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.

Transverse Vibrations of a Thin Rectangular Porous Plate Saturated by a Fluid

“…In this approximation, the solid frame is motionless because it is much heavier and more rigid than air. In reality, however, the solid can vibrate in the vicinity of resonance frequencies, resulting in a strong modi"cation of the surface impedance [1,2]. It has been shown [3] that Biot's theory [4,5] can be used to determine the displacement of the solid and its e!ect on the calculation of the surface impedance of rigid porous layers.…”

Dynamic Viscoelastic and Multiple Scattering Effects in Fiber Suspensions

“…Brodeur et al (1989) developed an instrument generating a stationary ultrasonic field to characterize the mean dimensions of softwood fiber samples diluted in water. Dahl et al (1990) experimentally studied the acoustic behavior of a flexible fibrous material consisted of cylindrically shaped aligned fibers. Brodeur (1991) presented a theoretical investigation on the motion of fluid-suspended fibers under the influence of a stationary ultrasonic wave field.…”

Fiber movements and sound attenuation in glass wool

“…The papers by Dahl et al 1 and Lambert 3,4 explain the measured acoustic properties by resonances in single fibers. However, it is difficult to see how single fiber could resonate, because all fibers are strongly coupled, and the wavelength of a sound wave are much longer than the distance between the fibers.…”

“…Dahl et al 1 measured the absorption coefficient of Kevlar samples with a thickness of 10 cm placed on a hard wall. They found a resonance in the absorption attributed to a mechanical resonance due to movement of fibers.…”