Xin Fengxian

Dynamic flow resistivity based model for sound absorption of multi-layer sintered fibrous metals

The sound absorbing performance of the sintered fibrous metallic materials is investigated by employing a dynamic flow resistivity based model, in which the porous material is modeled as randomly distributed parallel fibers specified by two basic physical parameters: fiber diameter and porosity. A self-consistent Brinkman approach is applied to the calculation of the dynamic resistivity of flow perpendicular to the cylindrical fibers. Based on the solved flow resistivity, the sound absorption of single layer fibrous material can be obtained by adopting the available empirical equations. Moreover, the recursion formulas of surface impedance are applied to the calculation of the sound absorption coefficient of multi-layer fibrous materials. Experimental measurements are conducted to validate the proposed model, with good agreement achieved between model predictions and tested data. Numerical calculations with the proposed model are subsequently performed to quantify the influences of fiber diameter, porosity and backed air gap on sound absorption of uniform (single-layer) fibrous materials. Results show that the sound absorption increases with porosity at higher frequencies but decreases with porosity at lower frequencies. The sound absorption also decreases with fiber diameter at higher frequencies but increases at lower frequencies. The sound absorption resonance is shifted to lower frequencies with air gap. For multi-layer fibrous materials, gradient distributions of both fiber diameter and porosity are introduced and their effects on sound absorption are assessed. It is found that increasing the porosity and fiber diameter variation improves sound absorption in the low frequency range. The model provides the possibility to tailor the sound absorption capability of the sintered fibrous materials by optimizing the gradient distributions of key physical parameters.

Sound Absorption Enhancement by Thin Multi-Slit Hybrid Structures

We report an extraordinary sound absorption enhancement in low and intermediate frequencies achieved by a thin multi-slit hybrid structure formed by incorporating micrometer scale micro-slits into a sub-millimeter scale meso-slit matrix. Theoretical and numerical results reveal that this exotic phenomenon is attributed to the noticeable velocity and temperature gradients induced at the junctures of the micro- and meso-slits, which cause significant loss of sound energy as a result of viscous and thermal effects. It is demonstrated that the proposed thin multi-slit hybrid structure with micro-scale configuration is capable of controling low frequency noise with large wavelength, which is attractive for applications where the size and weight of a sound absorber are restricted.

Vibroacoustic characteristics of micro-plates considering scale effect

Micro-plates of micron-size length, width and thickness are widely used in micro-electro-mechanical systems (MEMS), therefore, the analysis on the vibroacoustic characteristic of micro-plates is of paramount importance to ensure the stability of MEMS under acoustic excitation and the accuracy of acoustic sensors. The vibroacoustic performance of micro-plates with simply supported boundary condition is theoretically investigated by applying Cosserat theory and Hamilton variational principle, which has taken into account the scale effect of the micro-plate. The resultant equations are solved in conjunction with fluid-structure coupling condition. The developed model is used to investigate the influences of the scale effect and several key parameters, including length, width and thickness of the micro-plates, on the vibroacoustic characteristic of micro-plates. The present model hopes to provide a theoretical reference for the engineering optimization design of micro-plates in MEMS.

Acoustical properties of honeycomb structures filled with fibrous absorptive materials

The acoustical properties of the honeycomb structures filled with fibrous absorptive materials are theoretically investigated in the present paper. Since the fibrous materials are divided into periodical spaces, the model is developed based on a unit cell of the whole structure. The fibrous material in the honeycomb structure is modeled by applying the equivalent fluid theory with the obliquely incident sound existing on the periodical spaces in the form of standing wave. The energy flux is then obtained by the pressure and velocity at the fluid-structure interface, upon which the sound absorption coefficient and transmission loss is favorably calculated. The effects of the structure parameters on the acoustical properties are also discussed in the present paper. Results show that the hybrid structure has a better sound absorption ability than only fibrous material.