Separation And Reconstruction Of The Coherent/Non-coherent Sound Field Radiated By Multiple Sources

Nearfield acoustic holography (NAH) is an advanced technique for identifying the noise sources and visualizing the sound field. Through measuring the sound field close to the sound sources, the NAH technique can provide acoustic informations on the surface of the sound sources or in the three-dimensional sound field. These informations can help to realize low-level noise design, sound quality design and noise control of mechanical and electronic products. But the NAH requires that the measurement must be done in a free sound field, which is difficult to meet because the real sound field is usually complicated, for example, there are disturbing sources or reflections on the other side of the measurement surface, the noise source is closely to the ground, the noise source is inside an enclosed space, or the sound field is generated by non-coherent sources and so on. These complicated sound fields can be regarded as the superposition of sub sound fields radiated by multiple sound sources which are target sources, disturbing sources, reflections from ground or from the wall. These sub sound sources maybe coherent or not coherent. In such a real situation, the premise of reconstructing the target sources is to remove the influences of all the other sub sound sources.This dissertation studied the separation and reconstruction of the coherent /noncoherent sound fields radiated by multiple sources in different situations by using the equivalent source method (ESM), broadband acoustic holography from intensity measurement (BAHIM), sound field separation technique and particle velocity transducer. For the half-space sound field, two half-space NAH methods were proposed to improve the reconstruction accuracy; for the enclosed sound field, the double-layer patch NAH based on ESM was proposed, which can break through the restriction of hologram aperture and remove the influence of refletions from the walls; for the noncohrent sound field, the partial field decomposition method was proposed by using the particle velocity instead of pressure as reference to enhance the decomposition accuracy; the three-dimenrsional BAHIM technique was proposed by using a three-dimenrsional pressure-velocity (p-u) sound intensity probe, based on which the sound field on either side of the hologram plane can be separated without any reference signal. This dissertation was organized as follows:In chapter 1, the development of NAH was briefly reviewed, and then the existing problems and shortcomings were discussed, leading to the main research objectives.In chapter 2, two ESM-based half-space NAH techniques were proposed, one of which inserted the exact half-space green’s function into the regular ESM-based NAH, and the other one replaced the reflections caused by the boundary with a series of simple sources. Both numerical and experimental results verified the effectiveness of the two techniques, and the conditions in which the two techniques were effective were discussed with the boundary impedance.In chapter 3, by combining the sound field separation and patch NAH techniques, the ESM-based double-layer patch NAH was proposed, following by an error sensitivity analysis. The results of simulations and experiments indicated that this technique could remove the influence of disturbing sources or reflections and realize the patch reconstruction of target source.In chapter 4, the particle velocity reference was introduced into the partial field decomposition method, and numerical and experimental results verified its effectiveness and superiority over pressure reference. At the same time, the influence of the position and direction of particle velocity reference on the decomposition accuracy was studied. Moreover, a NAH-based sensitivity measurement method was proposed, and the sensitivities of a one-dimensional and a three-dimensional (3-D) p-u intensity probe were measured and compared with the references.In chapter 5, by using a 3-D p-u intensity probe, the BAHIM was extended to be 3D-BAHIM, which can measure the complex pressure and normal particle velocity on hologram surface without any reference signal, and finally realize the separation and reconstruction of the target sound field. The 3D-BAHIM is applicable to both coherent and noncoherent sound field. Furthermore, various kinds of solutions for dealing with the singularity problems inherent in the 3D-BAHIM were given, and their performances were compared.In chapter 6, the whole research work of the dissertation was summarized, and several topics needing further study were given.