Research On Acoustic Sources Identification And Acoustic Field Reconstruction For Middle And Low Frequency Based On Finite Aperture Array

Acoustic field measurement mainly includes two core tasks:acoustic source identification and acoustic field reconstruction.Among them,acoustic source identification can determine the main position of noise sources and control the noise from the source.The spatial acoustic field reconstruction can determine the noise radiation and attenuation characteristics,which can effectively control the noise in the transmission path.In order to achieve the above goal,the acoustic field is often measured and analyzed by using microphone array combined with acoustic imaging algorithm.However,in the cabin of ship,automobile,aircraft and other vehicles,the measuring space is often limited,and it is difficult to build a large microphone array for noise source identification and acoustic field reconstruction in the cabin.Therefore,convenient and efficient small aperture arrays are used for noise measurement in finite space.Due to the limited testing space,the microphone array can be built with small effective aperture and few measurement data,which directly affects the localization accuracy of acoustic source and increases the reconstruction error of sound field amplitude.More importantly,when the SNR of the measurement environment is low and there are multiple middle and low frequency noise sources distributed,it is difficult to effectively separate the positions of multiple acoustic sources whose spacing is less than half the wavelength of the signal due to the limitation of the half-wavelength theory.Therefore,it is difficult to determine the location of middle and low frequency acoustic source and assess the spatial sound field accurately.In order to solve these problems,this paper studies the acoustic sources identification and acoustic field reconstruction methods for middle and low frequency based on finite aperture array.The specific contents include:(1)A method for expanding the aperture of microphone array based on statistical optimal near-field acoustic holography is proposed.In order to solve the problem of limited array data and limited aperture caused by small space,a method for microphone array aperture expansion based on statistical optimal near-field acoustic holography is proposed.Firstly,the unit plane wave function matrix of measurement points in far-field and that of the expanded positions.Secondly,by solving the transformation coefficient of the plane wave function of the spatial acoustic field,the near-field acoustic pressure adjacent to the measured surface is reconstructed,so as to realize the virtual expansion of the array aperture.The stability of the array aperture expansion is improved by singular value truncation.Finally,the array data after the aperture expansion is applied to the beamforming method to improve the resolution,and a criterion of the array expansion validity is fitted by the simulation results,which can be used to determine the upper limit of array aperture expansion.The effectiveness of the method is verified by numerical simulation and localization experiments of two loudspeakers.The method can improve the spatial resolution of sound source localization based on far-field measurement under limited conditions and provide a new way for virtual expansion of array aperture.(2)A subwavelength acoustic source localization method based on evanescent wave correction is proposed.Aiming at the problem that high resolution identification of multiple lowfrequency acoustic sources with a spacing less than half wavelength is difficult for beam forming methods,a subwavelength acoustic source localization method based on evanescent wave correction is proposed.Firstly,based on the sound pressure data measured by the near-field,the cross-spectral time reversed field is constructed.Secondly,the time reversed focusing result is converted to the wave-number domain for correction to increase the evanescent wave content.Finally,the spatial inverse Fourier transform is used to obtain the final acoustic source identification result.Experimental results show that the proposed method can effectively improve the spatial resolution of the acoustic sources with sub-wavelength spacing under strong noise environment.The method provide a new scheme for high resolution of low frequency sound source localization in strong noise environment.(3)An acoustic field reconstruction method with off-grid model based on iterative weight l1-norm is proposed.In view of the difficulty of accurate reconstruction of acoustic source amplitude and spatial acoustic field in the noise environment,an acoustic field reconstruction method with off-grid model based on iterative weight l1-norm is proposed.Firstly,the acoustic field transmission function is established based on the off-grid model,and the optimization objective function based on the "block"sparsity constraint is constructed.Secondly,the l1-norm is weighted based on the"block" sparsity constraint,the weight is updated iteratively and the source strength vector is solved.After obtaining the final iterative solution result,the transmission matrix of spatial acoustic field reconstruction is established based on the off-grid coordinates to realize the spatial acoustic field reconstruction.The effectiveness of the proposed method is demonstrated by the reconstruction experiment of the amplitude and the spatial acoustic field of the two loudspeakers.The new method uses the optimized "block" constraint function for off-grid acoustic field reconstruction,which improves the reconstruction accuracy of acoustic source amplitude and acoustic field pressure.(4)The reconstruction of aeroacoustic field is carried out on the cylindrical structure under airflow environment.A microphone array measurement system with a small aperture is built in a small scale wind tunnel,and the three new methods are integrated into the reconstruction of cylindrical aerodynamic noise field.Firstly,the array aperture expansion method is used to expand the original array.Then the sub-wavelength acoustic source localization method is used to identify the distribution position of aerodynamic noise sources.Based on the above acoustic source identification results,the equivalent source configuration was optimized,and the spatial acoustic field was reconstructed by the acoustic field reconstruction method with off-grid model based on iterative weight l1-norm.Finally,the distribution of aerodynamic noise sources and the radiation characteristics of spatial aerodynamic noise field under the cylinder structure at 30 m/s were obtained.