CONTROL OF REVERBERATION TIMES IN DOME-SHAPED HALLS

Dr Hani ObeidSMIEEE, P.Eng.

Department of Electrical & Computer Engineering, Applied Sciences UniversityAmman – Jordan

Abstract

This paper outlines the approach used in solving the acoustic problems in a dome-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 doesn’t comply with the requirements of obtaining low RT. A computer simulation of the hall was 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.

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 with concave 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 where reflections 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 architectural and aesthetic aspects of the hall.

2. Architectural features of the hall

The hall is constructed from reinforced concrete and has a tremendous volume (43674 m 3), 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 distortion or 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. Program EASE

Fig. 1 Three-dimensional modeling of the hall

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Fig. 2 Plan of the hall

3.0 was 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 speech intelligibility, 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 can’t 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 to accepted 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.

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Table 1Absorption coefficients over the frequency range (125-8000) Hz for different materials

MaterialFrequency, Hz

125 250 500 1000 2000 4000 8000Concrete Block 0.02 0.03 0.03 0.03 0.04 0.07 0.08Gibson boards 0.25 0.10 0.05 0.04 0.07 0.07 0.0750% 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 0.75Perforated 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 0.80Roof fabric (12 oz/sq. yard)Foam 50 mm thick 0.09 0.29 0.64 0.97 1.05 0.97 0.75Sprayed cellulose fiber (75 mm) on solid baking

0.70 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

MaterialFrequency, Hz

125 250 500 1000 2000 4000 8000Concrete block 8.4 6.92 3.88 3.23 3.03 2.32 1.28Gibson boards 4.45 5.43 3.68 3.13 2.82 2.27 1.2750% concrete and 50% fiber glass

4.95 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 0.83Perforated Gibson board 3.12 2.05 1.58 1.49 1.31 1.13 0.81Fiber glass roof fabric (12 oz/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 0.73Sprayed cellulose fiber (75mm) on solid backing

2.21 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. 4. Architectural aspects of internal 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 matter after 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.

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Also a very reliable fixing of any material to the inside of the dome is of a great importance because any item falling down from that high will seriously injure persons underneath it.

Several different construction were tested and verified and it was found that the most appropriate 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, the cassette 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 applied by screen printing or multi-layer powder 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 holes shall 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. That means 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 be fixed 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 3Absorption coefficient of an aluminum cassette construction

Frequency, Hz

125 250 500 1000 2000 4000

Absorption coefficient

O.40 0.80 0.90 0.90 0.90 0.90

The fixing to concrete surface will be by special approved metal dowels and a support structure 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

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5. Conclusion

The 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.

6. References

1. David Egan. (1998). Architectural Acoustics. McGraw-Hill. Inc.2. K.B. Ginn. (1978). Application of B & K Equipment to Architectural Acoustics. 1st Edition. B & K publication, Denmark.3. Sound Advice. Sound system Handbook. Bureau Veritas, BOUYER.

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