168 Chapter 7
structured ceiling layout, prolongation of the reverbera-
tion time up to a factor of two results and is especially
large if long-delayed sound reflection groups are
produced by side wall surfaces that are inclined
outwards and upwards. But if these wall surfaces are
inclined towards the sound-absorbing audience area, the
shorter path lengths thus achieved may considerably
reduce the reverberation time as compared to the usual
calculating methods with vertical boundary surfaces.
Also with similar room shapes, different
room-acoustical conditions are obtained by just varying
the furnishing of the room (platform, audience areas).
All acoustically usable room shapes have in common
that the unhindered direct sound and energy-rich initial
reflections reach the listener. Deviations from this rule
occur through direct-sound shading in the orchestra pit
of an opera theater. Diffraction compensates this effect
partially and the listening experience is adapted to a
different sound impression which is similar to the case
of unhindered sound irradiation. The initial reflections
must arrive at the listener’s seat within a path difference
to direct sound of approximately 17 m (50 ms) for
speech and 27 m (80 ms) for music performances.
Decisive for an adequate spatial impression with
musical performances are, first of all, the lateral sound
reflections. The more the spaciousness is supported this
way, the more the orchestra sound gains, according to
Meyer,^30 in volume and width. The increase of sound
intensity perceptible with forte-play is thus enhanced
beyond the mere loudness effect so that the subjectively
perceived dynamic range is expanded. By the same
token, spaciousness is subjectively enhanced by an
increased loudness of the sound source.
From these general premises it is, for different
arrangement patterns between performers and listeners,
possible to derive universally valid guidelines for
fundamental room-acoustical problems of certain
typical room ground-plan layouts. In this regard one can
distinguish between purely geometrical layouts with
parallel boundary lines (rectangle, square, hexagon) on
all sides, with at least two mutually slanted boundary
lines (trapezoid) and generally curved boundary lines
(circle, semicircle, ellipse) and irregular layouts with
asymmetric or polygonal boundary lines.
7.3.3.2.1 Ground Plan
For obtaining lateral sound reflections, a room with a
rectangular ground plan is very well suited if the perfor-
mance zone is arranged at an end wall and the width of
the room is in the range of 20 m (66 ft), Fig. 7-20A.
This is the typical example of the linear contact in a
shoebox layout of a classic concert hall (Symphony Hall
Boston, Musik-vereinssaal Vienna, Konzerthaus
Berlin).If the performance zone is shifted from the end wall
towards the middle of the room, Fig. 7-20B, a circular
contact may come into being as a borderline case, where
audience or a choir may be arranged laterally or behind
the platform. Owing to the relatively pronounced
frequency-dependent directional characteristic of most
sound sources (singers, high-pitched string instruments,
etc.) there occur herewith, especially in the audience
area arranged behind the platform, intense balance prob-
lems which may even lead to unintelligibility of the
sung word and to disturbing timbre changes. On lateral
seats at the side of the platform, the listening experience
can be significantly impaired due to room reflections
where visually disadvantaged instruments are perceived
louder than instruments located at closer range. This
effect is even enhanced by lateral platform boundary
surfaces, whereas an additional rear sound reflection
area supports sound mixing. Often these acoustical
disadvantages are, however, subordinated to the more
eventful visual experiences.
If the performance zone is arranged in front of a
longitudinal wall, Fig. 7-20C, short-time lateral initial
reflections get missed especially with broad sound
sources (orchestras), whereby the mutual hearing and
consequently the intonation get impaired. Soloist
concerts or small orchestras (up to about six musicians)
may still provide satisfactory listening conditions, if
ceiling height and structure provide clarity-enhancing
sound reflections. By means of a sound-reflecting rear
wall combined with adjustable lateral wall elements
which do not necessarily disturb the visual impression,
it is possible to attain good room-acoustical conditions
with not too long rooms (up to about 20 m or 66 ft). For
spoken performances this way of utilization provides
advantages because of the short distance to the talker,Figure 7-20. Examples of so-called arrangement patterns
between performers and listeners in a room with a rectan-
gular ground plan.A. B. C.