The sound absorption coefficient(SAC)curves of the nonwovens,fabrics,and thick fibrous layers with the frequency in the range of 125-3 200 Hz and cavity distances of 5 cm,10 cm,20 cm,30 cm and 40 cm were measured.Based on analysis,it is found that the SAC is actually in direct proportion to the relative vibration amplitude of the sound wave,resulting in obtaining a mathematical expression showing that the SAC changes with the frequency for a given material.Additionally,a good corresponding relation between the maximal SAC and the permeability of the materials is discovered,thus an empirical formula is established.As a result,a complete SAC formula has been developed.In this formula,the SAC is solely a function of the sound wave frequency and permeability of the material.In comparison with sound absorption spectra of several materials,the calculated results and results measured with the formula coincide well with each other.
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