Evanescent Wave Filtering Technology And Application In Structural Acoustic Radiation

The acoustic radiation prediction of the structural vibration surface is an important prerequisite for the optimal design of the structure and noise control.Identifying which areas of the radiation source contribute a lot of acoustic energy propagating to the far field is a very important task and can point the way for subsequent vibration and noise reduction.In this paper,taking the common plate structure and cylindrical shell structure in the field of ship and ocean engineering as an example,based on wave-number domain filtering,the supersonic acoustic intensity theory is used to study the evanescent wave filtering technology of structural acoustic radiation and its application in detail.This research can provide ideas for vibration and noise reduction of ship structure and have practical research value.The main contents completed are as follows:Based on the related theory of finite element method,the finite element analysis of Mindlin plate element is carried out.The strain displacement relationship,displacement function and strain matrix,stress-strain relationship,mass matrix and stiffness matrix of Mindlin element are deduced.The natural frequencies and modes of several orders of the structure are calculated by means of a simply supported plate,and compared with the calculation result of the commercial software to verify the correctness of the program.Boundary element method of structural acoustic radiation are introduced,the Helmholtz integral equations are dispersed to calculate the sound pressure,normal vibration speed,sound intensity,radiated sound power,sound radiation efficiency,field sound pressure,etc.Using the simply supported plate as an example,the sound pressure,normal vibration speed,and sound intensity were plotted and the radiated sound power was calculated.The method of calculating the supersonic acoustic intensity in the wavenumber domain is described in detail and the rectangular thin plate and the cylindrical shell are taken as examples for calculation.The comparison chart of the normal sound intensity and the supersonic acoustic intensity displays that contributions of vibration regions in the structure surface separated by the nodal line to the sound pressure in the far field are offset by each other.The supersonic acoustic intensity can clearly indicate the un-cancelled region of the surface of the plate,that is the hot spot region that contributes to the far-field acoustic radiation.Combining the evanescent wave filtering technology of structural acoustic radiation with the free field recoery technology,it breaks through the bottleneck of the free-far-field measurement conditions.A method has been developed to obtain the free-far-field sound radiation characteristics of targets in water under non-free-field conditions and near-field measurements,and an example is given.The modal complexity index MSC is used to characterize the complex modal complexity of a inhomogeneous damping plate,and discussing the influence of damping on supersonic acoustic intensity under different complex modal complexities.The sound radiation characteristics of inhomogeneous damping plates are revealed.The results show that when the complex modal complexity is large,the hot spot area of the supersonic acoustic intensity image will be smaller or slightly blurred,and individual bright spots will appear in the middle area,but overall,different damping laying areas,different damping arrangements and different additional damping ratios all have little effect on supersonic acoustic intensity characteristics of simply supported plates,still can distinguish regions on a vibrating structures which radiate to the far field,and can be consistent with the popular theories of corner and edge mode radiation from plates.Based on the deficiency of complex modal complexity index MSC measure the complex mode acoustic radiation characteristics,the relationship between complex modal complexity and radiated sound power is also discussed and a complex mode radiation complexity index CMRC is proposed to measure the complexity and characteristics of complex mode acoustic radiation: the higher the complex mode radiation complexity index CMRC for a certain order of the vibration structure,the weaker the acoustic radiation ability of the complex mode.