Journal of The School of Marine Science and Technology,Vol.6 No.1
Nonlinear Acoustic Field of Composite-Type Two-Frequency
Coaxial Focusing Source
Shigemi SAITO, Iwaki AKIYAMA and Natsuki YOSHIZUMI
Abstract
To effectively eliminate the speckle noise in ultrasonic images, the present authors are studying a method of superposing many images generated with different frequencies. In order to easily achieve such a condition, the use of two sound beams emanated from a two-frequency coaxial focusing source has been presented, where six sound beams with different frequencies are strongly excited;two original primary sounds,two second harmonic sounds that are generated by nonlinear transmission of each beam,and the sounds with difference and sum frequencies that are generated by nonlinear interaction of two primary waves.In this paper,the linear or quasi-linear solutions for such six sound fields are derived by successively approximated solution for the Khokhlov-Zabolotskaya-Kuznetsov equation. The analysis is performed for the field divided into two regions;a layer of water coupler and another dissipative layer modeling biological medium. While the solutions for linear components are expressed by single integration, the solutions for nonlinearly generated components are complicated ones including quadruple integrations. To facilitate the numerical computation, the latter solutions are approximated to triple integrations using integral exponential function.The validity of the present analysis is confirmed through the comparison between the computation and experiment for the acoustic field generated in water with a coaxial focusing source of2and 8MHz. The analytical result for 2, 4, 6, 8and 10MHz sound fields agrees well with the experimentally observed ones.