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Vibration Mitigation

A metastructure with a membrane and arch masses
A feasible analytical model for accurately predicting the bandgap location of a metastructure is proposed. The interior resonating unit consists of a membrane and arch masses. Tuning the geometric properties of arch masses can effectively adjust the bandgap location. The multi-frequency wave filtering can be achieved by using a multi-celled structure. Experiments are performed to verify the simulated and predicted results.

A metashaft with shunted piezoelectric materials
The addition of shunted piezoelectric rings on an elastic shaft is proposed. Using the negative capacitance resonant shunt provides a feasible approach to lower bandgap frequencies and broaden bandwidth. Adjacent cells of different electric parameters can achieve multi-frequency torsional vibration suppression. The attenuation frequency highly relies on the inductance and capacitance but not on resistance which only affects the attenuation peak amplitude.

A metastructure with double-spiral local resonant systems
A novel frame-like beam with double-spiral local resonant systems is proposed. A stiffness matrix method is introduced to predict the resonant frequency of the resonating system. The bandgap location can be tailored by tuning mass thickness or the number of spiral beams. Masses of varied thicknesses across cells can create a bandgap with relatively broad bandwidth. Within the bandgap, negative mass density appears.

Metamaterial beams with membrane-mass structures
A metamaterial beam is comprised of a host beam containing periodic circular cavities filled with membrane-mass structures. Two kinds of bandgaps exist in the present structure: locally resonant bandgap (LRBG) and Bragg-type bandgap (BBG). The former originates from the resonant behavior of the resonator while the latter results from the structural periodicity. By altering the properties of the membrane-mass structure, the LRBG can be easily tailored. Multiple cells with different masses can create multiple bandgaps.

Sandwich structures with multi-resonators
Flexural wave propagation in a sandwich beam with multiple local resonators is presented. A two-resonator system connected in series or in array is introduced. Compared to the single-resonator system, the two-resonator system can offer richer bandgap characteristics. The array-type resonator is able to produce a boarder attenuation zone while the series-type resonator can create a bandgap with a frequency-multiplication relationship. The frequency range where effective mass density becomes negative coincides with the bandgap.

Sandwich beam on elastic foundation under moving loads
The propagation of sandwich structures with periodic assemblies on elastic foundation under external moving load is studied. Two types of periodic assemblies are considered, namely, a periodic core and a core with periodically embedded resonators. The sandwich beam is modelled as an equivalent Timoshenko beam. Wave numbers and travelling wave characteristics in the velocity field are analyzed by introducing a moving coordinate system. The Bloch theorem is employed to examine wave propagation of the structure with periodic assemblies. The critical velocities on both models are also determined.

Sandwich structures with internal absorbers
Flexural wave motion and power flow characteristics of sandwich beams with internal absorbers are investigated. The results show that the internal absorbers are able to damp out the unwanted waves generated by disturbances. A higher loss factor can create a wider attenuation zone but reduce the attenuation intensity. Frequencies corresponding to zero power flow lie in the bandgap.

Sandwich structures with resonators and periodic cores
Flexural wave propagation of a sandwich beam with periodic cores as well as local resonators is studied. Bandgaps where waves cannot propagate freely exist in the sandwich beam with embedded resonators or with periodic core properties. The effects of material properties of the resonator or the core on bandgap characteristics are investigated. It is found that the coupling of two mechanisms can enlarge gap width. Tailoring the material properties of the core/resonators enables to manipulate the location of the bandgap.

    Noise Reduction

Membrane-ring-orifice structure
A modified acoustic metamaterial consisting of membrane-ring structure with an orifice is presented. Two transmission loss (TL) peaks with the intensity over 40dB are created. Frequencies with unbounded effective mass density and sharp phase change coincide with the TL peak frequencies. When the membrane-ring-orifice structure is backed by a cavity, it can be served as a ventilated composite resonator possessing dual-frequency sound filtering capability in the low-frequency regime.

Coupled membrane-ring structures
Sound transmission of a coupled membrane-mass structure is presented. Unlike traditional membrane-type metamaterials only existing one transmission loss (TL) peak, an extra TL peak can be generated. The present structure possesses negative effective mass density or negative bulk modulus at certain frequency ranges. This coupling system can create a wider attenuation zone as well as a stronger attenuation.

Transmission analysis of a membrane with frame-shaped masses
Sound transmission of a membrane with multiple frame-shaped masses is investigated. A practical theoretical model is introduced to precisely predict transmission loss characteristics. The addition of frame mass to structure results in multi-peak profile, depending on the number of frames. Frequencies corresponding to negative dynamic mass lie in TL peak frequencies. Transmission loss peaks can be tailored by the width of the frame, the location of the frame, and the mass magnitude of the frame.

    Sound Absorption

A labyrinthine acoustic metasurface
A novel labyrinthine acoustic metasurface with nonuniform arc channels exhibits good absorption performance Changing the opening diameter or channel width can effectively adjust the target absorption frequency. Also, using a unit with multiple different branches can effectively increase the absorption bandwidth. Arc-shaped channels exhibits effectiveness and flexibility in the space usage.

A sound absorber with coiled Helmholtz resonators
A potential sound absorber is comprised of coiled Helmholtz resonators. This compact structure exhibits good sound absorption performance at low frequencies. An impedance-based method is proposed to capture the absorption characteristics of the present absorber. The manipulation of absorption band can be achieved by adjusting tube diameters and lengths. Resonators assembled in parallel can effectively broaden the absorption bandwidth.

Energy harvesting

A metastructure possessing vibration suppression and energy harvesting capabilities
A metamaterial beam is comprised of a host beam containing periodic square/circular cavities filled with membrane-mass structures. By altering the properties of the membrane-mass structure, the locally resonant bandgap can be easily tailored. With the proper design, the filtering bandwidth can be significantly broadened. The effects of the ring configuration and position on bandgap properties are illustrated. It is found that double-layer resonators can effectively broaden the bandgap width. The addition of the PVDF film on membrane surface is able to form a vibration-based energy harvesting device. Frequency with maximum energy output coincides with the vibration dip.

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