Theoretical And Its Application Investigation On Near Field Acoustic Holography For Underwater Structures In Confined Space

The radiation noise level of underwater vehicle is one of the main performances measuring its fighting capacity and is attracted attention widely in military.The research achievements can be used for torpedo and submarine weapon system for improving the concealment and raising its combat effectiveness and survivability.Therefore,the study on radiating noise measurement and noise sources identification of the underwater structures have very vital significance for the noise reduction and low noise design.Compared to the traditional acoustic holography technology or other noise measurement methods,the NAH technology has advantages of steady measurement,high resolution for sound field reconstruction and accurate and reliable vibration distribution of the source surface.In order to ensure the accuracy of the measurement data,the ideal measurement of the underwater structure sound radiation is in the free-field tank environment or open reservoirs and lakes,which is not easy to achieve for the high cost.Many tanks are not free-field and.the wall reflections can not meet the requirements of the reverberation field.Because of the reverberation sound field,the conventional NAH has significant influence on the reconstruction accuracy.In order to play the advantages of NAH,the appropriate ways to eliminate the interference of measurement points on the influence of sound field reconstruction must be applied for obtaining the approximate free field acoustic information in the non-free field environment.To solve these problems,the following researches are made in this paper:Wave superposition approach based NAH technique is studied on the radiation sound of the underwater structure for carrying on the reconstruction and prediction of the acoustic field.The method is robust and can be used for full wave number space sound field reconstruction algorithm by the simulation calculation and experiment verification,which breaks through the limitation of measuring aperture size at least two times as long as the object size in the conventional NAH technique and can quickly and accurately reconstruct the sound field of the source with large dimension structures.If the patch NAH is required,the holographic surfaces needn't surround the entire structure but as long as the concerning area and can be non conformal.The reconstruction results has significant influence on many factors such as the equivalent sources location,the number of the equivalent sources,holographic surface size,distance between measuring points and signal-to-noise ratio and so on.For the half free sound field,the sound field separation technique with double measurement surfaces based on wave superposition method is studied and used for underwater noise sources localization,radiation field reconstruction and prediction in half free space.Various parameters that affect the precision of reconstruction are analyzed,and the reconstruction results are compared with the conventional wave superposition method.The method gets rid of the restrictions that all sound sources must be on one side of the hologram surface in conventional NAH technology and can be used for the sound radiation sound field reconstruction and prediction under the condition of interference of other sound sources or in the half free space.The NAH technology for the confined spaces is studied owing to the ineffectiveness reconstruction with the measurement sound pressure.The acoustic field separation based on ESM and HELS are derived in detail.For different sound field input information,the double surfaces measurement using pressure sensors and single surfaces measurement using vector sensors with pressure and particle velocity are applied for analyzing the separation errors with different parameters such as the frequency,SNR and the number of measurement points and non conformal measurement.The second separation can be applied by the surface impedance when outward sound field contains the scattering filed for recovering the free field pressure on the measurement surface accurately,which is used for reconstructing the source surface pressure and calculating the radiation sound power level.By comparison with the theoretical value,the correctness of the method is verified.The simulations of NAH method for confined space are carried out by using acoustic finite element software.The non free-field tank model is established and the results resulting from the different boundary conditions(absolutely soft,absolutely hard,impedance)are compared with the theoretical values for the verification of effectiveness for calculating the sound field in the confined space.The sound field separation errors are compared by calculating the sound field of different sound sources(point sources and cylindrical wave sources)with different boundary conditions.The radiation sound power levels and the directivity diagrams calculating from the recovered free field pressure are compared with the theoretical value.When the number of measuring points is not enough,the reconstruction precision and spatial resolution of the acoustic field can be improved by the ELM interpolation technology effectively in the confined space,which can obtain the acoustical field detail missed due to the distance between two measuring points.A method for patch NAH based on neural network extreme learning machine(ELM)is studied.The network is constructed by treating the measurement data as training sample and the area outside the measured aperture are explored and interpolated.The mathematical model for the patch NAH technique based on ELM is built and the processes of extrapolation and interpolation are given for the reconstruction,and then the accuracy of the acoustic field extrapolation and interpolation results are analyzed.The influences on the error owing to the relative parameters are also analyzed.The sound field reconstructions from the virtual holographic surface by the ELM application are compared with the conditions that only the measurement points are used.The effectiveness of the method is verified by numerical simulations.The sound field reconstruction accuracy can be effectively improved by the data extrapolation from small aperture measurement data.The accuracy and the resolution of reconstruction with few sensors can also be effectively improved by the data interpolation.Finally,the underwater sources NAH experiment is carried out in the confined spaces.By double cylindrical sound pressure measurement surfaces,according to the above two algorithms respectively,the complex sound pressure of underwater sources radiation are measured in anechoic environment and noise elimination.The results illustrate that this approach can be precise and robust,broading the NAH technique application range.