ACOUSTIC PROPERTIES OF A FIBER ASSEMBLY

Abstract

A specific acoustic impedance of a fiber assembly was measured by the standing wave method, and an attenuation constant and a propagation velocity of a sound wave in the fiber assembly were computed by a following new method. Given the specific acoustic impedance Zl and Zl of fiber assemblies of l and 2l in thickness, Zl=W coth bl Z2l=W coth 2 bl where W is the characteristic impedance and b the propagation constant. As Zl and Z2l are measurable, b is calculated. The attenuation constant and the propagation velocity are obtained from the b as a function of frequency.The relation between these values and factors relating to the structure of fiber assembly were investigated. The results are as follows:(1) A sound absorption mechanism of a fiber assembly is explained by the frequency characteristic curve of specific acoustic impedance. The resistance part of the impedance varies largely with frequency under a certain structure of the assembly.(2) The relation between the attenuation constant β and the porosity P (%) may be expressed by; β=A(100-P) where A is a constant decided by a fiber fineness, β increases nearly in proportion to the total surface area of fibers in a unit volume of the assembly.(3) The sound propagation velocity in a fiber assembly of viscosity resistance type increases proportionally to log f (Hz) and decreases as the porosity of the assembly decreaese. A characteristic curve of sound propagation velocity in a fiber assembly of resonance type is considerably different from that of viscosity resistance type. In a low frequency range it is similar to the one of the viscosity resistance type, but at a critical frequncy in a middle or high frequency range the velocity increases discontinuously. The critical frequency becomes lower, as the porosity of the assembly is the smaller. Accordingly in a high frequency range the sound propagation velocity is larger in the fiber assembly of lower porosity than in that of higher porosity.