Theoretical investigation on the underwater sound absorption of a functionally graded anechoic coating under the phased arc array incidence

Laying anechoic coatings on the surface of underwater vehicles is an effective technology to improve the acoustic stealth performance. Meanwhile, the detection waves may have complex directivity, and it is important to study their influence on the sound absorption of the coating. This article introduces a functionally graded coating under a phased arc array incidence, which simulates a novel anechoic coating on an underwater vehicle to absorb sound waves emitted by active sonars. The incident field of the phased array is constructed by spherical harmonic expansion and the superposition principle. The wave propagation of the functionally graded coating is analyzed through an equivalent laminated model and the reverberation matrix method. Utilizing the angular spectrum method, the reflected sound field and absorption coefficient of the coating under the phased arc array incidence are predicted and the theoretical method is formed. Then the selections of parameters in Fourier transforms of the angular spectrum method are discussed for achieving converged results. A finite element analysis is applied to verify the effectiveness and calculation speed of the proposed method through the establishment of detailed quarter or half model. The effect of phased arc array parameters and the coating on the absorption and reflection performances are further studied. The angular spectrum decomposition of the incident sound field explains the changing trends of the sound absorption curves. The present work provides an important way to deal with this complex underwater sound propagation problem.