Research On Nearfield Acoustic Holography Based On The Equivalent Source Method And The Measurement Of Particle Velocity

Nearfield acoustic holography (NAH), which can be used to identify sound sources and to visualize sound field, is an advanced technique for the analysis and measurement of sound. It has been introduced into the industries of ship, vehicle, and home facility, and gradually becomes one of the most powerful tools for dealing with the noise problems of products of these industries. In this dissertation, after reviewing the development and the recent achievement detailedly, NAH technique was further investigated in view of four key processes, including reconstruction algorithm, regularization, measurement, and preprocess. The investigation on the reconstruction algorithm is that the NAH based on the equivalent source method (ESM) was extended to the application in unclosed sound sources, and in moving sound sources. In the field of regularization, a stabler method for the choice of regularization parameter was proposed in order to overcome the limitation of the L-curve method. A new measurement method using Microflown transducers was introduced into NAH, by which the particle velocity was measured instead of the pressure, and better reconstruction of normal velocities on the surface of sound sources could be obtained. Besides, the superiority of particle velocity measurement in the NAH was examined based on the Fourier transform and on the ESM, respectively. Two novel techniques for sound field separation were proposed based on the Fourier transform and on the statistically optimized method, respectively, by which the measurement becomes more effective, and the separation precision becomes higher. Some experimental investigations were carried out with the inner field of a car based on the proposed methods, such as sound field separation and particle velocity measurement. The results demonstrated that the NAH technique was implemented successfully in the inner field. The main contents of the dissertation can be divided into two parts, one (Chapters 2 and 3) concerned the NAH based on the ESM, whereas the other (Chapters 4, 5 and 6) concerned the NAH based on the measurement of particle velocity. The detailed arrangement of the dissertation is summarized as follows:In chapter one, the development history and the current status of NAH were reviewed detailedly from the points of reconstruction algorithms, error analysis, filter and regularization, and recent achievement. Then problems still existed in NAH were discussed, and the main research contents of this dissertation were determined.In chapter two, the NAH technique based on the ESM was extended to the case of unclosed sound sources, which provides a basis for the Patch NAH based on the ESM. A moving ESM was firstly proposed to calculate the sound field radiated from moving sources with arbitrary shape. By applying the moving ESM to NAH, the reconstruction and prediction of the field generated by arbitrarily shaped sources were realized. The validity of the proposed theory in this chapter was demonstrated by numerical simulation and by the experiment carried out with a clamped plate excited at its centre.In chapter three, the reason why the regularization parameter was wrong or improperly chosen in NAH based on the ESM, was deeply analyzed. Then a new method, called | cosθ| curve, was proposed for the choice of regularization parameter, which dealt with the limitation of the L-curve method. The validity and the superiority of the proposed method was proved by numerical simulations of simply supported plate and oscillating sphere, and by experiments carried out with clamped plate, box, and double speakers.In chapter four, the particle velocity transducer named Microflown was simply introduced from its principle and its application in the field of acoustics. In order to improve the reconstruction of normal velocities on the surface of sound sources, both conventional and Patch NAH techniques were proposed based on the measurement of particle velocity. The singularity in the process of velocity-pressure reconstruction was analyzed first, and then was eliminated by a deduced equivalent formulation, which guaranteed the successful application of particle velocity measurement to the NAH based on Fourier transform. The correctness and the validity of proposed method were verified by the numerical simulation as well as the experiment, and a conclusion that the particle velocity is more suitable as an input of Patch NAH was also demonstrated.In chapter five, the measurement of particle velocity was introduced into the conventional and the Patch NAH based on the ESM to enhance the reconstruction precision of normal velocities on the surface of sound sources with arbitrary shape. By error sensitivity analysis, it was shown in theory that NAH based on the ESM is stabler when it was from particle velocity measurement. Because the particle velocity is a vector, a p-u method can be obtained by combined measurement of pressure and particle velocity, which can eliminate the reflection and the influence from the disturbed sources on the opposite side of the holographic surface. The conclusions in this chapter were demonstrated by numerical simulations and experiments carried out with different actual sources.In chapter six, the existed problem in the current sound field separation techniques was analyzed first, and then an improved separation method was proposed based on the Fourier transform and on the measurement in a single surface. In order to reduce the errors from the Fourier algorithm, a new separation technique was further proposed based on the statistically optimized NAH, by which the measurement efficiency and the separation precision were enhanced. The validity and correctness of the two proposed separation techniques were verified by numerical simulation and experiments in different cases. Finally, the advantage and the disadvantage of these two proposed methods were illuminated through the comparison between them.In chapter seven, a practical experiment was carried out in the inner field of a car. The methods of particle velocity measurement, the sound field separation, and the Patch NAH based on the ESM were successfully applied to identify the noise source and to visualize the sound field in the inner field. The result of the experiment provides a basis for the application of NAH to the inner sound field, and it is significant to the spread of NAH.In chapter eight, all the investigations in this dissertation were summarized, and the topics studied further in the future were proposed.