穴あき板構造体の吸音特性に関する二,三の問題
1961 年 17 巻 1 号 p. 31-37
詳細
1961 年 17 巻 1 号 p. 31-37
Sound absorption of perforated panel absorbers is characterized mainly by the resonant frequency and the maximum absorption coefficient. Usually, the resonant frequency has been calculated by the formula f_0=c/2π・√<p/l'L>, where p is the open area ratio, and l' is the effective thickness of panel and L is the depth of air space. It was found that the measured resonant frequencies became lower than those calculated by the above formula, as the depth of air space becomes larger. Thus, the following corrected formula was derived, in which the impedance of air space Z=-i(ρc/S)・cotkL was expanded in series and taken into account until the second order term. f_0=c/2π√<p/l'L+pL^2/3> This new formula agrees fairly well with experimental results. Then, the effect of the position of fibrous materials on absorption characteristics was investigated, which were inserted between the perforated panel and rigid backing. In general, the absorption coefficient becomes larger as the fibrous material comes closer to the perforated panel, but it is not so critical as the case of thin cloth.
