1 Acoustics Instruments and Measurements July 2013, Caseros, Buenos Aires Province, Argentina REVERBERANTION CHAMBER DESIGN AGUSTÍN Y. ARIAS 1 1 Universidad Nacional de Tres de Febrero, Buenos Aires, Argentina. [email protected]1. INTRODUCTION A reverberation chamber is, basically, a room that has a long reverberation time and is designed as diffuse as possible. The construction of the room should realize a high performance of sound insulation from any noise that comes from outside, since the interior of the room is used primarily for acoustics characteristics of material testing, which requires complete independence of any unwanted outside sound. Furthermore, the materials on the surface of the inner walls must be carefully chosen, for minimum absorption of sound energy. Reducing the sound energy absorption means to increase the energy of the reflections, which leads to achieve a totally diffuse field and a long reverberation time. Thus the factors that dominate the sound attenuation are: air absorption, which is considerable regarding the size of the chamber, especially at high frequencies, and the low absorption coefficient of the room surfaces. In this report the design of a reverberation chamber is presented according to the requirements of ISO-354 “Acoustics - Measurement of sound absorption in a reverberation room” [1]. The construction details are specified and finally the simulation results are shown for evaluating reverberation time within the chamber. 2. ISO 354 REQUIREMENTS As mentioned above, the reverberant chamber design must meet certain essential characteristics defined in the Standard ISO-354. The most important are: The minimum volume of the chamber should be approximately 200 m 3 . The room should allow a large diffusion of the sound field, for which suspended diffusers are needed (large plates that hang from the ceiling to improve the sound diffusion). The relative humidity in the chamber should be greater than 40%, and temperature above 10 º C. The shape of the reverberation room shall be such that the following condition is fulfilled: (1) Where is the length of the longest straight line which fits within the boundary of the room (e.g. in a rectangular room it is the major diagonal), in meters. V is the volume of the room, in cubic meters. In order to achieve a uniform distribution of natural frequencies, especially in the low- frequency bands, no two dimensions of the room shall be in the ratio of small whole numbers. The equivalent sound absorption area of the empty room, A 1 determined in one-third octave bands, shall not exceed the values given in Table 1. If the volume V of the room differs from 200 m3, the values given in Table 1 shall be multiplied by (V/200 m 3 ) 2/3 . Table 1. Maximum equivalent sound absorption areas for room volume V = 200 m 3 Frequency [Hz] A 1 [m 2 ] Frequency [Hz] A 1 [m 2 ] 100 6,5 800 6,5 125 6,5 1000 7 160 6,5 1250 7,5 200 6,5 1600 8 250 6,5 2000 9,5 315 6,5 2500 10,5 400 6,5 3150 12 500 6,5 4000 13 630 6,5 5000 14 3. DESIGN 3.1. Reverberation chamber There is no ideal way to build reverberant chambers, but it's better to select non-uniform asymmetrical. In this manner, the reverberant field produced indoor will be as diffuse as possible. Figure 1 shows a 3D model of the reverberation chamber from an external and internal view.
A reverberation chamber is, basically, a room that has a long reverberation time and is designed as diffuse as possible. The construction of the room should realize a high performance of sound insulation from any noise that comes from outside, since the interior of the room is used primarily for acoustics characteristics of material testing, which requires complete independence of any unwanted outside sound. Furthermore, the materials on the surface of the inner walls must be carefully chosen, for minimum absorption of sound energy. Reducing the sound energy absorption means to increase the energy of the reflections, which leads to achieve a totally diffuse field and a long reverberation time. Thus the factors that dominate the sound attenuation are: air absorption, which is considerable regarding the size of the chamber, especially at high frequencies, and the low absorption coefficient of the room surfaces. In this report the design of a reverberation chamber is presented according to the requirements of ISO-354 “Acoustics - Measurement of sound absorption in a reverberation room” [1]. The construction details are specified and finally the simulation results are shown for evaluating reverberation time within the chamber.
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Acoustics Instruments and Measurements July 2013, Caseros, Buenos Aires Province, Argentina
REVERBERANTION CHAMBER DESIGN
AGUSTÍN Y. ARIAS
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1 Universidad Nacional de Tres de Febrero, Buenos Aires, Argentina.