A further development of underwater near-field acoustical holography: Toward a more quantitative measurement technique

Abstract

Near-field acoustical holography (NAH) is a strong experimental tool, which allows the visualization of the underlying physics in structural vibration, radiation, and scattering. NAH technology has been extending the scope of its application by algorithmic and experimental adaptations. However, it is recognized that NAH leaves behind some problems arising from the spatial FFT processing if one tries to extract more quantitative information for structural analysis. For example, it is not as easy to estimate the reflection coefficient of a rib attached on a plate as accurately as it is with the NAH measurement. Also, the precision in reconstructing the structural intensity seems to be relatively low. New methods to reconcile these difficulties are proposed in this paper: One is an ‘‘extrapolation method’’ to reduce the errors due to wraparound and zero padding; the other is an ‘‘adaptive K-space window method’’ to choose the maximum wave number of the measured evanescent wave depending on the differentiation order in the equation of the structural intensity.