Impedance Tube


Vibration Mitigation

Metamaterial beams with membrane-mass structures[1]
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[6]
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[8]
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[10]
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[11]
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 Insulation

Coupled membrane-ring structures[3]
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[7]
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.

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