穿孔パネル多孔質吸音層組合せ構造の吸音について

抄録

Perforated facings are now extensively used in practical applicaitons as acoustic material. In this paper are described measurements of the acoustic impedance of fairly thick porous materials with perforated facings, in particular the effect of the air-space between the two components upon the acoustic reactance; and the date have been compiled into a simple design chart. The acoustic reactance of perforated panels of various dimensions (Table 1) placed before a rigid wall is shown in Fig. 2 with the width of the air-space as parameter. The solid curves in this figure were calculated from Eq. (1) and are found to be in fairly good agreement with experiment. From these results we can conclude that the acoustic reactance of these structures may be considered to be consituted of two components: the mass reactance of the holes and the compliance of the air cavity. Secondly, the acousitc impedance of preforated panels supplemented by backing of porous material was measured for several combinations of panel perforation, porous material and air-space (Table 2). The results of measurement are as follows:(1) The acoustic resistance of this strucure is shown in Fig. 5. Here R__0 represents the resistance when the perforated panel is placed immediately in front of the porous material without air-space. It seems to be reasonable to consider as an approximation that, independently of the frequency of the sound and the variety of the porous material, the ratio R__L/R__0 decreases as the distance between the panel and the porous material increases, though the value of R__0 itself depends on the variety of the porous material and the arrengement of the preforated facings. (2) As examples of caluculation of acoustic reactance, Fig. 6(a) shows the measured results for a perforated panel(hole diameter d=0. 45cm, thickness l=0. 5cm, open area ratio p=0. 078) backed by a padding of hairfelt(1. 2cm thick) and separeated by a 1cm air-space. In this figure X__<m0> and X__<c0> are the mass and stiffness reactance of the holes and the air cavity respectively as caluculated from Eq. (1) and X' is the reactance of the hair-felt itself. The composite reactance X caluculated by means of the equivalent circuit of Fig. 6(b) and by using the values of X__<m0>, X__<c0>, X'(for this example) also is shown, by the solid curve, and this is in good agreement with measurement. Measured and caluculated results for other combinations of panel and porous material are shown in Figs. 7 to 10, while the absorption coefficents calculeted from the acoustic impedance are represented in Figs. 11 to 14. Combined absorption characteristics can thus be estimated when the acoustic impedance of porous materials are known.