Investigation On Effect Of Boundary To Incident Waves

Structural borne noise has been investigated usually from two considerations.One concerns with sound radiation and transmission with application of the theory of acoustic-structure coupling,and the other one focuses on propagation characteristics of structural waves caused by mechanical vibration with application of the wave theory.In order to avoid dealing with the complicated higher order non-linear differential equations,in the previous investigations,it is commonly assumed that the wave incidence or vibration wave propagation relates only with infinite dimension of panel or boundary.Obviously,a general conclusion cannot be obtained under the simplified assumption since finite boundary always exists in real situation.Through theoretical analysis and numerical simulation with finite element,in this thesis,the problem of coincidence effect of sound transmission through finite panel and the absorption of flexural wave by devised boundary structure is investigated.Firstly the coincidence effect of sound wave through a finite panel and the coincidence effect in case that the incident sound is spherical or cylindrical wave is investigated,the properties related to transmission loss,the coincidence frequency and the frequency domain of absorption are obtained.Then,the absorption of flexural wave by boundary is investigate;and in order to absorb flexural wave,a model boundary is constructed and its moment impedance with proper choice of material parameters is derived;the reflection coefficient and absorption frequency band of the flexural wave are calculated based on the impedance match principle.Finally,the numerical results for flexural wave absorption by two boundary formations are presented,that might extend the absorption frequency band,including the energy dissipation properties given in comparison to show the absorption effectiveness.Our theoretical analysis and numerical simulation leads the following conclusions.First,the existence of finite boundary leads to two valleys in sound transmission loss centered at coincidence frequency and critical frequency respectively,which results in sound insulation reduction of a panel;the size of a insulation panel does not obviously affect the sound insulation in the frequency band the coincidence happen;and the coincidence effect of a spherical incident wave or a cylindrical wave affects the sound insulation weaker than a plane wave does.Next,a properly devised boundary structure can absorb a flexural wave in a beam or a panel perfectly in principle at a given frequency,and a boundary with gradually varying impedance can enhance the absorption effect.The research results in this thesis are focus on the effect of boundary in vibration and noise control,and have the broad significance in sound insulation design and the technology of constructing the absorption boundary which can absorb the flexural wave in panels.