Methods For Recovering And Reconstructing The Free Radiated Field Of The Target Source In A Non-free Field

Near-field acoustic holography (NAH) is a powerful tool for sound field visualization. It can be used to identify sound source by reconstructing the acoustic quantities on the source surface, or to provide the radiation properties of products by predicting the radiating sound field. Till now, it has been widely used in the industries of vehicle, ship and home facility. However, the implementation of NAH requires the free-field assumption, i.e. the sources should be confined to one side of the measurement surface, and the other side should be source free. The free-field assumption is not easily satisfied in engineering since the measurement is usually implemented in situ and the disturbing source located at the other side of measurement surface may not be removed. For this kind of problems, a prevailing method is to use the field separation technique as the pre-processing technique of NAH to remove the influence of disturbing source, and then the free-field assumption is satisfied and NAH can be performed. However, the sound filed in a non-free field is composed of three components, which are the field that would be radiated by the target source in a free field (or the free radiated field), the disturbing field generated by the disturbing source and the scattered field that caused by the disturbing field falling on the target source. The field separation techqniue gives concerns to the first two fields but not the third field. When the frequency is high or performing NAH in a confined space, the scattered field may be strong and can never be neglected. Thus to ensure the accuracy of reconstruction in the non-free field, the free radiated field of the target source should be recovered first from the mixed field.In this dissertation, the recovery of the free radiated field and the accurate reconstruction using NAH in a non-free field is studied. The main content of the dissertation can be summarized as follows:In chapter1, the development of NAH was frst briefly reviewed, and then the current status of NAH in the non-free field was introduced and the problems existed were discussed, finally the main research objectives were determined.In chapter2, an extended formualtion of planar NAH was derived. In the derivation, the three components of the sound field in a non-free field were considered and thus the extended formulation can be used in a non-free field. For the singularity problem, an averaging method was proposed. The validity of the extended formulation was demonstrated by using simulations and experiments.In chapter3, a free field recovery technique was proposed based on spherical wave superposition method (SWSM). By using the free field recovery technique, the free radiated field that would be radiated by the target source was recovered from mixed sound field. A method for choosing the optimal number of spherical wave expansion terms was also proposed. The validity and necessity of the free field recovery technique was demonstrated by the simulation results. In the last, the application range of the SWSM-based free field recovery technique was also investigated.In chapter4, the equivalent source method (ESM)-based free field recovery technique was investigated. The adaptability of the ESM-based free field recovery technique was analyzed by three cases with different shapes of sources. The recovery technique was also used to indentify the active sources in a cavity, and the results of simualtions and experiments showed that the recovery techqniue could accurately indentify the positions of active sources.In chapter5, the field separation technique based on ESM and particle velocity measurement was first proposed, and it was shown that the field separation technique based on particle velocity measurement yileds better result of particle velocity than that based on pressure measurement. The factors that would influence the separation accuracy were also investigated. Further, the free field recovery technique based on ESM and particle velocity measurement was proposed and the validity and the advantage of the particle velocity measurement in recovering the free radiated field were proved by an experiment.In chapter6, the main work of this dissertation was summarized, and the topics needing further research were proposed.