Maximum Sound Absorption Coefficient of a Fiber Assembly

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When a sound wave is projected perpendicularly to a fibrous material in a tube the incident wave reflected by the rigid plate behind the fibrous material makes a stationary wave. Its characteristics vary with the porosity and thickness of the fibrous material, or with the back air space between the material and the rigid plate. By this method we have studied the sound absorption coefficients of fiber assemblies with the following results: (1) Sound absorption characteristics change from a viscosity resistance type to successively, a mixed type and a resonance type as they decrease in porosity. There are two kinds of absorption characteristics of the resonance type. One is a fibrous resonance type, with is for a thick sample. The other is a board resonance type, with is for a thin sample. (2) The relation between the most effective porosity Pe (%) at which a fiber assembly has the maximum sound absorption, i. e., 1.00 and thickness T(cm) is: (100−Pe)=α'T−β' where α' is a constant decided by fiber fineness and β' is a another constant. If T is constant, the relation between Pe (%) and the fiber fineness d (denier) is: (100-Pe)d−1⁄2=C where C is a constant decided by T. (3) The relation between the total surface area S (cm2) of fibers constituting a fiber assembly of porosity Pe(%) and T (cm) is: S=αTβ×104 where a and b are constants. A fiber assembly meeting this equation has the maximum sound absorption coefficient at a certain frequency if it has no back air space, and at a optional frequency if it has a back air space suited to the frequency.
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Maximum Sound Absorption Coefficient of a Fiber Assembly

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