Handbook for Sound Engineers

Acoustics for Auditoriums and Concert Halls 169

but has disadvantages due to timbre changes impairing
intelligibility. As the talker chooses to speak towards
the audience seats in the middle of the room, the lateral
seating areas are bound to be disadvantaged on account
of the frequency-dependent directional characteristic.
According to Meyer,^3 the sound pressure level reduction
laterally to the talker when articulating the vowels “o,”
“a,” and “e” is 0 dB, 1 dB, and 7 dB, respectively.
A special case of the rectangular room ground plan is
the square with approximately equal side lengths. Espe-
cially for multi-purpose use such halls allow the realiza-
tion of diverse forms of confrontation with the audience
for which good acoustical conditions are given in small
rooms with about 500 seats,^29 assuming some basic
principles are considered, Fig. 7-21A to C. Room
variant A represents the classical linear contact ensuring
a good direct-sound supply to the listeners, especially
with directional sound sources (talkers, singers, direc-
tional instrumental groups). Variant B offers an acousti-
cally good solution for sound sources of little extension
(talkers, singers, chamber music groups), since a good
lateral radiation into the room is given. It is true,
however, that in the primary structure there is a lack of
lateral sound reflections for supporting mutual hearing
and intonation in the performance zone. The amphithe-
atrical arrangement shown in variant C is suitable for
only a few kinds of performance, since apart from
visual specialities there are above all acoustic balance
problems to be expected. With directional sound
sources—e.g., talkers and singers—the decrease of the
direct sound by at least 12 dB versus the straight-ahead
viewing direction produces intelligibility problems
behind the sound source.

The trapezoidal ground plan enables, on principle,
two forms of confrontation: the diverging trapezoid
with the lateral wall surfaces diverging from the sound
source and the converging trapezoid with the sound
source located at the broad end side. The latter ground
plan layout, however, is from the architectural point of
view not used in its pure form, Fig. 7-22.
Variations of the first ground plan layout with a
curved rear wall are designated as fan-shaped or piece

of pie. The room-acoustical effect of the trapezoidal

layout depends essentially on the diverging angle of the

side wall surfaces. The room shape shown in Fig. 7-22A

produces room-acoustical conditions, which are simi-

larly favorable as those in a rectangular room used for

music, if the diverging angle is slight. With wider

diverging angles, the energy-rich initial reflections,

especially from the side walls, are lacking in the whole

central seating area, which is a characteristic condi-

tioned by this primary structure. A principal comparison

of the fraction of lateral sound energy produced merely

by the ground plan layout is shown in Fig. 7-23. As was

to be expected, the comparable, relatively narrow

ground plan of the rectangular shape shows a higher

lateral sound fraction than the diverging trapezoid. For

spoken performances this situation is relatively uninter-

esting, since in most cases the lacking early lateral

reflections can be compensated by early reflections

from the ceiling. If the performance zone is shifted in an

amphitheater-like solution to the one-third point of the

room ground plan, Fig. 7-22B, this variant is suitable

only for musical performances. The listeners seated

behind the performance zone especially receive a very

spatial, lateral-sound accentuated sound impression.

The room acoustically most favorable with a trape-

zoidal layout is that of a converging trapezoid with the

performance zone located at the broad end side, Fig.

7-22C. Already without additional measures on the side

of the platform, the audience areas receiving low early

lateral sound energy gets reduced to a very small area in

front of the sound source; almost all of the other part of

the audience areas receives a strong lateral energy frac-

tion, Fig. 7-23. Unfortunately, this room shape has only

perspectives for architectural realization in combina-

tion with a diverging trapezoid as a platform area. The

arrangement of so-called vineyard terraces constitutes

herewith a very favorable compromise solution in

which wall elements in the shape of converging trape-

zoids are additionally integrated in the seating area. The

effective surfaces of these elements direct energy-rich

initial reflections into the reception area,^32 Fig. 7-24.

Examples of projects accomplished in this technique are

Figure 7-21. Diverse platform arrangements in a room with

a square ground plan.

A. B. C.

Figure 7-22. Examples of so-called arrangement patterns

between performers and listeners in a room with a

trapezoidal ground plan.

A. B. C.