日大残響室における残響室法吸音率の測定について

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  • 4) THE MEASUREMENT OF SOUND ABSORPTION COEFFICIENT IN THE NEW REVERBERATION ROOM OF NIHON UNIVERSITY
  • ニチダイ ザンキョウシツ ニ オケル ザンキョウシツホウキュウオンリツ ノ ソクテイ ニ ツイテ

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In this paper, the characteristics of our new reverberation room and some problems of sound absorption coefficient measurements in this room are discussed. The reverberation room of Nihon University has an irregular pentagon shape with a volume of 170m^3. At 500c/s, the reverberation time in the empty condition is about 15 sec. As the sound source, a warble tone is used. The results of our experiments shows that the modulation frequency of the warble tone should be between 10/T and 20/T, that is 1c/s〜4c/s in this room, and the recommendation of ISO draft, where it is stated independently on reverberation time that modulation frequency should be "about 6c/s," is not reasonable. In order to get a diffuse sound field in the reverberation room with a very absorbent sample such as glaswool or rockwool, it is more effective to devide the sample in more areas. For example, the sample areas of 5, 10, 15 and 20m^2 should be devided into 2, 3, 4 and 5 areas respectively. In this case, the absorption coefficients α of glass wool, 25mm thick, 10kg/m^3, are proprotional to E, the length of free edges per unit area of the sample. And the absorption coefficient calculated from London's statistical formula using tube datas correspond to E=0. When a single area of 10m^2 of the glass wool is used, the values of α are too low due to the lack of diffusivity. For the sample of soft fiber board, whose α is less than 0.5, however, absorption coefficient does not depend on the position of the test sample.

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