Handbook for Sound Engineers

170 Chapter 7

the concert halls of the Gewandhaus in Leipzig and De
Doelen in Rotterdam.^33

This combination of a ground plan layout can also be
realized in the shape of a hexagon which is a common
application of a regular polygon ground plan. Elongated
hexagons show room-acoustical properties similar to
those of the combination of a diverging and a
converging trapezoid or, provided the converging or
diverging angle is slight, to those of a rectangular room.
If the ground plan is that of a regular hexagon,
Fig. 7-25, the necessary lateral sound reflections are
lacking especially with musical performances. Thanks
to its varied uses and the short distances between the
performance and reception zones it provides, this shape
is rather advantageous for congress and multipurpose
halls from the acoustical point of view. The amphi-
theaterlike arrangement of stage and audience of Fig.
7-25D shows acoustical similarities to the rectangular
variant of Fig. 7-20C. With sound sources having a

pronounced directional characteristic there occur timbre

and clarity problems for listeners seated at the sides and

behind the platform area, which cannot be compensated

by means of additional secondary structures along the

walls.

Ground plans with monotonically curved boundary

surfaces (circle, semicircle, Figs. 7-26A to D) produce,

due to their concave configuration towards the sound

source and especially if the tiers are only slightly sloped

or not at all, undesirable sound concentrations. On

account of the curved surfaces, the sound pressure level

may, in the concentration point, even surpass that of the

original sound source by 10 dB and thus become an

additional disturbing sound source. The resulting wave

front responses which depend on frequency, travel time,

and circle diameter, are shown in Fig. 7-27.^34 One

recognizes instances of migrating punctual and

flat-spread sound concentration (the so-called caustic),

which even after long travel times never do lead to a

uniform sound distribution. Without any structuring in

the vertical plane and without broadband secondary

structures, rooms having a circular ground plane are

acoustically suited neither for speech nor for musical

performances.

With asymmetrical ground plans, Fig. 7-26E, there

exists, for musical performances, the risk of a very poor

correlation between the two ear signals, an effect that

may give rise to an exaggerated spaciousness.

Energy-rich initial reflections are to underline the visual

asymmetry only as far as required for the architectural

comprehension of the room, otherwise the room

Figure 7-23. Principal portions of early lateral sound reflec-
tions in rectangular and trapezium rooms.

Figure 7-24. Lateral sound reflections produced by vine-
yard terraces.

lateral energy: high medium low

platform platform platform

L

L

L

S

L - Listener

S - Source

Figure 7-25. Diverse platform arrangements in a room of

hexagonal shape.

A. B.

C. D.