Theoretical And Experimental Studies On Transient Nearfield Acoustic Holography

Nearfield acoustic holography (NAH), developed in recent over20years, is an advanced acoustic technique with the powerful abilities of identifying noise sources and visualizing sound fields. It can provide the important guidance for the noise control, as well as the low-noise and sound quality design of product. Up to now, the domestic and foreign scholars have proposed a number of holography methods. However, these methods almost proceed in the frequency domain, and are only applied to stationary sound fields. In practice, sound fields are usually transient. If the NAH is used in transient sound fields to obtain the varying patterns of sound fields with time, it will be applied to the engineering more widely. Therefore, transient nearfield acoustic holography (TNAH) is developed. On the basis of analyzing the research significance and development of TNAH, it is divided into four kinds in accordance with the processing mode of pressure signals on the hologram surface:TNAH based on three-dementional transform, based on two-dementional transform, based on one-dementional transform, and without any transform.In this dissertation, problems existing in these four kinds of TNAH were investigated and discussed deeply, and then the corresponding solutions were provided. In the research of TNAH based on three-dementional transform, a method based on the Laplace transform for reconstructing the particle velocity in transient sound fields was proposed to solve the singularity appeared in time domain acoustic holography with the Fourier transform. In TNAH based on two-dementional transform, the impulse response function between pressure on the hologram surface and particle velocity on the reconstruction surface was deduced, based on which real-time acoustic holography processed the ability of reconstructing particle velocity. With respect to TNAH without any transform, two new approaches were presented:one is based on time domain plane wave superposition method, and the other adopted an interpolated time-domain equivalent source method (ESM). To realize the use of TNAH in nonfree sound fields, a separation technique of transient sound fields was developed. The detailed research contents of this dissertation are summarized as follows:In chapter one, the significance of TNAH was first elaborated, and then the development of calculating transient acoustic radiation at home and abroad was discussed to provide references and ideas for TNAH, finally by reviewing existing approaches of TNAH and analyzing their defects, the main research contents of this dissertation were determined.In chapter two, time domain acoustic holography based on three-dementional transform was investigated. Its reconstruction formulas of sound pressure were deduced based on Fourier transform, and its reconstruction errors as well as the corresponding control methods were analyzed in theory. To solve the singularity appeared in the process of reconstructing the particle velocity by time domain acoustic holography based on the Fourier transform, a method using the Laplace transform was proposed, whose validity was also confirmed by a numerical simulation with a baffled piston as source.In chapter three, real-time acoustic holography based on two-dementional transform was analyzed. The impulse response function between sound pressure on the hologram plane and sound pressure on the reconstruction plane was first deduced by Laplace transform from the wave equation of velocity potential and Neumann boundary condition, and then based on it the reconstruction process of sound pressure was shown. To reconstruct the particle velocity in real time, the impulse response function between sound pressure on the hologram plane and particle velocity on the reconstruction plane was also deduced by Laplace transform, and then based on it a reconstruction method of particle velocity using real-time acoustic holography was proposed and verified by a numerical simulation with a baffled circular piston as source.In chapter four, two approaches of TNAH without any transform were proposed by using the wave superposition idea:one is based on time domain plane wave superposition method, and the other based on an interpolated time domain ESM. They both avoided the use of two-dementional spatial Fourier transform, thus removing restrictions brought by two-dementional spatial Fourier transform. Besides, their reconstructions were proceeding in the time domain, which provided the ability of reconstructing sound fields in real time. The formulas of TNAH based on time domain plane wave superposition method were first deduced, and then its reconstruction process was elaborated, finally its validity and advantages were demonstrated respectively by numerical simulations and experiments, and by comparisons with time domain acoustic holography and real-time acoustic holography. Similarly, the formulas of TNAH based on an interpolated time domain ESM were deduced, and its reconstruction process was elaborated. Three simulation cases with different shape sources were carried out to demonstrate that TNAH based on an interpolated time domain ESM not only processed conventional abilities of TNAH, but also got rid of the limit from source shapes, thus having more wide application range.In chapter five, TNAH was extended to the nonfree sound fields. In the case of nonfree sound fields, a transient sound field separation technique was first developed to remove the influence of disturbing sources and separate out the sound field only radiated by objective sources in both time and space domains, and its separation formulas were deduced based on the propagation principle of sound pressure in the time-wavenumber domain. Then by utilizing the separated sound field radiated by objective sources and combining TNAH, the reconstruction could be realized in the case of nonfree sound fields.In chapter six, all the investigations in this dissertation were summarized, and the topics studied further in the future were proposed.