Experimental observation of bending wave localization

Abstract

Localization of bending waves has been observed for the first time for two-dimensional (2D) acoustic wave propagation in an inhomogeneous composite system consisting of a steel plate decorated with two sets of randomly attached Lucite blocks. A significant experimental feature of the localized modes is an exponentially decay of the mode intensity from their peaked centers, with a decay length that increases as (f0−f)−1 when the mode frequency f approaches a quasimobility edge f0. The minimum attenuation length is of the order of a block diagonal and is about 40% of the banding wave’s wavelength. The experimental data, as well as results of finite-element calculations, identify the source of the localization phenomenon as strong scattering of the bending wave by shear and flexural resonances of the Lucite blocks. This result supports theoretical predictions that resonant scattering enhances localization [cf. ScatteringandLocalizationofClassicalWavesinRandomMedia, edited by P. Sheng (World Scientific, Singapore, 1990)]. Recent experiments on acoustic wave propagation in rough composite steel/refractory walls also exhibit exponential localization at frequencies corresponding to 3-D propagation. The experimental data suggest that a composite plate is a unique vehicle for the study of classical localization: at low frequencies, in 2-D and, at higher frequencies, in 3-D. The generic nature of the localization phenomenon suggests its application as a tunable attenuation mechanism for bending waves