The absorption characteristics of anechoic coating embedding mixed-cavity structure with single or double shell backing

Rubber sheets embedding air cavities are wide used as anechoic coatings to reduce the energy of sound waves reflected by underwater structures, with a great military and use benefit to underwater vehicle sound stealth. According to the backing type, it can be divided into two kinds, namely rubber sheets-steel-air and rubber sheets-steel-water-steel-air, called the single and double shell backing coatings respectively. The most remarkable absorption characteristics of double shell backing coating is that its sound reflection coefficient frequency response curve increased on a series of sharp formants maxima, correspondingly, absorption coefficient curve appear a series of sharp minimum. This phenomenon is caused by the standing wave resonant which is formed in water interlayer between the double shells. Based on the finite element method, a non-rotating symmetrical numerical simulation model of coating embedding mixed-cavity, four small cavities around a big cavity, is established. Absorption characteristics of coating with single and double shell backing were studied. The results show that the sound absorption characteristics of mixed-cavity structure are significantly better than single-cavity structure´s under the condition of same perforation rate. The absorption coefficient peak can be improved 0.2.