Determination of sound velocity distribution using sectional near-field acoustical holography and simulated annealing

We propose methods for calculating sound field propagation in an inhomogeneous medium called ¿sectional near-field acoustical holography¿ (SNAH), and estimating a sound velocity distribution using SNAH and simulated annealing (SA). SNAH can be applied to an inhomogeneous medium. In contrast, a wavenumber must be constant in a calculated area in ordinary NAH theory. A space between two planes is discretized into many sections so that the sound velocity in each section becomes approximately uniform. Then, NAH is used to calculate each section. Calculation result using SNAH well agreed with the calculation result using finite element method (FEM). Then, the sound velocity distribution is estimated. Firstly, sound fields in three planes are measured. Secondly, a sound field at one of measured plane is calculated from the measured sound fields at the other measured plane using SNAH with an initial sound velocity distribution. Finally, difference of the real part of calculated and measured sound fields is minimized by optimizing the sound velocity distribution using SA. Sound velocity distributions as linear, quadratic, and gaussian distribution can be estimated. Validity of our proposal methods are confirmed by simulations.