Effects of fiber motion on the acoustic behavior of an anisotropic, flexible fibrous material

Abstract

The acoustic behavior of a flexible fibrous material was studied experimentally. The material consisted of cylindrically shaped fibers arranged in a batting with the fibers primarily aligned parallel to the face of the batting. This type of material was considered anisotropic, with the acoustic propagation constant depending on whether the direction of sound propagation was parallel or normal to the fiber arrangement. Normal-incidence sound-absorption measurements were taken for both fiber orientations over the frequency range 140–1500 Hz and with bulk densities ranging from 4.6–66.7 kg/m3. When the sound propagated in a direction normal to the fiber alignment, the measured sound absorption showed the occurrence of a resonance, which modified the absorption that was primarily attributed to viscous and thermal effects. When the sound propagated in a direction parallel to the fiber alignment, indications of resonances in the data were not present. From comparing these two sets of data and from considering the material structure, the resonance in the data for fibers normal to the direction of sound propagation was attributed to fiber motion. An analytical model for the acoustical behavior of the material displayed the same fiber-motion characteristics shown in the measurements.