An Investigation Of Scan-based Partial Field Decomposition Method

Nearfield Acoustic Holography (NAH) is a useful tool for identifying noise sourcesand visualizing sound field. By measuring the hologram sound pressure, the acousticquantities such as pressure, particle velocity at any point in three-dimensional sound fieldcan be reconstructed. Based on analyzing the domestic and foreign research status of NAH,it is studied that NAH requires that the measured data of acoustic quantities on hologramsurface is fully coherent. The incoherent sound field by scanning manner must bedecomposed into fully coherent partial fields before NAH reconstruction. In thisdissertation, the basic principles of the previous partial field decomposition methods andthe factors of its error are studied. A new method is proposed, which is partial fielddecomposition method based on pressure gradient references. A numerical simulation andan experiment are carried out to test the new method. By comparing the results of partialfields based on pressure gradient references with those based on the pressure references, itis shown that the partial fields based on pressure gradient references are better than thosebased on the pressure references.In Chapter One, the domestic and foreign research status of NAH is reviewed andanalyzed. The advantages and disadvantages of the hologram data measurement methodsand the partial field decomposition methods are discussed in detail. Finally the keyproblems and the main research contents of this dissertation are determined.In Chapter Two, the principle of virtual coherence method is derived in detail. Thefactors of its error are discussed and the manners are recommended to reduce the error. Anumerical simulation with two incoherent pulsating sphere sound sources is carried outaround the factors of the error. The partial fields are reconstructed with Fast FourierTransform based NAH and the results are good.In Chapter Three, the principle of partial coherence method is derived in detail. Thefactors of its error are discussed. A numerical simulation is also carried out and the resultsof NAH reconstruction are good.In Chapter Four, based on the researches of virtual coherence method and partialcoherence method, a new procedure based on pressure gradient references is proposed. Anumerical simulation and an experiment are investigated to test the feasibility of themethod. And it is shown that the partial field decomposition method based on pressuregradient references is robust and its results are better than those based on pressure references.In Chapter Five, researches in this dissertation have been summarized and the topicsfurther studied have been proposed.