Estimation of Sound Velocity Distribution Using Sectional Near-Field Acoustical Holography and Global Search

Abstract

We propose a method of estimating sound velocity distribution by the combined use of near-field acoustical holography (NAH) and optimization. In this fundamental research, the sound velocity varies only in the z-axis, and the sound velocity is a continuous distribution. In addition, the sound velocity varies in the range of a sound velocity that can be varied in water. For the estimation of sound velocity distribution, we proposed a modified method of NAH to calculate in an inhomogeneous medium called Sectional NAH (SNAH). In SNAH, a calculation space is discretized into a number of sections so that NAH can be used approximately in each section. The calculation result by SNAH in an inhomogeneous medium agreed well with the calculation result by finite element method (FEM). Here, the sound velocity distribution is estimated. Firstly, sound fields are measured. Next, the sound field at one of the measured planes is calculated from the other measured sound fields by SNAH with initial sound velocity. Then, the difference between the calculated and measured sound fields is minimized by optimizing the sound velocity distribution by the multi start downhill simplex method. Sound velocity distributions given as linear and Gaussian distributions were estimated in simulations by SNAH and FEM. The validities of our proposed methods were confirmed by simulations.