9 各種抵抗をもつ多孔質材料に接して穿孔板を張った吸音体の吸音特性 : 穿孔板の吸音特性に関する実験的研究

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タイトル別名
  • 9) SOUND ABSORBING CHARACTERISTICS OF PERFORATED PANEL ABSORBENTS WITH POROUS MATERIALS HAVING DIFFERENT FLOW RESISTANCES
  • 各種抵抗をもつ多孔質材料に接して穿孔板を張った吸音体の吸音特性
  • カクシュ テイコウ オ モツ タコウシツ ザイリョウ ニ セッシテ センコウバン オ ハッタ キュウオンタイ ノ キュウオン トクセイ

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抄録

Measuring the sound absorbing characteristics of resonant type absorptive structure consisting of a perforated panel and a porous material, placed with some air space behind them, by the reverberation room method, the effects of the flow resistance of the porous material (rl) and the open area ratio of the perforated facing (p) on the absorbing characteristics were found out. The perforation percentages of the panels used in this experiment were about 6, 9, 13, 20 and 30% with equal intervals on log scale, and the flow resistances of the porous materials used were 2.5, 13, 18 and 24g/cm^2・sec. The reverberation room was irregular pentagon shape with a volume of 86m^3. At 500c/s, the reverberation time of this empty room was about 9 sec. The air space was kept constant 90mm thick in this report and divided into several rectangles of 45cm×90cm by means of battens. From this experiment the following results were obtained. (1) For the perforated panels without porous materials, the measured maximum absorbing frequencies showed fairly good coincidence with the resonance frequencies calculated by the well-known Helmhorz resonance formula; while with porous materials, it showed better coincidence with the resonance frequencies calculated by the formula [numerical formula], in which the normal impedance of the air space was considered as that of the closed tube. (2) The maximum absorption coefficients were much influenced by the flow resistances of the porous materials rather than the open area ratio of the perforated panel. Thus, the effect of a perforated panel on the increase of the resistive part of the normal acoustic impedance of the facing plus porous layer was smaller than rl/p. As the results, the following experimental formulae were derived for the calculation of the resonance absorption coefficient, [table] where [table] n>0 and for a very small flow resistance [table] where n>0, n is obtained by equation (2). (3) By using the above values of R_0, the calculated value of the number of octaves in which the absoption coefficient is greater than d_<max>/2 showed fairly good agrreement with measured value.

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