| Measuring the acoustic power of cylindrical focusing transducers and studying dynamic focusing model of nonlinear acoustic fields under high frequencies and large aperture angles are of great significance to ensure the effectiveness and safety of interstitial thermal ablation. This dissertation addresses the research gap in accurate and effective acoustic power measurement for cylindrical converging waves. Based on radiation force balance(RFB) method, the theoretical model is derived, and the experimemt is conducted. The self-reciprocity method of acoustic power measurement for cylindrical converging waves is proposed systematically, according to the reciprocity theorem of electro-acoustics for transducers. Dynamic focusing models for a linear-phased array on a cylindrical focusing transducer and a multi-annular-phased array on a spherical concave transducer are explored and established. The main topics in this dissertation are listed as follows.Based on the theory of geometric acoustics, the model for acoustic power measurement of cylindrical converging waves is studied using RFB method. For a reflecting or an absorbing target, the formulas are obtained with a cylindrical focusing transducer and a linear array on a cylindrical concave surface. Comparison between ray acoustic model and far-field integration model is processed by numerical calculation. Applications for these two models are discussed. Acoustic power is measured on a convex-prism reflector and a planner absorber under different transducer-to-target distances and different driving electronic power sources using RFB system combined with the theoretical model. Applications for these two targets are also suggested.Based on the reciprocity principle of plane and spherical waves, a self-reciprocity method is proposed to measure the acoustic power for cylindrical converging waves. This study defines the electro-acoustic parameters of cylindrical converging waves, namely, the reciprocity coefficient, transmission current response, and the voltage sensitivity for the free field. Formulas of the self-reciprocity method for cylindrical converging waves are derived. Numerical calculations of reflection and diffraction correction coefficients are discussed. Expressions of acoustic power and radiation conductance for self-reciprocity method with a cylindrical focusing transducer are then obtained. An important correction to previous formulas for plane and spherical waves is proposed, and the measurement accuracy is improved. Two cylindrical focusing transducers are employed for the self-reciprocity experiment, whereby results are in agreement with those of RFB methods. Measuring acoustic power via the self-reciprocity method for cylindrical converging waves is found feasible and effective.Dynamic focusing model is studied for cylindrical converging waves based on the linear acoustic field. Acoustic field distributions in the radical, azimuthal, and axial directions are investigated theoretically by numerical simulation. Focusing performance is thoroughly studied under different focal distances, steering angles, element arc-widths, arc-spaces between adjacent elements, and numbers of elements. This study presents a theoretical analysis of the acoustic field experiment with a linear-phased array on a cylindrical focusing transducer.Source boundary conditions using spheroidal beam equation(SBE) are proposed for a multi-annular-phased array on a spherical concave transducer. The effectiveness of dynamic focusing model for linear SBE is validated by numerical computations. The pilot study for dynamic focusing model of nonlinear SBE is also implemented, whereby numerical results are in strong agreement with those of the Khokhlov-Zabolotskaya-Kuznetsov(KZK) model. Compared with the O'Neil and KZK models, the dynamic focusing model of SBE lays a theoretical basis for the study of axial dynamic focusing performance and nonlinear harmonic effects under high frequencies and large aperture angles. |
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