Introduction to Acoustics

Acoustics in Halls for Speech and Music 9.7 Room Acoustic Design of Auditoria for Specific Purposes 343
der a balcony. If the reverberation time suits classical
music concerts with the sloped seating in place, a substantial
area of variable absorption is needed to reduce
the T value when the chairs are absent. Another problem
may be that the telescopic systems offer only a rather
steep and linear slope, which is not optimal for classical
concerts. If the chairs are fixed on the riser steps, the
minimum step height is about 25 cm, corresponding to
a rake of about 25%.
If a change in T is accomplished by means of variable
absorption, G will be reduced along with lowering
T. However, if instead T is lowered by reducing the
volume, for instance by moving a wall or lowering the
ceiling, G will remain approximately constant or perhaps
even increase. This can be advantageous when the
low-T setting is to be used, for instance for unamplified
drama or for chamber music in a larger symphony
concert hall.
An example of a hall with variable volume is found
in Umeå, Sweden (Fig. 9.53). In this hall the auditorium
volume can be adjusted by moving the proscenium
wall between three positions. When the proscenium
Reverberation time (s)
2.5
2
1.5
1
0.5
125 250 500 1k 2k 4k
No absorbers
End wall blinds + curtains
End wall blinds
Side wall panels
All absorbers
Octave frequency (Hz)
is stored against the rear wall in the concert format,
the entire volume is available for concerts with large
symphonic orchestras on an open stage. In this situation,
the stage tower can be closed off by horizontal
panels at ceiling level. But when the proscenium is
moved forward to the opera setting, the auditorium
becomes smaller and a proscenium stage area is created,
while the ceiling panels are removed for access
to the fly tower above. In the third position, drama,
the auditorium volume is further reduced to create an
intimate theater. The variable elements also include
a hinged side-wall section to improve the shape of
the room for theater as well as moveable reflectors
and variable absorption curtains above the grid ceiling
level.
It should be mentioned that many of the problems
related to the successful design of speech and music
auditoria become more severe with increased size of the
room. In other words, it is much easier to design a hall for
less than 1000 people, like the two examples presented
above, than for 2000 plus. The problems become even
more complicated if multipurpose function is requested.
Fig. 9.52 Dronningesalen in the Royal Library in Copenhagen, Denmark; 1999, 400–600 seats. Reverberation time
curves, plan, section and wall elevations with hatched areas indicating variable absorption
Part C 9.7