Research On Patch Nearfield Acoustic Holography And Resolution Enhancement Of Nearfield Acoustic Holography

Nearfield acoustic holography (NAH) is a powerful technique for identifying noise sources and visualizing acoustic field. It has high resolution, because the evanescent waves which contain much detailed information of the acoustic field are utilized in the method. Therefore, it can be wildly used in mechanical equipment fault diagnosis and noise reduction.While NAH suffers from algorithmic errors, that are caused by finite measurement aperture effects and other factors. These errors would rise rapidly with the reduction of the measurement aperture. Therefore, NAH can not be applied when measurement aperture is small. This limitation is an obstacle to-extensive application of NAH technology. In order to overcome this shortage, a novel acoustic holography technique called as Patch NAH has been proposed in recent years. Compared with NAH technique, there exists a special procedure called as hologram pressure extrapolation in Patch NAH. By this procedure, the pressure measured in small aperture is extended or "continued" into a region which is larger than the original measurement aperture. That is why Patch NAH can sufficiently reduce algorithmic errors and keep the accuracy when the measurement aperture is small. Therefore, the hologram pressure extrapolation procedure is the key of Patch NAH, and the research work of this paper focuses on how to extrapolate hologram pressure. In this paper, three novel methods for extrapolating hologram pressure are proposed. And three kinds of Patch NAH technique based on these methods are also established. The validity of these methods has been verified by numerical simulations and experiments. Meanwhile, it is found that the proposed pressure extrapolation methods can also be used to interpolate the measured pressure for the NAH image spatial resolution enhancement. Therefore, the NAH image spatial resolution enhancing techniques were proposed in this paper. The experiment shows that these techniques can effectively improve the resolution of NAH image without increasing the measurement points. The main contents of the dissertation are shown as follows:In chapter one, the history of NAH was reviewed briefly. By analyzing the current status of NAH technique, the existing problem was discussed. To deal with this problem, the Patch NAH technique was introduced. By analyzing the research status of Patch NAH, the main research contents of this dissertation were determined. In chapter two, two kinds of the algorithmic errors of NAH were studied in detail. They are finite measurement aperture effects caused by truncating of acoustic field and wrap-around error related to dispersion in wave number domain. By analyzing the generation mechanisms of these errors, it shows that these errors have a close relationship with the measurement aperture size, and can be sufficient reduced by hologram pressure extrapolation. To prove the validity of hologram pressure extrapolation for reducing the algorithmic errors, numerical simulations and experiments were performed.In chapter three, to extrapolate the hologram pressure efficiently, the property of the hologram pressure was researched firstly. The band-limited property of the hologram pressure was illuminated. According to this property, the theoretical feasibility of hologram pressure extrapolation was proved and a novel hologram pressure extrapolating method based on a band-limited signal restoration method named Papoulis-Gerchberg algorithm (PGA) was proposed. The Patch NAH based on this method was also established. Furthermore, it is theoretically proved that the PGA-based pressure extrapolating method also can be used for hologram pressure interpolation. So that, a new method based on interpolation by using PGA was proposed for enhancing the resolution of the nearfield acoustic holography. The validity of the proposed methods was proved by numerical simulations and experiments.In chapter four, firstly, the theoretical base of acoustic radiation was reviewed and the basic equation of wave superposition approach (WSA) was deduced. According to the mechanism of WSA, the acoustic fields on and near the measurement surface can be approximated by the fields produced by fictitious sources placed inside the structure. Therefore, the pressure extrapolation can be realized by the superposition of fields generated by these fictitious sources. Based on this, the WSA was employed to realize hologram pressure extrapolation and interpolation. The Patch NAH and NAH image resolution enhancing method based on WSA were established. Then, the numerical stability of WSA was investigated. The model optimization and regularization strategy were proposed to deal with the ill-posed problem in solving the source strengths of fictitious sources.In chapter five, a new hologram pressure extrapolation method using orthogonal spherical wave source is proposed According to the solution of Helmholtz equation in spherical coordinates, any acoustic field can be approximately expressed as a linear sum of a series of orthogonal spherical wave functions. So that, the acoustic field generated by vibrator can be approximated, by superposing the fields generated by a series of orthogonal spherical wave sources of different orders, and the pressure exploration is realized by radiating process of these spherical wave sources. Because the orthogonal spherical wave sources of different orders are orthogonal with each other, they can be placed at one point inside the vibrator. So there is no need to design the position of all the fictitious sources.In chapter six, researches in this dissertation are summarized, and the topics which need to be further studied are proposed.