Study On Vibration And Structural Acoustics Of Structures Embedded With Acoustic Black Hole

The acoustic black hole has a concentration and manipulation effect on flexural waves in structures in a wide frequency range,and due to their easy-to-implement features,acoustic black hole structures have been a very important research hotspot in the field of vibration and acoustics which as a new method of vibration attenuation and noise reduction.The geometric acoustic method,semi-analytical method and finite element method were adopted to comprehensively and systematically investigate the flexural wave propagation in one-dimensional and two-dimensional acoustic black hole structures,the dynamic characteristics and influence mechanism of sandwich acoustic black hole beam,the sound insulation characteristics and influence mechanism of circular plates embedded with two-dimensional acoustic black hole,the vibration-acoustic coupling characteristics of a two-dimensional acoustic black hole plate-acoustic cavity and the application of acoustic black hole structure in engineering.Corresponding theoretical analysis models of flexural wave propagation in one-dimensional and two-dimensional acoustic black hole structures and the dynamics of sandwich ABH beams and a series of vibration-acoustic coupled finite element models embedded with two-dimensional ABHs were established.The effects of ABH parameters and damping layer on the system dynamics and vibration-acoustic coupling characteristics were analyzed in detail,and the accuracies of the established semi-analytical model and vibration-acoustic coupling finite element models were verified through experiments.The main contents and innovative research results of the thesis are as follows:1.The geometric acoustic approximation method was used to derive the motion control equation of the flexural wave in the one-dimensional acoustic black hole structure,and the calculation formula of the flexural wave reflection coefficient of the one-dimensional acoustic black hole structure with truncation which considering the influence of the damping layer on the system stiffness and mass and completely covered with the damping layer in the acoustic black hole area was obtained.The effect of the parameter changes on the reflection coefficient of flexural waves was analyzed,and the results show that when the one-dimensional acoustic black hole structure is not pasted with a damping layer,even a small truncation can sharply increase the reflection coefficient of flexural waves.It was also observed that the full coverage of the damping layer material in the one-dimensional acoustic black hole structure can significantly reduce the adverse effect of truncation on the flexural wave reflection coefficient,and the thicker the damping layer,the more beneficial it is to overcome the adverse effect of truncation on the flexural wave concentration effect.In order to help us more intuitively understand the aggregation effect of ABH structure on flexural waves,a finite element model was established to analyze the propagation of flexural waves embedded in a square plate embedded with two-dimensional ABH.The numerical calculation results clearly and intuitively show the propagation process of flexural waves in the square plate and the concentration effect of acoustic black hole on flexural waves.2.A semi-analytical model of sandwich truncated ABH beam structure with aluminum in the middle layer and steel in the upper and lower layers was constructed based on energy principle.The ABH effect of the sandwich beam under clamped-free boundary condition was analyzed.Meanwhile the effects of damping layer parameters,middle layer material and thickness on the vibrational acceleration response of the ABH region and the uniform beam region of the sandwich beam were also studied.It was observed that for the sandwich ABH beam structure,the influence of damping layer thickness on the acceleration response peak values of both the ABH region and the uniform region is very obvious in middle and high frequencies and the peaks at about 9 KHz are completely suppressed when the damping layer thickness reaching 3 mm.It also reveals that the use of aluminium as the middle layer material can bring a vibration attenuation at around 9 KHz both for the ABH region and the uniform beam region compared with using steel as the middle layer material.Experiments are carried out to verify the accuracy of simulation analysis.3.Series of vibro-acoustic coupling finite element models(FEM)for TL analysis of ABH circular plates were established by automatic matched layer(AML)method in this paper and an experimental platform for measuring TLs of ABH circular plates and uniform plates was constructed.The accuracy of the FEM analysis was verified by experimental measurements.To systematically analyze the influence mechanism of parameters of the ABH on TLs of ABH circular plates,the effects of diameter,number,and truncation thickness of ABHs on TLs of ABH circular plates were further studied.The effect of the damping layer on TLs of circular plates embedded with 1 and 19 ABHs was also analyzed and it reveals that the influence of damping layer mainly concentrates on the first-order resonance frequency and damping-controlled region of the plate,and at some frequencies,the greater the damping layer thickness,the worse the sound insulation performance,despite that the modal damping loss factor has been increased in the whole frequency domain.4.The vibration-acoustic coupling equation of the elastic plate-rectangular acoustic cavity under clamped boundary conditions was derived,and the solutions of the vibration velocity of the clampedly-supported elastic plate and the sound pressure in the rectangular acoustic cavity were given.A plate-acoustic cavity coupling finite element model composed of an elastic plate embedded with a two-dimensional acoustic black hole was established.Based on the acoustic mode theory,the difference between the acoustic modes of the plate-acoustic cavity coupling system composed of an elastic plate embedded with a two-dimensional acoustic black hole and a rectangular acoustic cavity was studied and the influence of the ABH structure on the acoustic mode of the acoustic cavity was also analyzed.The vibration-acoustic coupling frequencies of the plate-acoustic cavity coupling system composed of a two-dimensional ABH plate and the rectangular plate-acoustic cavity coupling system were obtained and the effects of the two-dimensional ABH on the system's vibration-acoustic coupling modes were compared.The sound pressure distribution of the acoustic cavity surface of the coupling system composed of the two-dimensional ABH plate and the rectangular acoustic cavity surface were calculated,and four monitoring points in the acoustic cavity were selected to investigate the effects of the two-dimensional ABH structure on the sound pressure distribution in the acoustic cavity.A two-dimensional ABH plate-acoustic cavity coupling finite element model with a damping layer attached on the ABH region was established and the effect of the damping layer on the sound pressure in the acoustic cavity of the plate-acoustic cavity coupling system composed of the plate embedded with a two-dimensional ABH was also studied.5.The application of acoustic black hole structure in vibration and noise reduction of frost-free refrigerator was explored.Through the vibration and noise test,the vibration acceleration spectrum of the compressor surface,the compressor feet,the connection parts of the supporting plate and the outer back wall of the refrigerator and the 1/3 octave frequency spectrum of the radiated sound of the outer back wall were obtained.The coherence theory was adopted to analyze and determine the main vibration source causing the radiation noise of the refrigerator box.The finite element model of the compressor support plate embedded with the 2D acoustic black hole structures and attached with the damping layers in the ABH areas was established.Through the analysis of the vibration modes and responses of the system and setting of monitoring points,the effects of the two-dimensional ABHs and damping layers on the vibration characteristics of key component of the refrigerator was studied.The results show that the two-dimensional ABHs and damping layers can suppress the vibration of the compressor at 1390.6 Hz,2087.5 Hz,and 2260.9 Hz transfering from the support plate to the refrigerator box,so that it can also suppress the radiated noise of the outer back wall of the refrigerator at 1250 Hz,2000 Hz and 2500 Hz.