Observation of acoustic wave localization.

Abstract

Acoustic wave propagation has been studied in a composite elastic plate consisting of a periodic square array (horizontal period=7 in.) of 190 lucite blocks (3.5×3.5×3 in.) glued to a 1-in.-thick, 6×6-ft steel plate. Elastic wave propagation is excited using a 0.2-ms force transducer and the vertical response is measured by an accelometer (0–25.6 kHz). The significant feature of the data is a 20- to 30-dB gap in the amplitude of the frequency response of the transfer function in the band between 2000 and 3800 Hz, which roughly corresponds to wavelengths coherently scattered by horizontal and diagonal arrays of lucite blocks, as well as to the first Brillouin zone of the square array. The decay length of the strongly localized bending wave mode is of the order of one unit cell ≊10 in. The decay length of the localized mode exponentially increases at the high-frequency side of the gap with an inverse logarithmic derivative of 120 Hz. It is believed that the interaction between the propagating bending mode of the plate and the coherent shear vibrations of the fixed lucite blocks is responsible for the observed elastic wave localization. Comparison is made to exact calculations of the transfer function for a uniform composite plate and calculations of the propagating bending and shear modes of an infinite periodic array of lucite blocks on a steel plate.