Methods For Non-stationary Acoustic Radiation Calculation And Sound Field Reconstruction

Near-field acoustic holography (NAH) is an advanced acoustic imaging technique with the powerful abilities of identifying noise source and visualizing sound field. By measuring the near-field pressure or particle velocity on the hologram surface, NAH can not only calculate the radiated sound field, but also can reconstructe any acoustic quantities both on the surface of source and in the three-dimensional sound field. In more than 30 years, researchers have proposed kinds of NAH methods. These methods provide the calculated results in the frequency domain and are mainly applied to the stationary source. However, most sources in the practical engineering are non-stationary sources whose statistical properties fluctuate in time. It is more important to expand NAH into the non-stationary source for analyzing the acoustic characteristics related to time. Therefore, non-stationary NAH (NS-NAH) is proposed.On the basis of studying the methods for the non-stationary acoustic radiation calculation and the sound field reconstruction, the methods of non-stationary acoustic radiation calculation are divided into three kinds according to the shape of non-stationary source,including the acoustic radiation calculation methods of non-stationary plane source, the acoustic radiation calculation methods of non-stationary curved surface source and the acoustic radiation calculation methods of non-stationary arbitrarily-shaped source. Accordingly, the methods for the non-stationary sound field reconstruction are divided into two kinds according to the shape of non-stationary source, including the sound field reconstruction methods ofnon-stationary regular-shaped source,the sound field reconstruction methods of non-stationary arbitrarily-shaped source. For realizing the non-stationary sound field reconstruction in a non-free field, the time-domain separation techniques are also analyzed. After deeply studying the problems in these calculation methods of non-stationary acoustic radiation and reconstruction methods of non-stationary sound fields, several solving methods are proposed in this dissertation. First, methods for the acoustic radiation calculation and the sound field reconstruction based on K-t method were proposed for the non-stationary plane source. Then, for a non-free field,separation techniques of non-stationary sound fields based on K-t method were proposed to extract the time-dependment pressure generated by a target source. Third,for the non-stationary arbitrarily-shaped source, methods for the acoustic radiation calculation and the sound field reconstruction based on interpolated time-domain equivalent source method were proposed. Lastly,for the non-stationary multiple sound fields, a separation technique based on the interpolated time-domain equivalent source method was proposed to separate the pressure field generated by every source alone. The detailed researches are shown in the following:In chapter one, the significance of NS-NAH was firstly described. Then, the development and research status of the acoustic radiation calculation methods and sound field reconstruction methods of non-stationary source were introduced in detail,and their problems were analyzed. Lastly, on the basis of these problems, the main research contents in this dissertation were given.In chapter two, the impulse response functions of pressure-acceleration (p-a),acceleration-acceleration (a-a) and velocity-velocity (v-v) were firstly derived for the non-stationary plane source by using Laplace transform. Then, on the basis of the p-a impulse response function, K-t method for the acoustic radiation calculation was proposed, and it was validated by using an experiment. Finally, by using the impulse response functions of p-a, a-a and v-v, K-t methods for the reconstructions of the normal acceleration and velocity were proposed, and an experiment of an impacted plate was used to validate the validity of the proposed methods of reconstructing the normal acceleraction and velocity.In chapter three, K-t method was applied to the non-free field. Firstly, a real-time sound field separation technique (RT-FST) with pressure and particle acceleration measurements was proposed to extract the desired non-stationary sound field generated by a target source in presence of disturbing sources. A numerical simulation has been carried out to show the separated results of RT-FST in time and space domains. An experiment further evidenced the feasibility of the RT-FST. Then,a time-domain sound field separation technique with single layer pressure-velocity measurements (SL-FST) was developed to separate the non-stationary sound field radiated by the target source from the mixed one in presence of disturbing sources. An experiment was carried out to validate the validity of SL-FST, and showed the necessity of high-pass filter in the time-domain calibration of p-u probe. Comparing with the separated results of RT-FSM, SL-FST can provide better separation.In chapter four, an interpolated time-domain equivalent source method was firstly proposed to calculate the acoustic radiation of the non-stationary arbitrarily-shaped source. Numerical examples of a baffled planar piston and an impulsively accelerating sphere demonstrated that the proposed method can model the non-stationary acoustic radiation very well. The experimental results also further verified the validity of the proposed method. Then, by using the relationships between the pressure (normal particle velocity) on the hologram surface, the normal acceleration (normal velocity)on the source surface and the equivalent sources, the interpolated time-domain equivalent source method was expanded to reconstrate the normal acceleraction and velocity. An experiment of an impacted plate was used to validate the validity of the proposed methods of reconstructing the normal acceleraction and velocity.In chapter five, on the basis of the interpolated time-domain equivalent source method, a separation technique of non-stationary multi-source sound fields with single layer pressure measurement was developed to separate the sound field radiated by every source alone from the mixed ones. A numerical simulation with two baffled circular pistons was used to validate the validity of the proposed separation technique,and the influences of distance between the hologram surface and the equivalent source surface, noise and equivalent source spacing were analyzed. An experiment with two speakers further evidenced the feasibility of the proposed separation technique.In chapter six, the whole research work of the dissertation was summarized, and several topics needing further study were given.