Measurements of the frame acoustic properties of porous and granular materials

Abstract

For porous and granular materials, the dynamic characteristics of the solid component (frame) are important design factors that significantly affect the material’s acoustic properties. The primary goal of this study was to present an experimental method for measuring the vibration characteristics of this frame. The experimental setup was designed to induce controlled vibration of the solid component while minimizing the influence from coupling between vibrations of the fluid and the solid component. The Biot theory was used to verify this assumption, taking the two dilatational wave propagations and interactions into account. The experimental method was applied to measure the dynamic properties of glass spheres, lightweight microspheres, acoustic foams, and fiberglass. A continuous variation of the frame vibration characteristics with frequency similar to that of typical viscoelastic materials was measured. The vibration amplitude had minimal effects on the dynamic characteristics of the porous material compared to those of the granular material. For the granular material, materials comprised of larger particles and those under larger vibration amplitudes exhibited lower frame wave speeds and larger decay rates.

Structural and Acoustic Damping Characteristics of Polyimide Microspheres

“…The microspheres can be successfully modeled as a porous material supporting both types of waves 4 , leading to the expectation that a similar dual behavior would be observed. An analysis taking the two wave behaviors into account would have to be done to deconvolve the results.…”

“…Although designed primarily for granular material (here polyimide and glass microspheres), this test setup was also used to compare results to more common materials, such as foam and fiberglass. See Park 4 for more details.…”

“…The third test is targeted specifically at characterizing the material's dynamic frame properties. This test (and the fourth test) uses the transfer function method as described by Park 4 to derive the dynamic properties of the material over the frequency band of interest. This is useful in that common techniques can return damping factors only at modal frequencies, thus obscuring the broadband characteristics of the material.…”

Damping of Structural Vibration Using Lightweight Granular Materials

“…However, when the thickness of the sample is much less than the wave length, the coupling between the two different waves is negligibly small [12]. When the coupling between frame airborne wave is minimal and assuming harmonic motion, i.e., u x; t ð Þ ¼ Re b u x ð Þe i wt f g , the equation of motion for the one-dimensional longitudinal vibration of the solid component is simplified as…”

“…The measurement procedures based on this longitudinal vibration of the specimen are well established [12][13][14]. In this measurement, a standing wave was induced by input displacement at x =0 (u 0 ).…”