| Noise vibration is one of the harmful factor in industrial production and mechanical movement,while the realization of flexible regulation of low frequency noise with large wavelength and super penetrating ability in the range of 20-200 Hz is still a challengeable acoustic problem.Classical sound absorption theories and technologies are limited by the acoustic properties bottleneck of traditional sound-absorbing materials.So it implies difficulties to achieve high-efficiency low-frequency sound attenuation with controlled frequency range while considering lightness and small size.Fortunately,this bottleneck can be broken by introducing a specially designed phononic crystal which enable to generate elastic wave bandgaps.This provides a new way for the prevention and control of noise and vibration which generated in industrial production and mechanical movement.This dissertation briefly introduces the basic concepts of phononic crystals and the latest investigation about the phononic crystal bandgap characteristics.Focusing on the two core issues of opening the low-frequency wide bandgap as well as regulating the pressure field and designing the tunable acoustic waveguide structure,the bandgap characteristics of the designed soft material phononic crystal are numerically simulated by using the finite element method.The generation mechanism of the low-frequency wide bandgap is analyzed,and the key influencing parameters for the design of models with low-frequency bandgap are summarized and given simultaneously.The reason for that the pre-stretch strain enables to open several new bandgaps in low-frequency range is explained by combining with the modal analysis,and meanwhile the regulation mechanism of pre-strain on the bandgap of phononic crystal model with soft material is revealed.Moreover,the adjustability of wave propagation path in line-defect acoustic waveguide as well as the change rule of defect mode frequency in point-defect waveguide under pre-strain are studied.In the same vein,the regulation mechanism of hydrostatic pressure up to 14 GPa on bandgap frequency and different resonance peaks is preliminarily explored.In this thesis,in view of the influence of variability of low-frequency noise frequency on noise prevention in real life,we present a novel two-dimensional phononic crystal plate,which consists of lead columns deposited in a silicone rubber plate with periodic holes.The bandgap characteristics of the novel structure are calculated by finite element method.Accordingly,the influence of geometric parameters of the phononic crystal plate on the bandgap characteristics is analyzed and the bandgap adjustability of the porous phononic crystal plate under pre-stretch strain is further studied.Similarly,the regulation mechanism of pre-compressive strain on the bandgap of three-component phononic crystal was explored.Furthermore,we have preliminarily simulated the bandgap characteristics under the pressure up to 14 GPa by finite element method.Note that compared with the traditional single-sided cylindrical phononic crystal plate,the new designed model possesses lower starting frequency and wider bandwidth,which is due to the coupling between the resonance mode of the scatterer and the long traveling wave in the matrix with the introduction of periodic holes.Secondly,considering the effects of in-plane and out-of-plane symmetry of the structure and its stiffness on the bandgap,the passive optimization of bandgap can be realized by changing the geometric parameters of the model,while applying pre-strain to the matrix of optimized unit cell enable to realize active real-time regulation of low-frequency bandgaps under small deformation.After further analysis of the corresponding mode,it is found that the localization degree of the porous phononic crystal unit cell vibration increases with the rise of the pre-stretch strain,which makes the multiple bands in the low-frequency range tend to be flattened,thereby opening and widening a plurality of local resonance bandgaps in the low-frequency range.As for the phononic crystal with cladding layer,the starting frequency of the first bandgap gradually decreases with the increase of hydrostatic pressure,and the bandwidth presents a regular upward trend.The designed two structures improve the real-time adjustability of sound isolation and vibration reduction frequency for phononic crystal in complex environments.Aiming at exploring the regulation mechanism of pre-strain on acoustic wave propagation path in soft material acoustic waveguide with line defects,multi-channel acoustic waveguide structures with three-defect and double-defect are constructed based on the designed phononic crystal plate.At the same time,for the purpose of investigating the stability of vibration energy recovery in complex force field environment,a point defect acoustic waveguide structure is constructed by introducing point defects,and the effects of pre-stretch strain on defect mode frequency is also analyzed.In order to clarify the mechanism of hydrostatic pressure on energy recovery efficiency of acoustic waveguide with geometric point defects,we have preliminarily simulated the influence of hydrostatic pressure up to 8 GPa on defect mode frequency and corresponding resonance peak value of waveguide with point defects.Based on the localization of elastic wave energy at specific frequency in point defect waveguide,we also designed a hydrostatic pressure sensor based on three-component phononic crystal with cladding layer.It turned out that the designed acoustic waveguide structure can derive acoustic waves with specific frequencies in different channels.Applying a certain degree of pre-strain to the soft material matrix can realize the orderly control of the defect mode frequency in the low-frequency range.Moreover,the resonance peak frequency decreases regularly with the increase of hydrostatic pressure,and the resonance peaks reach the maximum values at different pressures.The designed hydrostatic pressure sensor has a high Q value,up to 8830 in the studied pressure range,which indicates that it possesses good sensing performance.The results mentioned above are contributed to the establishment of theoretical prediction model of defect mode frequency in complex force field environment,and also provide ideas for the design and optimization of vibration energy harvesting system with strong robustness. |
