Fengxian Xin

Analytical and experimental investigation on transmission loss of clamped double panels: Implication of boundary effects

The air-borne sound insulation performance of a rectangular double-panel partition clamp mounted on an infinite acoustic rigid baffle is investigated both analytically and experimentally and compared with that of a simply supported one. With the clamped (or simply supported) boundary accounted for by using the method of modal function, a double series solution for the sound transmission loss (STL) of the structure is obtained by employing the weighted residual (Galerkin) method. Experimental measurements with Al double-panel partitions having air cavity are subsequently carried out to validate the theoretical model for both types of the boundary condition, and good overall agreement is achieved. A consistency check of the two different models (based separately on clamped modal function and simply supported modal function) is performed by extending the panel dimensions to infinite where no boundaries exist. The significant discrepancies between the two different boundary conditions are demonstrated in terms of the STL versus frequency plots as well as the panel deflection mode shapes.

Erratum: Hybrid acoustic metamaterial as super absorber for broadband low-frequency sound

A hybrid acoustic metamaterial is proposed as a new class of sound absorber, which exhibits superior broadband low-frequency sound absorption as well as excellent mechanical stiffness/strength. Based on the honeycomb-corrugation hybrid core (H-C hybrid core), we introduce perforations on both top facesheet and corrugation, forming perforated honeycomb-corrugation hybrid (PHCH) to gain super broadband low-frequency sound absorption. Applying the theory of micro-perforated panel (MPP), we establish a theoretical method to calculate the sound absorption coefficient of this new kind of metamaterial. Perfect sound absorption is found at just a few hundreds hertz with two-octave 0.5 absorption bandwidth. To verify this model, a finite element model is developed to calculate the absorption coefficient and analyze the viscous-thermal energy dissipation. It is found that viscous energy dissipation at perforation regions dominates the total energy consumed. This new kind of acoustic metamaterials show promising engineering applications, which can serve as multiple functional materials with extraordinary low-frequency sound absorption, excellent stiffness/strength and impact energy absorption.

Tensional acoustomechanical soft metamaterials

We create acoustomechanical soft metamaterials whose response to uniaxial tensile stressing can be easily tailored by programming acoustic wave inputs, resulting in force versus stretch curves that exhibit distinct monotonic, s-shape, plateau and non-monotonic snapping behaviors. We theoretically demonstrate this unique metamaterial by considering a thin soft material sheet impinged by two counter-propagating ultrasonic wave inputs across its thickness and stretched by an in-plane uniaxial tensile force. We establish a theoretical acoustomechanical model to describe the programmable mechanics of such soft metamaterial, and introduce the first- and second-order tangential stiffness of its force versus stretch curve to boundary different behaviors that appear during deformation. The proposed phase diagrams for the underlying nonlinear mechanics show promising prospects for designing tunable and switchable photonic/phononic crystals and microfluidic devices that harness snap-through instability.

External Mean Flow Effects on Sound Transmission Through Acoustic Absorptive Sandwich Structure

A theoretical model is developed to investigate the influence of external mean flow on sound transmission through an infinite double-leaf panel filled with porous sound absorptive materials. The sound transmission process in the porous material is described by using the equivalent fluid model, while fluid-structure coupling conditions are employed to ensure displacement continuity at fluid-structure interfaces. In order to verify the theoretical model, the model predictions are compared with existing experimental results. Numerical investigations are subsequently performed to quantify how a set of systematic parameters affect the sound transmission loss. It is demonstrated that the porous material affects the transmission loss in terms of both the absorption effect and the damping effect. Besides, thematerial loss factor and the thickness of the face plates also have an influence on the coincidence dip of the transmission loss curve. At frequencies below the coincidence frequency, the mean flow increases the transmission loss values due to the added damping effect of themean flow, whilst shifts the coincidence frequency upward because of the refraction effect of the mean flow. In addition, the coincidence frequency decreases with increasing azimuth angle between the sound incident direction and mean flow direction.

Acoustomechanical giant deformation of soft elastomers with interpenetrating networks

We demonstrate giant deformation caused by ultrasound waves in soft elastomers with interpenetrating networks and reveal the physical mechanisms underlying the snap-through instability and phase transition. The snap-through instability can be harnessed to generate large deformation when the elastomer is subjected to combined mechanical and acoustical loading. We further demonstrate that the preserved stresses can enhance not only the mechanical tangential stiffness but also the acoustical tangential stiffness of the elastomer. However, with fixed acoustical loads, the preserved stresses reduce the mechanical tangential stiffness because the dependence of acoustic radiation stress on the stretch state overturns the effect of the preserved stresses. Our findings enable new strategies of device designs based on acoustomechanical soft elastomers having interpenetrating networks.

Ultrathin multi-slit metamaterial as excellent sound absorber: Influence of micro-structure

An ultrathin (subwavelength) hierarchy multi-slit metamaterial with simultaneous negative effective density and negative compressibility is proposed to absorb sound over a wide frequency range. Different from conventional acoustic metamaterials having only negative real parts of acoustic parameters, the imaginary parts of effective density and compressibility are both negative for the proposed metamaterial, which result in superior viscous and thermal dissipation of sound energy. By combining the slit theory of sound absorption with the double porosity theory for porous media, a theoreticalmodel is developed to investigate the sound absorption performance of the metamaterial. To verify the model, a finite element model is established to calculate the effective density, compressibility, and sound absorption of the metamaterial. It is theoretically and numerically confirmed that, upon introducing micro-slits into the meso-slits matrix, the multi-slit metamaterial possesses indeed negative imaginary parts of effective density and compressibility. The influence of micro-slits on the acoustical performance of the metamaterial is analyzed in the context of its specific surface area and static flow resistivity. This work shows great potential of multi-slit metamaterials in noise control applications that require both small volume and small weight of sound-absorbing materials.

Sound Absorption Optimization of Graded Semi-Open Cellular Metals by Adopting the Genetic Algorithm Method

Built upon the acoustic impedance of circular apertures and cylindrical cavities as well as the principle of electroacoustic analogy, an impedance model is developed to investigate theoretically the sound absorption properties of graded (multilayered) cellular metals having semi-open cells. For validation, the model predictions are compared with existing experimental results, with good agreement achieved. The results show that the distribution of graded geometrical parameters in the semi-open cellular metal, including porosity, pore size, and degree of pore opening (DPO), affects significantly its sound absorbing performance. A strategy by virtue of the genetic algorithm (GA) method is subsequently developed to optimize the sound absorption coefficient of the graded semi-open cellular metal. The objective functions and geometric constraint conditions are given in terms of the key geometrical parameters as design variables. Optimal design is conducted to seek for optimal distribution of the geometrical parameters in graded semi-open cellular metals.

Deep subwavelength acoustic metamaterial for low-frequency sound absorption

A novel class of low-frequency sound absorbers based on a honeycomb sandwich panel is theoretically designed and numerically demonstrated. The absorber with a remarkably small thickness (e.g., 1/131 of wavelength) is comprised of a perforated facesheet, a perforated honeycomb core and a non-perforated back panel. Built upon the classical microperforated panel absorber (MPPA), the idea of introducing a perforated honeycomb core which creates a double-layer perforated absorber (DLPA) without adding to the total thickness greatly enhances the low-frequency absorption performance. Theoretical predictions of the sound absorption coefficient are obtained and compared with numerical simulations obtained using the finite element method (FEM). A good agreement is achieved. The proposed sound absorber is promising for low-frequency noise absorption especially when limited space and high mechanical stiffness/strength are simultaneously demanded.

Modeling of surface roughness effects on Stokes flow in circular pipes

Fluid flow and pressure drop across a channel are significantly influenced by surface roughness on a channel wall. The present study investigates the effects of periodically structured surface roughness upon flow field and pressure drop in a circular pipe at low Reynolds numbers. The periodic roughness considered exhibits sinusoidal, triangular, and rectangular morphologies, with the relative roughness (i.e., ratio of the amplitude of surface roughness to hydraulic diameter of the pipe) no more than 0.2. Based upon a revised perturbation theory, a theoretical model is developed to quantify the effect of roughness on fully developed Stokes flow in the pipe. The ratio of static flow resistivity and the ratio of the Darcy friction factor between rough and smooth pipes are expressed in four-order approximate formulations, which are validated against numerical simulation results. The relative roughness and the wave number are identified as the two key parameters affecting the static flow resistivity and the Da...

Tunable acoustic absorbers with periodical micro-perforations having varying pore shapes

Circular pores with sub-millimeter diameters have been widely used to construct micro-perforated panels (MPPs), the acoustical performance of which can be predicted well using the Maa theory (Maa D.-Y., J. Acoust. Soc. Am., 104 (1998) 2861). We present a tunable MPP absorber with periodically arranged cylindrical pores, with their cross-sectional shapes systematically altered around the circle while maintaining their cross-sectional areas unchanged. Numerical analyses based on the viscous-thermal coupled acoustical equations are utilized to investigate the tunable acoustic performance of the proposed absorbers and to reveal the underlying physical mechanisms. We demonstrate that pore morphology significantly affects the sound absorbption of MPPs by modifying the velocity field (and hence viscous dissipation) in the pores. Pore shapes featured as meso-scale circular pores accompanied with micro-scale bulges along the boundaries can lead to perfect sound absorption at relatively low frequencies. This work not only enriches the classical Maa theory on MPPs having circular perforations, but it also opens a new avenue for designing subwavelength acoustic metamaterials of superior sound absorption in target frequency ranges.