Ships detect underwater targets such as submarines and mines mainly through sonar systems. In addition to the acoustic signals emitted by sonar, the ship's own noise is also a signal source. Therefore, acoustic stealth coatings should include sonar wave absorbing coatings and damping noise reduction coatings.
(1) Acoustic-absorbing nano-wave coating materials
The "quiet" submarine currently in service is covered with 3-5cm thick anechoic tiles, which can solve the problem of absorbing sonar waves. The absorption rate can reach 70% (above). The anechoic tile is integrally , and there is a specially designed cavity structure inside, so that the sonar waves entering the tile are reflected multiple times in the cavity. Absorption. However, the sound-absorbing tiles need to be pasted on the outside of the boat one by one. It is difficult to paste parts with complex shapes , and tiles often fall off. Therefore, the coating material is a good technical means. The principle of the sound-absorbing nano-wave coating is to first adjust The impedance of the coating material is as consistent as possible with seawater, so that the reflection of sonar waves on the surface of the coating is reduced as much as possible , so that most of them enter the coating.In the coating, sonar waves pass through multiple layers similar to the cavity structure , Structural materials with different densities are absorbed by mechanisms . What is currently needed is an ultra-thin sound-absorbing coating for mines, with a thickness of less than 2mm, and a thick coating for submarines, with a thickness of less than 3cm and a frequency range of The low-frequency part is less than 3Khz, and the absorption rate is greater than 70%. The development of sonar wave stealth coatings is very difficult, and it is being explored at home and abroad.
(2) Damping noise reduction coating
The damping and noise-reducing coating adopts viscoelastic materials, through the internal friction of the polymer's internal molecular chain, the vibration is converted into heat energy and dissipated, so as to achieve the purpose of reducing noise. In the 1990s, a constrained damping . After coating the damping layer on the substrate, adding a rigid constrained layer with a large Young's modulus can increase the loss factor several times. The damping layer can use specially cross-linked elastic polyurethane and fillers . The constraining layer uses reinforced epoxy coating and steel plate. There is a strict relationship between the thickness of the damping layer and the constraining layer. In order to achieve a good damping effect, damping structure design is required. The technical requirements of damping coatings are developing in the direction of low frequency and high temperature. Now the frequency is required to be less than 3kHz, and the temperature is required to be from room temperature to 60°C.
