Introduction to Acoustics

F b
d
F n
I
S
Fb Fn S
d
F n/F b
d/d I
Acoustics in Halls for Speech and Music 9.6 Geometric Design Considerations 325
F b
II III
I II III IV
0.63
1
0.55
0.83
Fig. 9.19 Mean stage–listener distance and net efficiency of floor area for four different room shapes. Fn is the net
area occupied by the audience, Fb is the total floor area, d is the average source–listener distance; dI = d for case I
(after [9.28])
0.64
0.78
d
0.67
0.79
1
F n
S
s 3Modell
Fig. 9.20 Calculated distributions of early lateral reflection levels in rooms of different plan shape: fan, hexagon,
rectangular and reverse fan. Darker areas represent higher LEF levels (after [9.5])
When building an auditorium, efficient use of the
available floor space and the average proximity of the
audience to the performers are very important parameters.
Depending on the plan shape of the room different
values appear as shown in Fig. 9.19.
Low average distance is the main reason for the frequent
use of the fan shape (III and IV in Fig. 9.19),
although the directivity of sound sources (like the human
voice) as well as the quality of lines of sight sets
limits on the opening angle of the fan.
Another limitation of the fan shape is that it does
not generate the strong lateral reflections so important
in halls for classical music. This was already seen in
the empirical equations in Sect. 9.5.6 and confirmed in
Fig. 9.20, showing the calculated distributions of early
lateral reflection levels in rooms of different plan shapes.
The reason is that sound waves from the source hitting
the side walls will produce reflections that will run
almost parallel to the direct sound and so hit the listener
from an almost frontal direction, therefore not contribut-
F b
d
IV
F n
S
Part C 9.6