1967 年 23 巻 6 号 p. 415-424
The influence of Received March flow resistance of fibrous materials on their sound absorption characteristics has been investigated by measuring the flow resistance and the normal incident sound absorption coefficient. To deal with this subject from the point of view of several factors relating to the structure of fibrous materials, glass wool boards of various dimension made by three manufacturers(Table 1), five different non-woven fabrics sold on the market(Table 4), 31 different kinds of cotton fabrics woven out of the same yarn(Table 2)and eight kinds of felt made by a needle punch machine(Table 3)were used as sample. The apparatus shown in Fig. 1 was used to measure the specific flowresistance which is difined by the speed of pressure differential △P(dyn/cm^2) between two faces of a sample and the ratio of current V(cm/sec). The relation between the specific flow resistance R_v(dyn・sec/cm^3) of fibrous materials and V in a range of the low speed can be expressed by the experimental formula Eq. (2), where A_i and B_i are constants decided by the structure and the yarn density of a fabric or the thickness and the bulk density of a glass wool board. A_i can be suggested as a specific flow resistance R_s of the sample. The relation between the specific flow resistance R_vs(dyn・sec/cm^4) and the bulk density D_a(g/cm^3) of a fiber assembly can be given theoretically by Eq. (7). However, as a result of measurement it is evident that the specific flow resistance of the glass wool board of 2. 5cm thick was in proportion to D^<5/3>_a and, in case of thickness of 5. 0cm the power of D_a varied according with the manufacturers, as shown Fig. 3. The relation between the specific flow resistance R_s of glass wool board having the same thickness and the sound absorption coefficient for each frequency is shown in Fig. 6 and 7. Accordingly the absorption characteristics of a glass wool board can be detarmined by measuring the specific flow resistance. The maximum absorption coefficient on the frequency characteristic curve of the viscous resistance type is not affected by the depth of air layer behind the sample. The relation between the specific flow resistance R_s of fibrous materials and the maximum absorption coefficient is shown in Fig. 12. The maximum absorption coefficient of a fibrous material can be obtained from the specific flow resistance by using this curve, whatnever the structure of the fibrous materials may be.