The New Structure Of Broadband Low-frequency Sound Absorbing And Insulation Performance Based On Porous Materials

In this thesis,we explore new structures based on the porous materials for low frequency sound absorption and insulation by using theory,simulation and experiment methods.Through structure design,we examine systematically the sound absorption and insulation properties of porous material-based structures.The objective is focused on broadband and low-frequency sound absorbing materials with subwavelength thickness.The main work can be summarized as: 1.The sound absorption and insulation properties of sintered fibrous metals(SFM)are investigated.We propose a semi-analytical model of acoustic wave propagation in the SFM,and derive the equivalent density and bulk modulus of the SFM and in turn their sound absorption coefficients.The results show that the sound absorption coefficient of the SFM at low-frequency is small,and it increases with the increase of frequency.The relation between the sound aborption coefficient of the SFM and their microstructure parameters is also exmianed.In the same way,the transmission loss of the SFM is also analyzed.The influences of the sample thickness,porosity and fiber diameter on the sound insulation performance are also investigated systemaically.2.We propose a hybrid composite(CSF)made of the SFM and film,and another hybride composite(CSFA)made of the SFM,film and aluminium piece.Through this hybridization,we demonstrate their super sound absorption performance over the SFM at low-frequency.The influence of the film damping,cell size and stacking layer number on the sound absorption is analyzed using theory,simulation and experiment.We find that the aluminium piece attached on the film enhances the vibration intensity of the film,the sound absorption coefficient of CSFA is therefore increased at the resonance frequency.In order to design a structure with high sound absorption over a broad frequency band,we propose to sandiwich different unit cells with varying sizes targeted at different frequencies,a new material with a broadband sound absorption characteristic are then designed.3.We propose a super-absorption structure based on porous material-metamaterial(PMC),and derive its absorption coefficient analytcially.It is found that the absorption coefficient of PMC can be higher than the sum of the individual components,i.e.,porous material and metamaterial,called super-sbsorption phenomenon.In addition,we also show that the metamaterial placed behind of the porous material can improve the sound absorption coefficient of the porous material at the resonant cavity,and reduce greatly the apparent thickness of the resonant cavity.4.We design a new sound absorbing structure with a broadband efficiency and deep subwavelength thickness particularly for low frequency.The material is mainly composed of two axially coupled tubes in series which are co-planarly coiled in a plane perpendicular to incident waves.By carefully designing the geometric parameters of the coupled tubes,we can overlap the absorption coefficient curves of each individual tubes,and are therefore able to broaden the frequency bandwidth within which the absorption coefficient is larger than a designed value.And the effects of geometrical parameters of the composites are also analyzed.In addition,another four types of sound absorbing materials,such as coupled expansion tubes,tubes with continuous variable cross-section,parallel tubes and mixed tubes,are designed and their sound absorption performances are discussed.