Abstract
The normal incident absorption coefficient α_0 of many cloths placed at a certain distance from a rigid wall was measured. The peak value of α_0 due to energy loss by viscous resistance of air at the distances of 1/4λ and 3/4λ from the rigid wall was obtained and the flow resistance R_f of the cloths was measured. The relationship between the peak value of α_0 and the flow resistance R_f was investigated experimentally and theoretically. The following results were obtained. 1) The samples are classed into three groups in relation to absorption mechanisms; viscous resistance type, resonance type, and intermediate type between the viscous resistance type and the resonance type. 2) In case that R_f of a cloth is in the range between 10 and 40 rayls, α_0 increases with R_f, and when R_f is larger than 50 rayls, α_0 decreases with increasing R_f. When R_f of a cloth is 40〜50 rayls, α_0 therefore shows the maximum value (about 100%). 3) Theoretically, in case that R_f of a cloth is equall to 42 rayls, the peak value of α_0 becomes 100% under the assumption of modelized cylindrical narrow pores distributed homogeneously.
