By the spherical wave spectrum transform,the sound pressures on the two spherical surfaces surrounding the scattering objects with arbitrarily-shaped sur-faces are decomposed into spherical wave components that propagate in a known manner,the relationships between the spherical wave components of the same order on the two spherical surfaces are established by the wave field extrapolation theorem,and the formula of the separation theory in the spherical coordinate is established in the wave-number domain. After separating the scattered pressure,the total scattered field can be obtained by holographic reconstruction and predic-tion. In order to overcome the instability of acoustic inverse problem,a new wave-number domain filter technique is proposed. It is proved that,as long as the two holographic spherical surfaces are taken to be close enough,the singularity of the separation formula can be avoided. The results of numerical simulation dem-onstrate the feasibility and validity of the separation theory.
LI WeiBingLIAN MeiZhuanBI ChuanXingCHEN JianCHEN XinZhao
A new method based on interpolation using band-limited signal restoration method was proposed for enhancing the resolution of the nearfield acoustic holography. According to the band-limited property of the pressure on the hologram surface, a band-limited signal restoration method named Papoulis-Gerchberg algorithm was used to realize the interpolation of acoustic pressure. Therefore acoustic pressure data on the hologram surface were increased, the sampling interval was reduced, the information on evanescent waves which was lost because of the large sampling interval was partially recovered, and the resolution of nearfield acoustic holography image was improved. The experimental result shows that the method can enhance the resolution of the nearfield acoustic holography image efficiently.
On the basis of nearfield acoustic holography (NAH) based on the equivalent source method (ESM), patch NAH based on the ESM is proposed. The method overcomes the shortcoming in the conventional NAH that the hologram surface should be larger than the source surface. It need not to discretize the whole source and its measurement need not to cover the whole source. The measurement may be performed over the region of interest, and the reconstruction can be done in the region directly. The method is flexible in applications, stable in computation, and very easy to implement. It has good potential applications in engineering. The nu- merical simulations show the invalidity of the conventional NAH based on the ESM and prove the validities of the proposed method for reconstructing a partial source and the regularization for reducing the error effect of the pressure measured on the hologram surface.
