Introduction to Acoustics

floor can be tilted to reduce volume and allow the rear
seats benefit from being closer to the reflecting ceiling.
If for architectural reasons the ceiling is higher than
necessary in a room for speech or rhythmic music, substantial
areas of the room surfaces must be treated with
sound-absorbing material to control the reverberation
time. However, more sound absorption will reduce the
overall sound level in the room so this is only recommended
if the sound can be amplified.
If the ceiling is to be partly absorbing, the ceiling area
that should be kept reflecting in order to distribute useful
first-order reflections to all listeners can be generated
geometrically by drawing the image of the source in
the ceiling surface and connecting lines from this image
point to the boundaries of the seating area. Where these
lines cross the actual ceiling we find the boundaries for
the useful reflecting area, as shown in Fig. 9.35.
In larger auditoria, a more-detailed shaping of the
ceiling profile may be needed to ensure even distribution
of the reflected sound to the listeners. Notice in
Fig. 9.36 that the concave overall shape of the ceiling
can be maintained if just the size of the ceiling panel
segments is large enough to redirect the reflections at
suitably low frequencies.
Local reshaping of surfaces while maintaining the
overall form is also illustrated in Fig. 9.37,inwhichthe
weak lateral reflections in a fan-shaped hall are improved
by giving the walls a zigzag shape with almost parallel
sets of smaller areas along the splayed side walls. In
the example shown in the photo, the panels are even
separated from the wall surface and tilted downward.
9.6.8 Suspended Reflectors
In many cases it is advantageous to reduce the delay
and increase the level of a reflection without changing
the room volume. In this case it is obvious to suggest
individual reflecting surfaces suspended within the room
boundaries, as shown to the right in Fig. 9.38. Ifthe
reflector is placed over an orchestra pit, it can improve
mutual hearing among the musicians in the pit as well as
increase the early energy from the singing on the stage
to the seating primarily in the stalls area.
Suspended reflectors are also suitable for acoustic
renovation of existing, listed buildings, because the main
building structure is not seriously affected (as in the case
of the Royal Albert Hall in London [9.3]).
For a given source position, a small reflector will
cover only a limited area of receiver positions. There-
Acoustics in Halls for Speech and Music 9.6 Geometric Design Considerations 331
a) d)
b) e)
c) f)
Fig. 9.34 Means of controlling early reflections in auditoria (after
[9.28])
q 1'
q 1
q'
S e
b
q
a
a'b'
c'd'
f
c
d
1.2m
Fig. 9.35 Sketch identifying the area responsible for covering
the audience area with a first-order ceiling reflection
(after [9.32])
Part C 9.6