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The echo reduction of an underwater vessel is one of the major concerns of its concealing problem. Recently, using scatterers in the doubly periodic distribution has attracted enormous interest. This is because of their relatively good performance and the ability to tune in the effective frequency range. In this study, a model of an acoustic echo reducing coating with internal scatterer cavities is considered. Subsequently, the major parameters that affected the acoustical performance of the layer, such as the size, the number and the location of cavities along with the thickness, for two types of spherical and cylindrical cavity geometries, were analyzed and presented. The results showed that in the layer, the larger the cavity size, the greater the echo reduction effect. Regarding the lattice constants, considering the smaller one, decays the echo reduction. However, will increase transmission loss. Also, the location of the cavity, if close to the wave entry plane, demonstrates a better echo reduction effect.

The absorption performance of anechoic coatings depends on the material properties, layer thicknesses and cavity distribution density and cavity size. In this paper a design method based on numerical simulation was presented by combining FEM and acoustic duct method (ADM). Analyzing of anechoic coatings was performed under active sonar impinging plane wave by normal incident angle. In this approach considering a unit cell of anechoic coating and two ducts of the fluid on its both sides, the reflection characteristics, transmission and reflection coefficients of anechoic coating were calculated using acoustic duct theory. Validation was performed by comparing the results of simulation with the available experimental data. Also a design code in ANSYS software was developed. Finally using this code and the proposed model, the acoustical behavior of anechoic coatings was investigated. The results show that sound transmission coefficient for coating with cylindrical cavity is larger than that of spherical cavity. Furthermore unlike the transmission coefficient, for all frequencies upper than 1500 Hz, coating with cylindrical cavity has a greater echo reduction and therefore its anechoic performance is better than that of spherical cavity.