Research On Algorithm Of Near-field Source Visualization In A Non-free Field

Near-field acoustic holography (NAH) which only records the pressure on the source surface can predict and reconstruct any acoustic quantity in the whole 3-D field. It has huge potential in sound field visualization. However, the implementation of NAH requires the free-field assumption, i.e. the dorsal of the measurement surface must be for free, which greatly limits its application in non-free environment, for example in car or submarine cabin.Now for implementing the non-free field acoustic transform, it often adopt the sound field separation technique to remove the disturbing source behind the measurement surface,However which neglects the scattered sound by the surface of the source. When the intensity of the disturbing is very strong or measurement in confined space will result in failure of the separation. By using the free field recovery technique based on spherical wave expansion with a single origin (SOSWE) recover the free field including the cabin reflecting and the scattering on the source surface, which provide the free field condition of the reconstruct.Five chapters can be summarized as follows:1. The development of NAH was first briefly reviewed, and then the current status of NAH in the non-free field was introduced.2. Based on the Helmholtz equation, the formula of the NAH at the Cartesian coordinate,the cylindrical coordinate and the spherical coordinate are derived. The algorithm errors of implementing NAH are discussed,and give the measures to restrain them. By numerous numerical simulations demonstrate the high resolution of the NAH on the sound source identification.3. Sound field separation technique has been derived according to superposition principle of the small amplitude wave equation. Numerical simulations demonstrate the necessity and the availability of the sound field separation in removing the disturbing source behind the measurement surface, then prove the applicability of the spherical sound separation on restraining reflect in the enclosing space.4. Consider the scattered by the sound surface, an extended formulation of planar NAH was derived in the non-free field. By using the free field recovery technique based on SOSWE implement the piston source identification in the rigid sphere cavity, by way of simulation. A further argument that spherical sound separation is accurate and steady on the sound source identification in enclosing cavity is proved.5. Some algorithms of near-field source visualization in my article are feasible and effective by designed planar acoustic array.