Acoustic Reflection Charactertristics Of Layered Absorber With Inhomogeneous Medium Layer

In order to increase the absorption of the sound waves, submarines are often covered with viscoelastic layers. In this paper, the finite element method is applied to analyze the sound characteristics of thin layered acoustic structures, containing fluid layer, elastic layer and viscoelastic layer with periodic porous medium, at low frequency (0-2000Hz). It is considered that the fluid can be modeled as a viscoelastic medium with equivalent Lame constants. In addition, for thin layer at low frequencies, the porous frame may be assumed as motionless. In this case, the reduced control equation of pore fluid in porous material is obtained by Biot's theory. Similarly, the pore fluid is also equivalent to a viscoelastic medium. The finite element formulation is derived by using the Galerkin technique. Influences of material constants and the structure both of the porous medium and the heavy particles on reflection characteristics of the structure is analyzed. In addition, we also considered the corporate influences of the porous medium and the heavy particles through the double cell structure. The results show that existence of the inhomogeneous medium layer has obvious effect on the reflection coefficient of the structure at lower frequency. This research provides important reference for the practical engineering.