Analysis Of Acoustic Radiation Based On The Propagation Wave

In the field of naval architecture, the propagation character of the noise is a big problem. Steady-state plane wave solution of acoustic wave equation are divided into propagation wave and evanescent wave. In terms of structure vibration and sound radiation, evanescent wave is non-radiation wave that caused by the subsonic wave of structure vibration and only contributes to the near-field sound pressure. Propagation wave is the superposition of ordinary plane waves, and is caused by the supersonic wave of structure vibration. It’s not only contributes to the near-field sound pressure, but also to the far field sound pressure. In the far field, we can only measure propagation waves. So the work of isolating the propagation waves and the evanescent waves has high sense of value. In this paper, all contents are as follows:We review the relevant literactures and summarize the main research results about the acoustic numerical calculation and the supersonic intensity caused by the propagation wave.Combining with the finite element method(FEM), the Mindlin plate bending theory is presented in detail. Then a simply supported Mindlin plate’s finite element model is established, through which calculating the natural frequencies and the vibration modes. The results are compared with the analytical solutions to prove the reliability of the finite element method (fem).Introducing the boundary element method(BEM) and the Rayleigh integral which apply to the infinite baffled plate structure. Using rigidly supported plate model to verify the Rayleigh integral program’s reliability.Explain the cause of the propagation and the evanescent wave, especially the origin of "supersonic intensity" produced by the propagation wave. Simply supported rectangular plate’s three radiation modal-surface radiation modal, corner radiation modal and edge radiation modal are introduced. Different forms of radiation modals correspond to different regional distribution of supersonic sound intensity in the plate. The supersonic sound intensity computing method based on the angular spectrum theory and the computing expression in spatial domain is then presented.Make a study of the vibration modesof the plate atructure based on the propagation wave to get the acoustic radiation characteristics(normal vibration velocity, pressure, sound intensity) only contributed by the propagation wave. Combine the BEM method with the spatial filtering process to realize the function of separating the propagation sound field from evanescent wave field. Realize the filtering process in frequency-spatial domain directly using the filtering function.Considering a simply supported rectangular plate, one dimensional radiation modal s are calculated and discussed using supersonic sound intensity expression in the spatial domain. Then certain mode shapes are selected to represent a velocity distribution pattern for the plate, The velocity patterns are equal to the first four (m×m) modes of the plate. A two-dimensional convolution is computed between surface normal velocity,surface sound pressure and the filter function respectively to obtain the active sound intensity which consider both of the propagation wave and evanescent wave and the supersonic intensity which only consider the propagation wave, thus isolating the propagation wave and the evanescent wave’s contribution to the sound radiation. To verify the radiation modal theory, we change the thickness of the plate. The radiation modes get changed with the variation of the natural frequencies under the same mode. For a number of natural frequencies under a point force excitation, comparing the active acoustic intensity with the supersonic sound intensity, then calculating the radiation sound power on the surface of the plate. We can see it is constant before and after the filtering process which proves that the evanescent wave radiates no acoustic power.