Study On Reconstructing Three Dimensional Interior Sound Field Based On Spherical Acoustic Holography

Microphone array with acoustic holography method is a classical sound field reconstruction technique;it can be applied in noise source identification and sound quality improvement of mechanical devices,etc.Most studies are focused on planar array and corresponding planar acoustic holography method at present.However,the planar array is not able to identify whether the source locates in front of the array or the opposite,thus this technique is inadaptable to apply in the 3D space where multiple sources or sound reflection and scattering exists.Considering the limitation of planar acoustic holography,this dissertation carried out the study of spherical microphone array and spherical acoustic holography.The sound field of surrounding space and sound pressure distribution on source structure can be reconstructed by the sound pressure data measured by spherical array with spherical acoustic holography method.Thus the technique is suitable to identify noise sources in 3D space,particularly in enclosed space.Spherical acoustic holography method can be divided into spherical harmonic beamforming method and spherical nearfield acoustic holography method.The performances of the two methods in sound field reconstruction and sound source identification were analyzed through numerical simulations and experiments,and the comparison of results were made.In the spherical harmonic beamforming method,the reconstructed sound field was supposed as planar waves or spherical waves generated by monopoles outside the array.The space directivity of beamforming output behaved well in middle and high frequency range.However,the method was adapted to the open space environment due to the selected sound field model.Moreover,the beamforming output can not reflect the true sound pressures but the relative magnitudes.In the spherical nearfield acoustic holography method,the above sound field model was not adopted,so this method was applied to open space as well as the enclosed space environment.And the process of sound field reconstruction complied to the spatial transform strictly,so it can reconstruct the true sound field by using the obtained sound pressures.The disadvantage of the method was that the reconstruction error would amplify quickly along with the increase of the radius of reconstruction sphere so that it can only be applied to the low frequency and nearfield cases.Besides the researches on the spherical nearfield holography methods,the dissertation also dealt with the theory and design method of the spherical microphone array front end.The detailed research contents of this dissertation are summarized as follows:1.The research significance and progress of sound field reconstruction in enclosed space as well as the spherical acoustic holography system has been reviewed.It shows that the spherical acoustic holography system is an effective tool for the interior sound field reconstruction.2.The general solution of acoustic wave equation in spherical coordinate system has been deduced,and some common sound field models have been presented.Then the principle of sound field reconstruction based on spherical microphone array with spherical acoustic holography including spherical nearfield acoustic holography method and spherical harmonic beamforming method has been introduced.3.It makes the sound field reconstruction based on spherical microphone array with spherical acoustic holography transferring from theoretical to practical by solving the following two problems.Firstly,the optimum microphone distribution on the spherical array at different points has been determined.Secondly,the incident sound field has been reconstructed at high accuracy with a rigid spherical microphone array,based on spherical acoustic holography with the mathematical model of scattered sound field.4.The sound field reconstruction based on spherical microphone array with spherical acoustic holography has been simulated in the plane wave and monopole source circumstance respectively;the accuracies of reconstruction are examined at different parameters.And the comparison between spherical nearfield acoustic holography method and spherical harmonic beamforming method has been made.5.The sound field reconstruction based on spherical microphone array with spherical acoustic holography has been simulated in the enclosed space of a vibrating sphere;the accuracies of reconstruction are examined at different parameters.And the regularization treatments are introduced into the process of sound field reconstruction to suppress the divergence of the reconstruction error along with the increasing of the truncated term number or the radius of reconstruction surface.6.Sound field reconstruction and sound source identification based on spherical microphone array with spherical acoustic holography has been experimented in the anechoic chamber,so as to validate the analysis in the simulations.And two practical applications of noise source identification by the spherical microphone array in the enclosed space have been introduced.7.Researches in this dissertation have been summarized,and the topics for further study have been proposed.