The Absorption Mechanisms Of Anechoic Tile

Anechoic tile has been widely used in the submarine as acoustic stealth equipment, which can reduce the reflection intensity of active sonar effectively. The intensive study on the absorption mechanisms of anechoic tile, which can be used for parameters optimization of material and structure, and anechoic tile type design for a specific region or band, has great significance in improving the survival rate and success rate of surprise attack of a submarine.This paper aims at achieving an acoustic character of efficient and broadband in anechoic tile. Based on the finite element method and multiple-scattering method, the effect of material loss factor and structure parameters on absorption characteristics of underwater anechoic tile with steel plate and air backing were investigated in depth. The main research contents and results are as follows:Based on the finite element method, a new mixed-cavity structure, a large cavity surrounded by some small cavities, was proposed. This structure increases the total inner surface area of cavities, so the range of radial vibration of the surface is expanded. It can cause overall vibration in anechoic tile, and make its resonant absorption work better. The absorption characteristics of anechoic tile embedding frustum-of-a-cone cavities or elliptic-cylinder cavities with single and double shell backing were studied. Under the condition of same perforation, the absorption performance in full band of anechoic tile embedding frustum-of-a-cone mixed-cavity can be enhanced; the greater oblateness, the lower resonant frequencies of anechoic tile embedding elliptic-cylinder mixed-cavity. Anechoic frequencies are consistent with the development direction of lower and wider broadband frequencies.Using the theory of Mie scattering, the varying of the size and distribution of the dynamic stress concentration factor and normalized displacement on the inner surface of an infinite rubber medium embedding isolated spherical cavity, thin spherical shell or local resonance structure with frequency was investigated. The different contributions of different waveforms in the resonant absorption mechanisms of the cavity were revealed. The effect of structural parameters such as the radii, locations, collocations of spherical cavities, thickness of spherical shell, periodic intervals of scatterer, thickness of rubber and thickness of steel on the absorption performance of anechoic tiles were studied using multiple-scattering theory. By discussing the effect of Laméconstant's loss factor on the absorption performance of anechoic tile, it can be found that the shear modulus loss factorημ plays a leading role in the cavity resonance absorption whileηλdetermines the sound loss at high frequency band. Based on the absorption mechanisms obtained, the structures of anechoic tiles with multiple layers of spherical cavities were designed. Though the loss factors of the thin anechoic tiles were not high, they had efficient broadband anechoic characters in particular high-frequency band or low-frequency band.The effect of material and structural parameters of anechoic tile embedding multi-layer spherical scatterer on its resonance frequency was investigated. Once the material parameters were decided, the stress matrix and displacement on the inner surface of an infinite rubber medium embedding isolated locally resonant scatterer were only concerned with fro and ri /ro . In coreless structure, when the silastic layer is so thin that it's kri larger than rubber's kro , the structure's resonant frequency can be reduced. When the local resonance structure has solid core, its first-order absorbing peak will shift to the higher frequency. So this structure is not conducive to the absorption of low-frequency sound waves.The Mie scattering matrix of eccentric spherical scatterer was derived. And the absorption characteristics of anechoic tile embedding eccentric spherical scatterer were studied. The results show that eccentricity can broaden the absorption peak of anechoic tiles and mitigate its absorption valley. This type of anechoic tile has better absorption characters than the one with concentric spherical scatterers in most conditions...