a sound-reflecting structure behind the actors)
and also reduced the attenuation suffered by sound energy
as it passed over the audience. With regard to a modern,
indoor theatre, we may say that measures that improve
sight lines will also improve the acoustics. In this respect
a straight-line slope (often insisted upon by fire codes) is
not as efficient as a sloped floor that increases in angle to
the rear of the hall.

Introduction
The following paper, prepared by David L. Klepper,
Chairman of the USITT Engineering Subcommission on
Audio and Acoustics, with the assistance of C. Paul Boner,
Russell Johnson, Ranger Farrell, Richard Thompson,
Clyde Nordheimer, Chris Jaffee, and Dennis Paoletti
reflects the opinions of the committee.
Note that music may be performed in theatre and the
acoustical requirements for music may be different than
speech. These conflicts and their resolution are to be
discussed in a forthcoming paper on multi-purpose halls.
Goals

Theatre Plans

The acoustical design of any theatre should have, as its
first goal, high speech intelligibility. A second goal should
be speech naturalness and lack of distortion or harshness;
and a third goal should be a wide dynamic range, permitting an actor's whisper or a yell to be understood. These
goals should apply to all seats within a theatre, and for
any likely acting position; therefore good sound distribution is essential in any theatre. Finally, intruding noises
can be annoying, even before they actually interfere with
intell igibility; and, consequently, a low background noise
level (almost inaudible) from the heating and air conditioning system and the exclusion of exterior and adjacent
sounds are most important. These are subjective goals, and
the task of th is paper will be to relate them to objective
considerations in design and construction. Th is paper is
not a substitute for detailed consultation by a trained and
qualified acoustical consultant for a specific project, but
is intended as an aid to the theatre designer's understanding of the consultant's work.

Similarly, a moderate fan plan that puts a larger number of the audience relatively close to the stage is usually
superior with respect to distance to a rectangular plan that
places a larger number at a greater distance from the stage;
and a one-balcony basic plan will usually be superior to
one level of seating for the same reason. Bringing all
listeners as close as possible to the stage would seem to
favor the arena stage concept, or theatre-in-the-round
over the uni-directional theatre (either proscenium or
open-stage); but, unfortunately, actors are not bu ilt with
mouths at the backs of heads. The directivity of the
human voice has been studied exclusively, and we know
that in the speech range where much intelligibility lies
(the s's and t's and f's), 2,000-4,000 Hz., levels are
roughly 10 db lower (one-tenth the actual sound intensity
or subjectively half as loud) behind the head as compared
with in front. The resu It is that arena stage theatres of
equivalent acoustical excellence tend to be considerably
smaller than uni-directional theatres, with thrust-stage and
three-quarter round theatres somewhere in between. As a
practical guide for indoor theatres not employing electronic speech reinforcement, the following maximum
sizes are suggested:
1800 seats maximum
Unidirectional:
Thrust Stage:
1200 seats maximum
Arena Stage:
450 seats maximum
These are acoustical criteria only: assume actors with
good diction, and there may be abundant reasons outside
of acoustics to further Iimit the size of theatres for
excellence. Conversely, successful theatres of larger size
are possible, but only at some sacrifice to the actor's
dynamic range, or with the use of electronic reinforcement. (Again, there may be important non-acoustical considerations against larger sizes).

Outdoor Theatre
The simplest form of theatre, acoustically, is an outdoor arena, where the listener must rely primarily on
direct sound transmission from the actor's mouth to the
listener's ears. There are limitations on the size of such a
theatre. We must remember that sound energy outdoors
decreases 75% in intensity (or decreases 6 decibels) for
every doubling of distance. Sound energy will be attenuated further as it passes over a sound-absorbing audience.
While we have the evidence of some fairly large Roman
and Greek outdoor theatres, we may assume that actors
functioning in those theatres stayed fairly close to the top
of their dynamic range, and there was no attempt to have
listeners understand a whisper, even a stage whisper.

THEATRE DESIGN AND TECHNOLOGY

OCTOBER. 1973

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