A Study On The Errors Existing In Measurement And Reconstruction Of Nearfield Planar Acoustic Holography

In this dissertation, the development and present status of nearfield acoustic holography is summarized. Analyzing the unsolved problems both in research and application, it is found that the errors existing in reconstruction and measurement of NAH have not been yet studied sufficiently. It is one of the important reasons for limiting the application of this technique and requires a great deal of further study.Analyzing characteristics both of the discrete real-space Green's function and the discrete reconstruction algorithm of planar acoustic holography based on 2-dimensions Discrete Fourier Transform, the error of the discrete reconstruction algorithm is confirmed when the real space Green's function is applied to reconstruct sound field. It is proved theoretically that the wraparound error can be eliminated by adding zeros to holographic sequences and uncertainty of value at edge of Green's function sequences does not influence result of reconstruction. It is also proved that the wraparound error is unavoidable as K-space Green's function is applied. It is suggested that the wraparound error can be illustrated by the amplitude decrement ratio 8 of the real-space Green's function. Influences of reconstruction's frequency and distance on the amplitude decrement ratio 8 and reconstruction's precision are discussed. It is pointed out that the k-space Green's function can only be used to nearfield reconstruction.The method for obtaining the real-space Green's function and its influence on reconstruction's precision are studied systematically. It is pointed out that if reconstruction's distance is very small, then the singularity of the sampled Green's functions lead to over-estimation of the angular spectrum of the sampled Green's functions and reconstruction error is remarkable. Thus the real-space sampled Green's function is not applicable to small distance holographic reconstruction. For the real space integrated Green's function, calculating process is given and a few parameters are corrected. The Astringency and approximate precision of the real space integrated Green's function based on Taylor series expansion are studied. Theoretical analysis and simulation show that more accurate reconstruction result can be gained by using the real space integrated Green's function both in nearfield and farfield.The types and characteristics of errors existed in holographic signal based on sound pressure measurement are analyzed, and error model of this measurement form is established. Influence of random error resulted from environment noise and electric noise on precisions both of holographic data and reconstruction is analyzed. The influence of the systematic errors due to the distortion of amplitude-frequency characteristic of the measuring channel on holographic data precision was analyzed quantitatively. Error expression of holographic reconstruction with respect to the systematic errors was educed. A correction method of holographic data based on transform function measurement between measuring channels was proposed. The results of theoretical analysis and simulation show that the above-mentioned correction method is effective.The singularity in calculating expression of holographic complex sound pressure's phase in planar acoustic holography based on intensity measurement is studied. The discrete algorithm of complex sound pressure reconstruction on the holographic plane based on acoustic intensity measurement is educed. It is pointed out that the phase fluctuating error will arise due to violation of approximate assumption in discrete algorithm. Influence of measurement parameters on phase fluctuating error is discussed. A window function is proposed to reduce phase fluctuating error. The effectiveness of the window function above-mentioned is presented by the result simulated. Methods of calculating amplitude approximately of complex pressure are discussed, and their precisions are compared. The influence of phase mismatch and amplitude mismatch between channels of acoustic intensity measurement s...