Theatre Design & Technology - Feb 1974 - 17

DESIGN CONSIDERATIONS FOR A DEMOUNTABLE
CONCERT ENCLOSURE (SYMPHONIC SHELL)
CHRISTOPHER JAFFE
PREFACE

The following paper, prepared by Christopher Jaffe, Member of the USITT Engineering Subcommission on Audio and
Acoustics, with the assistance of Richard Wangerin and the following Commission members: Ranger Farrell, Harold
Burris-Meyer, Russell Johnson, Ed Kook, Richard Thompson, and the Subcommission Chairman, David Klepper, touches
an area still regarded as controversial by some practicing acoustical consultants. However, every effort was made to
reconcile different points of view, and the paper represents a consensus to the extent possible. Previous papers prepared
by the Subcommission dealt with Theatre Sound and Communications Systems and Speech Acoustics for the Theatre,
and future papers will discuss Orchestra Pits and the acoustics of Multi-Purpose Auditoria. We wish to emphasize the
important point that good concert enclosure design must complement, and not replace, good acoustical design
throughout the multi-purpose auditorium.
I n developing design criteria for a demountable concert
enclosure or symphonic shell, a basic understanding of
acoustic criteria for the source and listening areas of
concert halls is important.
The source area of a hall can be defined as that portion
of a room or space where the orchestra is located, and the
listening area as that portion of a room or space where the
audience is located.
Many architectural acoustic texts utilize single-figure
average criteria as gu ides in room acoustic design.
However, most practising acousticians would agree that
physical acoustic criteria for a concert hall vary from the
source to the listening area and that a key factor in the
success of a hall is the interrelationship between criteria in
these locations.
The criterion for the architectural acoustic design of
each element of a hall is related to specific musical
acoustic goals or design objectives. Design objectives for
the source area may be enumerated as follows:
1. Balance the acoustical energy output of the various
sections of the orchestra.
2. Counteract the directional characteristics of the
individual instruments and blend them into a single
musical entity.
3. Distribute the balanced and blended sound
efficiently as possible throughout the listening area.

4. Maintain the greatest possible dynamics for mid and
high frequency sound in the early arriving sound field
and for low frequency in the reverberant field.
5. Enable the musicians to hear themselves and other
sections of the orchestra.

THEATRE DESIGN AND TECHNOLOGY

6. Provide a warm reverberant environment on stage.
These design goals are related to physical acoustic
criteria such as source area reverberation time, the ratio of
the direct to reflected sound in the source area, the size,
shape, mass and composition of reflecting and diffusing
surfaces surrounding the stage, as well as the relative
distances of these surfaces from the orchestra.
Design goals in the listening area can be described as
follows:
1. Enable the listener to receive a well blended and
balanced orchestral sound.
2. Enable the listener to receive a maximum number of
early secondary reflections at sufficiently high sound
levels for intimacy and definition.
3. Provide a secondary sound field having a sufficient level
of the familiar bass-rich traditional concert hall
frequency spectrum.
4. Avoid any acoustical faults that might interfere with
the reception of the direct and reflected sounds, such as
obstructions or echo and focusing producing surfaces.
The design goals in the listening area are related to the
same physical acoustic criteria as those in the source area.
However, the criteria' in the listening area will often have
different numerical values than their counterparts in the
source area, and the relationship between the two sets of
conditions will determine the suitability of the
E:nvironment for musical presentation.
When one studies the design of a single-space concert
hall in the traditional shoebox style, such as Boston's
Symphony Hall or the Musikvereinssaal, one observes that
source area design goals related to balance, blend,

FEBRUARY. 1974

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