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Control of reverberation times in dome-shaped halls
HANI S. OBEIDDepartment of Electrical & Computer Engineering
Applied Sciences UniversityP.O.Box 950674, Amman 11195
JORDAN
Abstract: - This paper outlines the approach used in solving the acoustic problems in adome-shaped hall with high ceiling, which constitutes the main challenge in this work.
The reverberation time RT of such a huge volume without treatment is large and doesnt
comply with the requirements of obtaining low RT. A computer simulation of the hallwas done and a series of calculations were performed by program EASE 3.0 for various
materials with different absorption coefficients in order to control the acoustical
environment within the hall and to obtain the required RT.
Key-words: - Reverberation times, absorption, and dome-shaped halls.
1. Introduction
The architectural shape of any construction
plays a vital role in its acoustical
performance. The basic architectural shapes
that are problematic are (in order of
difficulty) domes, round rooms, rooms withconcave surfaces and cubical rooms. The
problem with all of these shapes is their
ability to focus reflected sound. The
geometry of each of these shapes causes
focal points and lines to form in space wherereflections tend to arrive simultaneously.
The simple solution is to destroy the
acoustic symmetry of the space while
maintaining its visual symmetry. This is
done by treating the surfaces with diffusive
and absorptive components [1]. This study
was performed to solve acoustical problems
in dome-shaped hall designed for parliament
meeting without affecting the architecturaland aesthetic aspects of the hall.
2. Architectural features of thehall
The hall is constructed from reinforced
concrete and has a tremendous volume
(43674 m3), height (40 m), and dome-shaped
ceiling. Since the hall is used mainly for
speech, the goal is to obtain high speech
intelligibility besides other acoustical design
consideration. The other acoustical goals are
speech naturalness and the lack of distortionor harshness [2]. The hall was simulated by
using computer program EASE 3.0. Fig.1
shows a three dimensional modeling and
Fig. 2 shows a plan of the hall. ProgramEASE
Fig. 1 Three-dimensional modeling of the
hall
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Fig. 2 Plan of the hall
3.0 were used to calculate the reverberation
time RT of the hall. It was found that the RT
of the hall without any treatment of the
surfaces (ceiling and walls) is equal to 3.6 s
at mid frequencies (500-1000) Hz (Fig. 3).
Fig. 3 Reverberation time of the hall without
any treatment
This excessive RT will destroy the speechintelligibility, and it is due to echo, which is
a potential problem in concave shaped
ceiling as in our case. The recommended RT
for such halls is in the range of 0.6 to 1.3 s
[3], while the optimum RT is (0.7-0.9) s [1].
3. Treatment of the internal
surface of the dome
The reflections from the internal ceiling
surface cant be used as a reinforcement of
the sound because the ceiling is very high
and the reflections will be heard as echo.
Therefore, in such cases, it is recommended
to treat the ceiling with absorptive material
in order to prevent reflections from reaching
the listeners and to decrease the RT toaccepted level. Different absorptive
materials were used as a cladding to the
ceiling of the dome to explore the required
one. Table 1 shows the absorption
coefficients of these materials over the
frequency range (125-8000) Hz.
Table 1
Absorption coefficients over the frequency range (125-8000) Hz for different materials
Frequency, HzMaterial
125 250 500 1000 2000 4000 8000
Concrete Block 0.02 0.03 0.03 0.03 0.04 0.07 0.08
Gibson boards 0.25 0.10 0.05 0.04 0.07 0.07 0.07
50% concrete and 50%fiber glass
0.20 0.20 0.22 0.40 0.38 0.38 0.38
Acoustic spray 0.08 0.29 0.75 0.98 0.93 0.76 0.75
Perforated Gibson boards 0.45 0.70 0.70 0.65 0.75 0.80 -
Fiber glass 0.65 0.71 0.82 0.86 0.76 0.62 0.80
Roof fabric (12 oz/sq.
yard)
Foam 50 mm thick 0.09 0.29 0.64 0.97 1.05 0.97 0.75
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Sprayed cellulose fiber (75
mm) on solid baking0.70 0.95 1.0 0.85 0.85 0.90 -
The reverberation times of the hall were calculated by EASE 3.0 for treated ceiling with different
absorptive materials and the results of the calculations are shown in table 2.
Table 2Reverberation times of the hall for various materials
Frequency, HzMaterial
125 250 500 1000 2000 4000 8000
Concrete block 8.4 6.92 3.88 3.23 3.03 2.32 1.28
Gibson boards 4.45 5.43 3.68 3.13 2.82 2.27 1.27
50% concrete and 50%
fiber glass4.95 4.36 2.80 1.96 1.92 1.65 1.05
Acoustic spray 6.72 3.68 1.50 1.09 1.12 1.17 0.83
Perforated Gibson board 3.12 2.05 1.58 1.49 1.31 1.13 0.81
Fiber glass roof fabric (12oz/sq. yard) 2.35 2.03 1.4 1.22 1.30 1.32 0.90
Foam 50 mm thick 6.53 3.68 1.68 1.1 1.05 0.98 0.73
Sprayed cellulose fiber
(75mm) on solid backing2.21 1.56 1.18 1.23 1.20 1.04 0.76
The results of the calculations indicated that
the sprayed cellulose fiber if implemented
would be the right absorptive material and
the reverberation time of the hall as a result
of ceiling treatment is equal to 1.2 s for mid
frequencies.
3. Architectural aspects ofinternal cladding
The right selected absorptive material
constitutes the internal cladding of the
dome, therefore, it must not only maintain a
high degree of sound absorption but must be
incombustible, non-aging even at high
temperatures (no drizzling of fine matterafter a few years), resistant against high
temperatures and be securely fixed. The
other aspect is the material should be
molded to obtain the required ornamentation
suggested by the interior designer.
Due to the high illumination load (1000 lux)
in the hall much heat is generated by the
lamps. This heat is accumulating inside the
dome with the result that the temperature in
the dome will be very much high. High
temperature accelerates aging of material,
especially organic material. It is therefore
not advisable to work with sprayed cellulose
fiber on the exposed surfaces.
Also a very reliable fixing of any material to
the inside of the dome is of a greatimportance because any item falling downfrom that high will seriously injure persons
underneath it.
Several different constructions were tested
and verified and it was found that the mostappropriate construction to fulfill the stated
requirements is to manufacture the internal
claddings in the form of cassettes. The best
material to construct the cassette from is
perforated aluminum surface backed up by
sound absorbing material. In particular, thecassette can be formed by an outer casing of
perforated aluminum sheets pressed in a
form as required by the interior designer or
architect. The finish of the visible surface of
cassette will be powder coating in the base
color; further motives or decorations can be
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applied by screen printing or multi-layerpowder coating.
The aluminum surface shall be
manufactured of 1 mm perforated sheet
aluminum. The diameter of the holes shall
be 2.5 mm and the percentage of the holesshall be equal or more than 16%. The
critical frequency for this type of perforation
above, at which the absorption will drop
dramatically, is equal to 6500 Hz. Thatmeans the high absorption of this cassette
will be effective over a wide range of
frequencies from 125 Hz to 6500 Hz. That
range of frequencies is more than enough for
the type of activities that will be performed
in the hall.
40 mm thick mineral wools slabs shall befixed behind the aluminum surface. Textiles
or tissues shall be laminated the surfaces of
the mineral wool to prevent trickling of
glass fibers (Mineral wool slabs according to
DIN 18165). 500 mm air space shall be left
between the cassette and the concrete
surface of the dome. The degree of
absorption of this construction is shown in
table 3.
Table 3
Absorption coefficient of an aluminumcassette construction
Freque
ncy,
Hz
12
5
25
0
50
0
10
00
20
00
40
00
Absorption
coeffic
ient
O.4
0
0.8
0
0.9
0
0.9
0
0.9
0
0.9
0
The fixing to concrete surface will be by
special approved metal dowels and a supportstructure to maintain that required distance
from the concrete surface, which is
necessary for the correct sound absorption.
The whole structure shall be rigid and
sufficiently strong. Fig. 4 shows the
construction of the suggested aluminum
cassettes.
Fig. 4 Construction of aluminum cassette
4. ConclusionThe reverberation time of dome-shaped hall
was studied by using a computer simulation
program EASE 3.0. It was found that the
ceiling treatment with the right absorptive
material would yield the required
reverberation time. Due to architectural
constraints a special construction was
proposed, which has the required absorption
coefficient and fulfill the architectural needs.
5. References1. David Egan. ArchitecturalAcoustics. McGraw-Hill. Inc., 1998
2. K.B. Ginn. (1978). Application of B& K Equipment to Architectural Acoustics,
1978. 1st Edition. B & K publication,
Denmark.
3. Sound Advice. Sound systemHandbook. Bureau Veritas, BOUYER.