control of reverberation times in dome1

8/8/2019 Control of Reverberation Times in Dome1

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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

[email protected]

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.

control of reverberation times in dome1

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